ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-3609-2016Trapping, chemistry, and export of trace gases in the
South Asian summer monsoon observed during CARIBIC flights in 2008Rauthe-SchöchArminarmin.rauthe-schoech@mpic.dehttps://orcid.org/0000-0001-5738-8112BakerAngela K.https://orcid.org/0000-0001-7845-422XSchuckTanja J.https://orcid.org/0000-0002-1380-3684BrenninkmeijerCarl A. M.ZahnAndreasHermannMarkusStratmannGretaZiereisHelmuthttps://orcid.org/0000-0001-5483-5669van VelthovenPeter F. J.LelieveldJoshttps://orcid.org/0000-0001-6307-3846Max Planck Institute for Chemistry (Otto Hahn Institute), Department of Atmospheric Chemistry, Mainz, GermanyKarlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research, Karlsruhe, GermanyLeibniz Institute for Tropospheric Research (TROPOS), Leipzig, GermanyGerman Aerospace Center (DLR), Institute for Atmospheric Physics, Oberpfaffenhofen, GermanyRoyal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlandsnow at: Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, GermanyArmin Rauthe-Schöch (armin.rauthe-schoech@mpic.de)17March2016165360936296February201510March201522February20161March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/3609/2016/acp-16-3609-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/3609/2016/acp-16-3609-2016.pdf
The CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere
Based on an Instrument Container) passenger aircraft observatory performed in
situ measurements at 10–12 km altitude in the South Asian summer monsoon
anticyclone between June and September 2008. These measurements enable us to
investigate this atmospheric region (which so far has mostly been observed
from satellites) using the broad suite of trace gases and aerosol particles measured
by CARIBIC. Elevated levels of a variety of atmospheric pollutants (e.g.
carbon monoxide, total reactive nitrogen oxides, aerosol particles, and
several volatile organic compounds) were recorded. The measurements provide
detailed information about the chemical composition of air in different parts
of the monsoon anticyclone, particularly of ozone precursors. While covering
a range of 3500 km inside the monsoon anticyclone, CARIBIC observations show
remarkable consistency, i.e. with distinct latitudinal patterns of trace
gases during the entire monsoon period.
Using the CARIBIC trace gas and aerosol particle measurements in combination with the
Lagrangian particle dispersion model FLEXPART, we investigated the
characteristics of monsoon outflow and the chemical evolution of air masses
during transport. The trajectory calculations indicate that these air masses
originated mainly from South Asia and mainland Southeast Asia. Estimated
photochemical ages of the air were found to agree well with transport times
from a source region east of 90–95∘ E. The photochemical ages of the
air in the southern part of the monsoon anticyclone were systematically
younger (less than 7 days) and the air masses were mostly in an ozone-forming
chemical mode. In its northern part the air masses were older (up to 13 days)
and had unclear ozone formation or destruction potential. Based on analysis
of forward trajectories, several receptor regions were identified. In addition
to predominantly westward transport, we found evidence for efficient
transport (within 10 days) to the Pacific and North America, particularly
during June and September, and also of cross-tropopause exchange, which was
strongest during June and July. Westward transport to Africa and further to
the Mediterranean was the main pathway during July.
Introduction
During boreal summer the South Asian monsoon dominates atmospheric
circulation over Asia, and has a strong influence on atmospheric transport
and chemistry of much of the Northern Hemisphere . The
monsoon is characterised by a persistent large-scale anticyclonic structure
in the upper troposphere, centred over Pakistan and northern India, framed by
the subtropical eastward jet in the north and a westward equatorial jet in
the south. This upper troposphere anticyclone (UTAC) is not static but
oscillates in strength, shape, and position . At the same
time it features a strikingly distinct composition signature throughout the
monsoon season. Observations from satellites have shown an enhancement of
mixing ratios of a number of trace gases in the UTAC, most prominently
methane (CH4) and carbon monoxide
(CO) . Since the monsoon is accompanied by strong
convection, upper tropospheric trace gas mixing ratios are directly linked to
surface emissions in this densely populated region. In addition, polluted air
masses can be trapped and accumulate inside the UTAC, where they can be
chemically isolated for several days . The UTAC
can also play a governing role in the dispersion of volcanic plumes, e.g.
after the June 2011 eruption of the Nabro volcano in
Eritrea . Outflow occurs predominantly westward towards
northern Africa and the Middle East, where a summertime ozone (O3)
maximum due to ozone formation in monsoon outflow has been
reported , and to the Mediterranean
region . Not only does the South Asian
summer monsoon influence trace gas and aerosol particle loadings in the upper
troposphere, it also affects cross-tropopause transport into the lowermost
stratosphere . An
extensive review on southern Asian pollution outflow in all seasons is given
by .
While most observations in the South Asian summer monsoon UTAC are from
satellites, CARIBIC Civil Aircraft for the Regular Investigation of
the Atmosphere Based on an Instrument Container,
http://www.caribic-atmospheric.com/;
phase 2 provides first in situ observations over the Indian subcontinent,
performed during the 2008 monsoon season. During the summer monsoon period
from June through September, 14 flights between Frankfurt, Germany, and
Chennai, India, were conducted, crossing the western part of the UTAC at
altitudes between 10 and 12 km. The CARIBIC observatory thus
contributes to the measurements within the UTAC called for by experts on
atmospheric chemistry and the Asian monsoon .
The CARIBIC in situ observations in the upper troposphere aim to contribute
to the understanding of the monsoon and its impacts on atmospheric
composition through this work and previous studies which dealt with the
elevated mixing ratios of a range of trace gases that were measured by
CARIBIC within the UTAC, for example CO, CH4, nitrous oxide (N2O),
sulfur hexafluoride (SF6) , several non-methane
hydrocarbons NMHCs;, and methyl
chloride CH3Cl;. Furthermore, as
earlier measurements from CARIBIC phase 1 have shown, aerosol particle number
concentrations are enhanced in the UTAC . Trajectory
calculations indicated that the air masses originated mainly from South Asia
and mainland Southeast Asia and had been transported up to cruise altitude by
deep convection associated with the summer monsoon. High mixing ratios of
water vapour at southern latitudes confirmed that recent convection had
occurred. An analysis of tracer correlations, namely CO, CH4, and ethane
(C2H6), revealed that, in addition to enhanced vertical transport of
polluted boundary layer air, emissions of methane from biogenic sources, such
as wetlands, open landfills, and rice paddies, increase during the summer
months , resulting in disproportionately high mixing
ratios. Carbon dioxide (CO2) mixing ratios were found to be lower inside
the UTAC. A model study using CARIBIC data found that, at least in 2008, the
region was a net sink of CO2 because of strong uptake by the
terrestrial biosphere . Independently, observed low CO2
mixing ratios with increased δ13C(CO2) values combined with low
δ18O(CO2) values indicate photosynthetic uptake of CO2 and
oxygen atom exchange with soil and leaf water .
CARIBIC2 flights to Chennai, India, in 2008.
Listed are the flight number, date of departure and flight direction.
Column 4 gives the northern cut-off for the trajectory analysis which was
selected for each month to exclude trajectories belonging to the eastward
jet stream. Column 5 lists the latitude of the wind reversal along
each flight track which is used as an indicator of centre latitude of the
monsoon UTAC at flight altitude (see Fig. ).
The last column indicates whether air samples have been collected or not.
Flight no.DirectionDate of departureNorthern cut-offMonsoon centre latitudeAir samples?236southward18 June 200836.5∘ N27.9∘ Nyes237northward18 June 200836.5∘ N25.5∘ Nyes238southward19 June 200836.5∘ N25.8∘ Nno239northward19 June 200836.5∘ N24.4∘ Nno240southward15 July 200840.0∘ N26.7∘ Nyes241northward15 July 200840.0∘ N26.1∘ Nyes244southward13 August 200840.0∘ N26.6∘ Nyes245northward13 August 200840.0∘ N24.6∘ Nyes246southward14 August 200840.0∘ N25.8∘ Nno247northward14 August 200840.0∘ N25.3∘ Nno248southward10 September 200835.5∘ N23.5∘ Nyes249northward10 September 200835.5∘ N22.7∘ Nyes250southward11 September 200835.5∘ N24.9∘ Nno251northward11 September 200835.5∘ N24.0∘ Nno
Interestingly, the CARIBIC observatory also encountered air masses with the
typical monsoon signature of elevated mixing ratios of CH4, N2O,
SF6,
and some NMHCs, accompanied by relatively low CO2 mixing ratios far away
from the South Asian monsoon region, namely over eastern Canada in the
vicinity of Toronto in September 2007. This raises the question about the
whereabouts of air masses that are exported from the UTAC. Air mass
trajectories pointed to export from the monsoon region (see Sect. S1 for
details), although transport of Asian pollution over the Pacific towards
North America occurs predominantly in the northern hemispheric winter and
spring , whereas in summer westward outflow is prevalent.
While no comparable case was observed during CARIBIC flights to North America
in subsequent years, plumes of photochemically processed air originating from
Asia have been probed over different parts of North America during INTEX-NA
(the Intercontinental Chemical Transport Experiment – North America) flights
in July and August 2004 .
CARIBIC measurements yield a fairly detailed description of the chemical
composition of air in different parts of the UTAC, including mixing ratios of
ozone precursors like the sum of reactive nitrogen oxides (NOy), CO, and
NMHCs. Using this information and the Lagrangian particle dispersion model
FLEXPART we investigate the characteristics of the
trapping of pollution, its distribution and the evolution of chemical
composition in the UTAC. Furthermore, based on analyses of air mass forward
trajectories several receptor regions are identified and their relative role
for monsoon pollution export is tentatively quantified. Additional material
is presented in the Supplement.
Methods
The CARIBIC observatory phase 2 started its routine operation in 2005 using a
Lufthansa Airbus A340-600 to make measurements during sequences of typically
four long-distance flights per month. This specific aircraft was
retrofitted in 2004 with a permanently mounted air and aerosol particle
inlet system which is connected via (partially heated) stainless steel tubing
(some of which lined with perfluoroalkoxy alkane (PFA) to reduce wall
effects) to the CARIBIC container when installed .
The 1.6 t container houses instruments for in situ measurements and remote
sensing as well as systems for air and aerosol particle collection. In this
study, results from both the in situ measurements providing a high data
density along the flight track and from the air sampling equipment are used.
The latter gives a low data density (28 samples per monthly flight sequence)
but with more detail because many more trace gases are analysed
retrospectively in the collected air samples. Flights start in Frankfurt
(since August 2014 from Munich), Germany, to various destinations around the
globe. After the final return flight, the container is unloaded, the
measurement data are retrieved and the air samples are analysed for a suite
of different trace gases in several
laboratories .
From April to December 2008, the CARIBIC container took measurements during
32 flights between Frankfurt, Germany, and Chennai, India (see list on
http://www.caribic-atmospheric.com). For
this study, we consider the 14 “monsoon” flights conducted between June and
September 2008 (see list of flights in Table ). These
months represent the core of the monsoon period over India as previously
discussed by and . More information
about the non-monsoon months can be found in these two previous studies and
will not be discussed here. All 14 flights crossed the western part of the
monsoon UTAC between the western coast of India and Chennai at the southeast
coast. The UTAC was probed at altitudes between 10.3 and 11.9 km (see
Fig. S4 in the Supplement). Wind fields obtained from the European Centre for
Medium-Range Weather Forecasts (ECMWF) at the 250 hPa model level,
which corresponds approximately to the flight altitude, are shown as 10-day
means in Fig. . In the north, the meandering
subtropical jet is shifted northwards and slows down as the monsoon
progresses northwards from June to August, while it regains strength and moves
south in September as the monsoon retreats. In June, the wind arrows show the
UTAC centred over northern India and Pakistan. July shows an expansion of the
UTAC towards eastern China and a split of the UTAC with a second “eye” over
northern Africa. In August, the centre of the UTAC is spread out over
northern India, Pakistan, and the Arabian Peninsula, and with the retreat of
the monsoon in September, the UTAC slowly fades and is less clearly visible
in the wind field.
CARIBIC flight tracks across the
South Asian monsoon region in June–September 2008. The panels show the
flight tracks for each month and 10-day mean ECMWF horizontal wind fields at
250 hPa ending at the day of the last flight in each month. The
colour code shows the wind speed, while the white arrows indicate the wind
direction. Note the anticyclonic monsoon circulation centred over the
Indian subcontinent.
Emissions of CO (left panel) and
non-methane volatile organic compounds (right panel) for August 2008 from the
Regional Emission inventory in ASia (REAS) v2.1
including the update from 14 November 2013. The red lines mark the CARIBIC
flight tracks for June to September 2008.
In addition to a suitable wind field structure provided by the monsoon UTAC,
trapping pollutants also requires sources for these which are typically
located at the ground e.g..
Figure shows emission data for CO and the sum of all
non-methane volatile organic compounds (NMVOCs) from all sources listed in the
Regional Emission inventory in ASia (REAS) version 2.1 including the
November 2013 update for August 2008 in India and the
surrounding region below the UTAC and in its source region. The geographical
distributions for CO (left panel) and NMVOCs (right panel) are similar in the
region. The Indo-Gangetic plain in northern India and southern central China
show high CO emissions together with parts of Thailand and Vietnam. NMVOC
emissions are somewhat higher in southern India and lower in southern central
China compared to CO emissions. Detailed modelling studies with the Weather
Research and Forecasting with Chemistry (WRF-Chem) online regional chemistry
transport model to link emissions and transport have been published
separately .
CARIBIC trace gas and aerosol measurements
The CARIBIC container houses a number of different instruments for measuring
a spectrum of trace gases and also aerosol
particles . Discussed extensively in this work are
CO, O3, NOy, and aerosol particles (N4-12 and N12). Brief
descriptions of these measurements are presented next.
Carbon monoxide is measured with an AeroLaser AL 5002 resonance fluorescence
UV instrument modified for use onboard the CARIBIC passenger aircraft. The
instrument has a precision of 1–2 ppbv at an integration time
of 1 s and performs an in-flight calibration every 25 min. A
technical description of the CO instrument can be found
in .
Ozone measurements are based on a fast, commercially available dry
chemiluminescence (CL) instrument, which has a precision of 0.3–1.0 % at typical ozone mixing ratios
between 10 and 100 ppbv and a measurement frequency of 10 Hz
. The performance of this instrument
has been characterised in detail by . The absolute ozone
concentration is inferred from a UV photometer designed in-house which
operates at 0.25 Hz and reaches an accuracy of 0.5 ppbv.
The sum of reactive nitrogen oxides (NOy) is determined using an
NO chemiluminescence detector (Eco Physics AG) after conversion of
NOy to NO. The time resolution is 1 s.
The overall uncertainty for the NOy measurement at 10 s time
resolution is about 7 % at 450 pptv
NOy.
The total water mixing ratio (gaseous, liquid, and ice phase) is measured by
means of a chilled-mirror frost-point hygrometer (CR-2, Buck Research
Instruments L.L.C.) with a time resolution of 5 (high humidity) to
300 s in the dry lowermost stratosphere with an uncertainty of
around 0.3 ppmv at cruise altitude. A modified two-channel
photoacoustic diode-laser spectrometer (Hilase, Hungary) measures both
gaseous (H2Ogas) and total water mixing ratios with a time
resolution of 3 s and a precision of 1 ppmv. These
measurements are calibrated to the absolute water mixing ratios determined
with the frost-point hygrometer. A detailed description of the CARIBIC
humidity measurements is available in . An overview of the
H2O data can be found in .
Integral aerosol particle number concentrations are measured with three
condensation particle counters (CPC, modified TSI model 7610) with lower
threshold diameters (50 % counting efficiency) of 4, 12,
and 18 nm, respectively, and an upper detection limit of around
2 µm at 200 hPa operating pressure .
The difference in counts between the 4 and 12 nm channels is given as
N4-12 and corresponds to the nucleation mode. In the upper
troposphere, where the CARIBIC aircraft is taking measurements, the 12nm
channel (N12) mainly corresponds to the Aitken mode. All aerosol
particle number concentrations used in this paper are given at standard
pressure and temperature (STP) of 273.15 K and 1013.25 hPa.
Whole air samples are collected in two units, each of which contains 14 glass
sampling flasks of 2.7 L volume, pressurised to
∼4.5bar during collection. Samples were collected at
pre-determined, evenly spaced intervals of roughly 35 min
(∼480km) with filling times between 0.5 and 1.5 min
(∼7–22 km). In months with four flights between Frankfurt and Chennai,
i.e. June, August, and September 2008, sample collection only took place
during the first two flights in order to achieve higher spatial and temporal
resolution (see last column in Table ). Greenhouse gases
(GHGs) and NMHCs were measured post-flight in the laboratory at the Max
Planck Institute for Chemistry in Mainz, Germany. GHGs are separated with a
gas chromatography (GC) system and then measured by flame ionisation
detection (CO2 and CH4) and electron capture detection (ECD,
N2O and SF6; ,
), while the NMHCs are separated on a second GC system
and then measured by ECD .
FLEXPART trajectory calculations
FLEXPART is a widely used Lagrangian particle dispersion model in ongoing
development at the Norwegian Institute for Air Research (NILU). It simulates
long-range and mesoscale transport, diffusion, dry and wet deposition, and
radioactive decay of various tracers see detailed description
in. An updated description of the FLEXPART model is available
from . In the current study, FLEXPART version 9.02 was used together
with ECMWF meteorological input data with a temporal resolution of 3 h and a spatial resolution of 1.0∘×1.0∘ and
91 model levels.
We started trajectories every 3 min along the flight tracks and
calculated them 14 days forward and backward in the single-trajectory mode of
FLEXPART with enabled parameterisations for the sub-grid terrain effect and
sub-grid convection. The trajectory positions were recorded every
30 min. The southernmost point of our flight track is Chennai
(12.99∘ N, 80.18∘ E). The northern cut-offs for the
trajectory analysis are 36.5∘ N in June, 40.0∘ N in July
and August, and 35.5∘ N in September (see
Table ) and were chosen to exclude the subtropical jet
stream and thus concentrate the analysis on the monsoon region. The
continuous trace gas measurements were averaged over 60 s centred
around the start points of the trajectories along the flight track to obtain
representative trace gas concentrations for the air characterised by the
trajectories.
Results and discussion
We will first present the CARIBIC observations of trace gases and aerosol
particles (Sect. ) with their latitudinal and
photochemical distributions and characteristics
(Sect. ). The vertical profiles measured by
CARIBIC over Chennai are presented next
(Sect. ). Then we show the origins of these
air masses (Sect. ) and continue with a description of
where the air is transported to after it has spent time in the monsoon UTAC
and how long it remains trapped inside it (Sect. ).
Latitudinal profiles of wind direction
(top panel, 90∘ means wind from the east, 180∘ from the south,
270∘ from the west) and wind speed (bottom panel) at aircraft cruise
altitude for the first flight from Frankfurt to Chennai in each of the four
monsoon months.
Latitude of the aircraft at the
transition between eastward and westward flow in the UTAC for each flight
during the summer monsoon 2008. Arrows pointing to the right indicate
flights from Frankfurt to Chennai; arrows pointing to the left indicate
return flights. Filled symbols represent the first round-trip flight
series; open symbols indicate the second (when applicable). For each of
the seven pairs of flights the wind change (transition point) is about
1∘ further north on the flight to Chennai (when the aircraft
cruise altitude is 1–2 km higher) than on the return flight.
Latitudinal profiles of trace gases and
meteorological parameters during the second flight in August from Frankfurt
to Chennai on 14 August 2008. In addition to colour coding of axes, scales
for parameters represented by solid lines are on the left and scales for
parameters represented by dashed lines are on the right. Aerosol particle
number concentrations are given at STP (273.15 K,
1013.25 hPa). Note that all data are shown versus geographical
latitude and that the water vapour shown in blue in (b) is on a
logarithmic scale (see also Figs. S9–S21 in the Supplement).
CARIBIC observations
The following sections discuss the CARIBIC observations and how the
latitudinal and vertical variations in the trace gas mixing ratios can be
used to infer information about the chemical properties of the observed air
masses and their origin.
Measurement position in the upper troposphere anticyclone (UTAC)
Along routes between Chennai and Frankfurt during the monsoon season
(June–September 2008), the aircraft passed from the southern section of the
UTAC, where winds are from the east, through the centre into the northern
section, where winds are from the west or vice versa for flights from
Frankfurt to Chennai. The transition between the two regimes is evident in
the wind data recorded by the aircraft (Fig. ),
which show abrupt wind direction reversals in the vicinity of 26∘ N.
Wind speeds are low in the centre of the UTAC (reaching near zero at the
transition point) and increase strongly at the northern edge of the UTAC and
upon entering the Northern Hemisphere subtropical jet stream (see also flight
tracks in Fig. ). The latitude at which the
aircraft crosses between the two sections or regimes varies from month to
month, with the northernmost average location in July and the southernmost in
September (Figs. , , and
Table ). It is noted that there are differences observed
between flights in each single month, with the point of transition on the
flights to Chennai being systematically further north than on the
corresponding return flights. The systematic differences in altitude expose a
vertical structure in the UTAC. The cruise altitudes of the aircraft at
22–28∘ N for flights to Chennai are 1–2 km higher (since
most aircraft fuel has already been burned) than during the return flights
(when the aircrafts fuel tanks are still full shortly after take-off; see
Fig. S4). However, the flight routes to and from Chennai between 22 and
30∘ N for each month were identical with the exception of one return
flight in August (Fig. S5 in the Supplement). This implies that the
differences observed between individual months (at the same altitude)
document changes in the structure of the monsoon circulation (in the
horizontal and/or the vertical direction). The observed northwards shift of
the UTAC centre at higher altitude means that we observe an interesting
northward tilt of the monsoon axis, consistent with previous
work e.g.their Fig. 2. In general, the centre of the
UTAC is observed to be furthest north during July
(Fig. ). This is consistent with meteorological
studies of monsoon development and its northward propagation and
recession .
To compensate for biases attributable to variations in flight routes and/or
horizontal movements of the monsoon UTAC, and to normalise for position
within the UTAC between months, we give our data an additional latitudinal
coordinate. Relative latitude (Δlat) defines the location
relative to the latitude at which the aircraft crosses from the northern to
the southern section of the UTAC or vice versa (i.e. relative to the
“centre” of the UTAC indicated by the reversal of the zonal wind).
Positive Δlat describes measurements in the north, while
negative Δlat describes measurements in the south, with values determined
on a per flight basis.
Latitudinal transects
As an example for the UTAC being fully developed, Fig.
gives the individual trace gas, aerosol particle, and wind latitude transect
profiles for the second flight from Frankfurt to Chennai in August
(14 August 2008) at altitudes of 10.3 to 11.9 km between
approximately 14 and 40∘ N. The southern section of the flight track
(14–25∘ N) is characterised by low O3
(Fig. a, average value 39 ppb for
Δlat-7.5 to -2.5∘), considering that the mean
concentrations over all flights lie between 51 ± 11 (Δlat-7.5 to -2.5∘) and 81 ± 13 ppb (for the northern
part, Δlat 2.5 to 7.5∘) and low NOy
(Fig. b, average value 0.33 ppb with corresponding
mean concentrations of 0.57 ± 0.38 and 0.83 ± 25 ppb)
concomitant with high CO (Fig. a, average value
96 ppb with corresponding mean concentrations of 101 ± 8 and
92 ± 10 ppb), high water vapour (Fig. b,
average value 299 ppm with corresponding mean concentrations of
409 ± 219 and 35 ± 41 ppm) and high aerosol particle
number concentrations (Fig. c). In the northern section
(27–40∘ N), the opposite is observed; mixing ratios of O3
(average value 86 ppb for Δlat 2.5 to 7.5∘) and
NOy (average value 0.81 ppb) are elevated, while levels of
CO are a little higher for this flight (average value
102 ppb). Here especially the levels of water vapour and aerosol
particle number concentrations are much lower, with the exception of some
brief instances of elevated nucleation-mode (N4-12) particle number
concentrations. These distinct and clear patterns are observed for all
flights under investigation in all months and are attributable to the
influence of different air mass source regions, different chemical regimes,
and varying transport times within the different regions of the UTAC (see
also Fig. below and Figs. S9–S21 in the
Supplement).
Profiles of trace gases
(a–d) and aerosol particle number concentrations (e and
f, at STP 273.15 K, 1013.25 hPa) vs. relative
latitude (Δlat) coordinates for all flights. The colours
indicate the flight month, the black line is the mean over all flights, and
the grey shading indicates the 1σ standard deviation around the
mean (both calculated over moving 2∘Δlat bins). Note
the logarithmic scale in the water vapour plot in (a).
An important feature of the monsoon is persistent deep convection over South
Asia, which is strongest over the Bay of Bengal and the Indian
subcontinent . South of ∼26∘ N
(Δlat<0) the aircraft encountered air that had passed over
these highly active convective regions and had become burdened with emissions
from South Asia. This accounts for enhancements in the primary pollutant
CO and relatively low levels of the secondary pollutant O3, as
well as the enhancements in water vapour. Likewise, high levels of nucleation-mode aerosol particles (i.e. freshly formed particles, N4-12)
persisted. These aerosol particles are very short-lived relative to the other
species (hours to a few days) and can be regarded as indicators of
convection . Interestingly, mixing ratios of NOy are
lower in the south than in the north (0.37 vs. 0.72 ppb),
possibly as a result of lower NOx emissions in the south combined with the
conversion to water-soluble components of NOy in the highly reactive
polluted air masses followed by enhanced removal under the high-temperature,
high-humidity conditions of the monsoon. In the north, the air parcels
encountered by CARIBIC have been transported for several days within the UTAC
and show signs of chemical processing and aging. Water vapour and nucleation-mode particles have been depleted by experiencing low temperatures during
transport and washout, respectively. CO levels are reduced and
O3, given more time to form photochemically, is elevated. Similar
changes in composition could have been caused by in-mixing of stratospheric
air. However, care has been taken to identify and remove all data points that
have stratospheric influence based on the measured ozone concentrations and
the potential vorticity (PV) values from the ECMWF model using thresholds of
150 ppb and 1.3 PVU for ozone and PV, respectively. The
distinction between freshly polluted air in the southern section and more
processed, aged air in the northern section is also supported by a previous
study of CARIBIC NMHC data .
Results for all individual flights are plotted against relative latitude in
Fig. . The standard deviation calculated over
moving 2∘ relative latitude bins is shown as grey shading. Water
vapour in Fig. a is shown on a logarithmic
scale. The differences of the means for the southern and northern section of
the flights are larger than the 1σ standard deviation (see numbers
given above) for all trace gases except for CO, which shows a maximum
slightly south of the wind reversal (Δlat ∼3∘) decreasing
southwards and northwards. In general, trace gas
(Fig. a–d) and aerosol particle concentration
(Fig. e and f) profiles in June, July, August,
and September are similar, although the overall patterns are interrupted by
certain events. Notable features are the much higher NOy
(Fig. d) and lower CO
(Fig. b) in the north during September (with
no notable difference for O3), and the abrupt gradient in CO
concentrations between north and south during June. The large variability in
aerosol particles compared to the trace gases is caused by the strong
dependency of the particle number concentration on clouds whose position
relative to the CARIBIC flight tracks was different during each flight.
Monthly mean aerosol particle distributions by latitude are shown in Sect. S5
in the Supplement. A detailed investigation of clouds and particle number
densities is beyond the scope of this work and the interested reader is
referred to the CARIBIC-based work by .
Monthly distributions of O3 vs.
CO during the summer monsoon season 2008. Points are colour-coded by
relative position within the UTAC, as determined by Δlat (see
also Sect. S6 in the Supplement).
As our analysis focuses on the monsoon as a transport mechanism for polluted
air masses from South Asia and mainland Southeast Asia and the influence on
other regions, understanding the composition and chemistry of air parcels as
they move through the UTAC is critical for evaluating the potential impact on
downwind regions. This includes understanding not only of primary pollutants
in transported air masses but also of the tendency to form secondary
pollutants, particularly ozone. A previous CARIBIC data-based study of the
relationship between ozone and NMHCs, which are indicators of pollution and
act to some degree as ozone precursors, showed that air masses in the south
have a greater ozone formation potential than in the north, where air masses
have diminished formation potential or show the beginnings of ozone
destruction . Ozone formation potentials in the troposphere
can also be qualitatively understood through the relationship between ozone
and carbon monoxide using the enhancement ratio ΔO3/ΔCO, as determined from
correlation plots, in addition to the information provided by the coarser
resolution NMHC data. Positive correlations, i.e. ΔO3/ΔCO>0, indicate the formation of ozone. Negative correlations can
(outside of stratospheric influence) indicate destruction of ozone or
in-mixing of different air masses, which may also apply when a lack of
correlation is observed and references
therein.
Wind speed (coloured lines, lower
x axis) and wind direction (grey lines, upper x axis, 90∘
means wind from the east, 180∘ from the south, 270∘ from
the west) as recorded by the CARIBIC aircraft for the descent into (solid
lines) and ascent from (dashed lines) Chennai. Only data south
of 16∘ N and above 2 km altitude are plotted. Chennai is
at 12.99∘ N, 80.18∘ E.
Figure shows scatter plots of O3 vs.
CO during each of the four monsoon months with colour coding
indicating relative latitude. First of all, the low-latitude data
(-10∘<Δlat<-5∘) combine low O3
values (< 50 ppb) with about 75–100 ppbCO. With
the exception of September (which has few statistically significant slopes at
all in the low-latitude band) the ΔO3/ΔCO slopes
are significant and positive. At the northern edge of the UTAC
(Δlat∼ 15∘), again with the exception of
September, slopes are negative, whereas August has positive values throughout
except for Δlat< 15∘. The correlation slopes north
of -5∘ vary considerably. Here O3 values are definitely
higher; they are, however, not well correlated with the generally similar
CO values between 75 and about 110 ppb. These relationships
for the monsoon months indicate that ozone-forming regimes dominate in the
south, while in the northern part of the UTAC there is no clear potential for
ozone formation (see also Sect. S6 and Fig. S25 in the Supplement, which shows
the values of the slope as a function of relative latitude). All these
tendencies are determined from the slope of ΔO3/ΔCO over a few degrees latitude, which agrees with the range of
200–500 km used in an earlier study by their
Fig. 10. As expected, correlations for the individual flights
(not shown) yield similar qualitative results since the time between the
individual flights through the monsoon in one month is short
(12–36 h) compared to the transport time of the air inside the UTAC
(many days).
Vertical profiles over Chennai
The majority of flight time through the UTAC was spent at cruise altitude in
a narrow altitude range between 10.3 and 11.9 km (see Fig. S4 in the
Supplement), providing limited insight into the vertical distribution of
trace species in the UTAC. However, the descent into and ascent from Chennai
airport provide us with some information in the vertical in the southern part
of the UTAC, as the aircraft passes between ∼11km and ground
level over a relatively short distance (∼200–250 km) in
approximately 30 min (Fig. S4 in the Supplement). During this time the
aircraft moves from being within the UTAC into the free troposphere below and
eventually into the boundary layer and vice versa. In situ data are available
for most species down to ∼2km, with the exception of water
vapour, which is available down to ∼5km only, and NOy,
which is not available below ∼10km. Aerosol particle number
concentrations during ascent are only available above 6 km. No whole
air samples were collected on ascent and descent. Both ascent and descent
took place during night, with landing times around 23:30 LT (local time) and
take-off between 02:00 and 03:40 LT the following morning (Fig. S4 in the
Supplement). Descents into the airport were from a west–north-west direction
towards the Bay of Bengal and final approaches from the east while the
ascents were over land west of Chennai (except for the September flights,
which took off to the east). Similar flight times and patterns were followed
for all flights except for July, when the approach into Chennai followed a
route further north of Chennai before the final approach to the airport from
the east (see Fig. S6 in the Supplement).
Vertical profiles of CO(a), O3(b), water vapour (c), and number
concentration of Aitken-mode aerosol particles N12 at STP
(273.15 K, 1013.25 hPa) (d) between 2 km
and 12 km during descent into (solid lines and filled symbols) and
ascent from (dashed lines and open symbols) Chennai. The colours indicate
the flight month, the black line is the mean over all flights, and the grey
shading indicates the 1σ standard deviation around the mean (both
calculated over moving 1.5 km altitude bins). Note the logarithmic
x axis for water vapour (c) and aerosol particles (d).
While descending from cruise altitude, the aircraft moved from the westward
flow of the UTAC into the eastward flow of the low-level Somali Jet with its
wind speed maximum around 850 hPa. This is
clearly visible in the wind direction recorded by the CARIBIC aircraft shown
in Fig. . In June and August this transition
occurred between 6 and 7 km, while in September it was around
9 km altitude. In July, tropospheric wind speeds were much lower than
in the other months and there was no wind reversal evident from the
measurements. This was a general feature of the meteorology on 15 and 16 July
when the flights took place. Over a large region in southeastern India and out
over the adjacent Bay of Bengal, wind speeds were very low from the surface
up to 500 hPa (∼ 5km), which is about the height of
the wind reversal seen in Fig. in June and
August. The corresponding ECMWF wind fields during the days of the flights
are shown in Fig. S7 (745 hPa winds) and Fig. S8 (510 hPa
winds) in the Supplement. Over central India and the Indian Ocean south of
India there were eastward winds in the lower troposphere extending the Somali
Jet over the Indian subcontinent and into the Bay of Bengal. But just around
Chennai, winds were very calm. This has probably led to a trapping of the
local pollution from the city of Chennai during these days in July 2008,
whereas higher wind speeds in the lower troposphere were encountered in the
other months.
Vertical trace gas and aerosol particle profiles during descent and ascent
show a fairly consistent picture, although trace gas data for the two July
flights (see orange lines in Fig. ) reflect the
unique meteorological situation described above, particularly during the
descent into Chennai. Very dry conditions
(Fig. c) were measured while O3 was
strongly enhanced (Fig. b) and further increased
with decreasing altitude, reaching 116 ppb at the lowermost point
(not shown). These profiles were also characterised by high CO
(Fig. a) between 6 and 9 km, which is
above the level of wind direction change. There was no recognisable change in
the number concentration of Aitken-mode aerosol particles
(Fig. d). In the free troposphere there was a
weak north-westward flow (see Fig. ) which
would transport the pollution of Chennai and its industries right to the
region where the July flight descended into Chennai airport (see Fig. S6 in
the Supplement), while the other flights descended from the west–north-west,
i.e. mostly upstream of the pollution sources in Chennai.
In the other months, vertical “C-shaped” profiles of CO (see
Fig. a) reflect the position change outside and
inside of the UTAC as the aircraft ascends into the upper troposphere. Moving
upward from 2 km, mixing ratios of CO decrease until reaching
the point where the wind direction changes, marking entry into the lower
levels of the UTAC. Here, CO begins to increase, often reaching, and
even exceeding, maxima observed at lower altitudes. The difference between
maximum and minimum is most pronounced in June, with mixing ratios in the
middle troposphere being higher than in the other months, when mixing ratios
are fairly similar. No significant differences between ascents and descents
are observed. The standard deviation calculated over moving 1.5 km
altitude bins is shown as grey shading and supports the “C-shaped” profile.
Profiles of O3 (Fig. b) show increasing
mixing ratios with altitude, with no clear transition between free
troposphere and UTAC and no significant differences between ascent and
descent. A monthly trend is present, with the lowest values in August and the
highest values in June, when the vertical gradient was also steepest. Monthly
differences are most pronounced above 6 km. Conversely, water vapour
(Fig. c) decreased with altitude, with a similar
vertical gradient during each month. June profiles were the driest, with the
exception of the second flight into Chennai in September and the flight into
Chennai in July (see above). June is at the beginning of the monsoon season
in India, when precipitation is not yet as strong as during the subsequent
months.
Concentrations of the Aitken-mode aerosol particles (N12,
Fig. d) show a positive vertical gradient,
except for one descent in August, which is variable with altitude but shows no
consistent trend. There are no significant monthly differences in
concentration or gradient. Also, N12 particle number concentrations
during ascent and descent are more or less equal considering their general
variability with altitude. This is expected since both descent and ascent
occur well after nightfall, when all direct influence from the daytime
convective activity has ceased. Part of the variability of the vertical
N12 profiles is due to crossing of contrails of other aircraft with
locally very high aerosol particle number concentrations.
Measurements of O3 profiles with MOZAIC (Measurement of OZone by
Airbus In-service airCraft) aircraft over Chennai in 1996 and
1997 and Hyderabad in central India in
2006–2008 showed similar concentrations and a similar
positive gradient with altitude. However, none of these observations
documented a large increase towards the surface in the monsoon months as
observed by CARIBIC in July 2008. MOZAIC CO profiles measured over
Hyderabad in 2006–2008 were more or less constant at
∼ 100ppb throughout the free and upper troposphere and only
showed an increase towards the surface below ∼ 4km. A
pronounced increase in CO in the free troposphere as measured by
CARIBIC in July 2008 was not observed over Hyderabad in the summer
months of these years. This corroborates that 15/16 July 2008 was an
exceptional case with respect to the CO and O3 profiles over
Chennai.
Ten-day backward trajectories for
CARIBIC flight 244 to Chennai on 13 August 2008. Upper panel: trajectories
north of the wind reversal at 26.6∘ N. Lower panel: trajectories
south of the wind reversal. Every second trajectory is plotted for
clarity. The colour indicates the trajectory altitude (in km above
sea level) and the black dots mark 24 h time steps along the
trajectories. The CARIBIC flight track is shown by the thick red line.
Distribution of source regions for all
the CARIBIC flights. The colour code shows the percentage of trajectory
points that were below 5 km altitude in the preceding 10 days per
4∘×4∘ grid box. Upper left panel: source regions
for trajectories starting north of the wind reversal. Lower left panel:
source regions for trajectories starting south of the wind reversal. Right
panels show the corresponding standard deviation of the percentages in the
monsoon months June to September per grid box for all grid boxes reached by
trajectories from at least 2 months (see also Figs. S26–S29).
Receptor regions used in the analysis of
the FLEXPART forward trajectories.
NameRegionNorth America30–50∘ N, 150–70∘ WCentral Pacific10–34∘ N, 130∘ E–150∘ WCentral Africa0–25∘ N, 10∘ W–35∘ EMediterranean28–40∘ N, 0∘ E–40∘ WSouth Asian monsooncentre: 80∘ E∗; radius: 38∘ longitude, 16∘ latitudeTP stratospherehigher than thermal tropopause along trajectoryPV stratospherehigher than dynamical tropopause (PV >2.5PVU) along trajectory
∗ Latitude of monsoon centre is flight-dependent (see second-to-last column in Table ).
Source regions of monsoon air
Before dealing with the fate of the air masses probed by CARIBIC
(Sect. ) we investigate the origin and subsequent
processing of pollution during the trapping in the UTAC at low temperatures,
pressures, and water vapour under high insolation. In previous CARIBIC papers
we have discussed the issue of the origin of certain
pollutants .
Figure shows trajectories calculated with the
FLEXPART model for a representative case to identify the origin of the air
that was sampled by the CARIBIC aircraft when flying through the UTAC. The
backward trajectories for the flight to Chennai on 13 August 2008 are
colour-coded with trajectory altitude above sea level. As the trajectory
reliability decreases with time into the past (and future), we use only the
first 10 days for the source and receptor region analysis and in all
subsequent plots.
Receptor regions as determined for
CARIBIC flight 244 to Chennai on 13 August 2008 (flight track shown with
thick red solid line). The panels show all FLEXPART forward trajectories
for this flight south of the northern cut-off (40∘ N) in grey. For
each receptor region, the corresponding trajectories are shown in colour.
The dashed lines indicate the boundaries of the receptor regions
(boxes and ellipse). Trajectories with stratospheric parts are shown in the lower
right panel in yellow for the thermal and pink for the dynamical tropopause
criterion.
For the northern section of the flight track most of the observed air masses,
approaching from the west, have resided for 4 days or longer inside the
UTAC at altitudes (indicated by the trajectory colour in
Fig. ) at or above the CARIBIC flight altitude. For
the southern section, the air masses reached the CARIBIC flight track from
the east after having been convected over or east of the Bay of Bengal,
originally having followed an eastward flow in the lower troposphere with the
Somali Jet over the Arabian Sea and central and southern India. Only when
approaching Chennai airport at low altitudes we observed air coming directly
from the west without having passed over the Bay of Bengal.
A summary of the source regions for all flights is shown in
Fig. by means of the geographical distribution
of all points of the backward trajectories below 5 km altitude, i.e.
in the lower troposphere (left panels). The right panels show the
corresponding 1σ standard deviation of the percentages over the
monsoon months June to September. The limit of 5 km was chosen
because it is well below altitude of the wind reversal which separates the
upper troposphere region with the monsoon UTAC from the lower troposphere
(see also Fig. ). Lower-altitude thresholds
(down to 1 km, not shown) do not change this picture significantly.
This points to rapid more or less directly vertical convection from the lower
troposphere into the UTAC region. For the northern part of the UTAC (upper
left panel), the air came from eastern India, the Indo-Gangetic Plain, and from the northern parts of the Bay of Bengal and mainland Southeast Asia.
The air measured in the southern part of the UTAC (lower left panel)
originated in central and southern India, the western part of the Bay of
Bengal, and the Arabian Sea off the western coast of India. The 1σ
standard deviations per grid box (right panels in
Fig. ) scale with the mean distribution – i.e. a
high mean value is most often accompanied by a high standard deviation. This
shows that the position of the convective events is variable – i.e. a grid box
with a high percentage in one month may have a lower mean percentage in the
next month when the high percentage may be found in the neighbouring grid
box. While the general pattern was similar for the all flights in
June–September 2008, the exact location of the convective uplift and hence
the detailed pattern of the source regions varied (see Figs. S26–S29 in the Supplement).
Outflow from the UTAC
The following subsections describe the fate of the air and the contained
trace species observed with CARIBIC after they have been processed in the
monsoon UTAC.
Receptor regions of monsoon air
FLEXPART forward trajectories were used to determine to where the air masses
observed by CARIBIC at cruise altitude in the South Asian monsoon UTAC were
transported. We defined seven receptor regions, listed in
Table . Over North America, central Pacific, central
Africa, and the Mediterranean, we defined boxes reaching from the ground to
the thermal tropopause. The trajectories are detected over these regions if
they are within the defined boxes using the local thermal tropopause height
output from the FLEXPART model along the trajectories. The South Asian
monsoon region itself is defined as an ellipse with the centre at
80∘ E, while its latitude is between 22.7 and 27.9∘ N
depending on the actual flight (see second to last column in
Table ). The monsoon centre latitude is determined from
the zonal (west–east) wind reversal along the flight track using the wind
speed and direction recorded by the aircraft (see
Sect. ). To qualify as a monsoon trajectory, the
trajectory has to stay for at least 70 % of the 10-day period within
the monsoon ellipse and below the thermal tropopause. Trajectories which
stayed for at least 24 h above the thermal or dynamical tropopause
(PV > 2.5 PVU) were counted as TP (tropopause) or
PV stratospheric trajectories, respectively. As expected, all but one of the
trajectories which met the criterion for the dynamical tropopause also met
the criterion for the thermal tropopause.
As each forward trajectory may cross several receptor regions, we determined
the fraction of time the trajectory spent inside each receptor region from
the number of 30 min trajectory time steps that were inside the
receptor region. An example for this analysis is shown in
Fig. for the flight on 13 August 2008. Each
panel shows all the forward trajectories (south of the northern cut-off) for
this flight in grey. The trajectories which cross the different receptor
regions are shown in red (North America), magenta (central Pacific), green
(central Africa), blue (the Mediterranean), and orange (South Asian monsoon).
The lowermost right panel shows the trajectories, with stratospheric parts in
yellow (for thermal tropopause criterion) and pink (dynamical tropopause
criterion). As mentioned above, the dynamical tropopause is usually somewhat
higher than the thermal tropopause. Therefore the pink trajectories form a
subset of the yellow trajectories except for one trajectory which is
classified as stratospheric according to the dynamical tropopause but not
according to the thermal tropopause.
Receptor region fractions
averaged over all CARIBIC flights to Chennai in June to September 2008.
Since trajectories may reach multiple receptor regions, the totals
exceed 100 %.
The aggregate results for the receptor region analysis are shown in
Fig. . It shows the percentages of
trajectories reaching the seven receptor regions defined in
Table averaged for each month between June and
September 2008. Since a trajectory may reach more than one receptor region
(e.g. central Africa and Mediterranean, central Pacific and North America, or
dynamical PV stratosphere and thermal TP stratosphere), the totals
exceed 100 %. The least receptor region overlap is observed in
September, while the trajectories in July reach the largest number of receptor
regions. July is also different from the other months in that it has the
largest fraction of outflow to the west, i.e. to central Africa (green bar)
and further on to the Mediterranean (blue bar). This may be a peculiarity for
15/16 July 2008, when the flights took place. Note that in July there were
only two flights and not four as in the other months.
The latitudinal distribution of the receptor regions (not shown) for the air
sampled by the CARIBIC aircraft varies from flight to flight, but the general
tendency is that air sampled in the north is mostly either exported towards
the east or entering the stratosphere, while most air masses that are
exported towards Africa and the Mediterranean have been sampled in the
southern part of the flight track through the UTAC. Importantly, air sampled
in the middle of the UTAC, i.e. at the location of the wind reversal, and a
few degrees south appears to have the highest likelihood of being transported
to the stratosphere. In September, transport is somewhat different, with less
air sampled in the middle of the UTAC moving to the stratosphere, but instead
more air remains in the UTAC and the region where air exported towards the
east is expanding southwards. This shift in the latitudinal distribution may
be the result of a weakening monsoon circulation towards the end of the
monsoon season. The actual monsoon withdrawal as diagnosed by the India
Meteorological Department, however, started in 2008 at the end of
September . One should also keep in mind that the monsoon
contributes only partially to the air and the pollutants which reach the
receptor regions. Especially for receptor regions far away, e.g. North
America, the monsoon will only have a limited influence. But in special cases
the monsoon may even influence air mass composition as far away as the east
coast of Canada (see Supplement Sect. S1 for a case documented by CARIBIC in
September 2007).
Estimating transport times
An important cross-check of the FLEXPART trajectory calculations is possible
by comparing with the photochemical age of air pollutants estimated from
ratios of NMHCs measured in the whole air samples collected during the two
flights in July and the first two flights in August 2008. Since sample
collection only took place during the first two flight per month, no samples
are available for the last two flights in August. The samples from June and
September did not give consistent photochemical age estimates. The
photochemical age is used as a proxy for the time elapsed since the
pollutants had been emitted into the air mass. Details of the NMHC age
calculation have been published previously by . For air
samples collected south of the wind reversal that had recently
been convectively lifted to cruise altitude, we use an estimated mean
OH concentration 〈[OH]〉 of 2.48×106moleculescm-3. For samples
collected north of the wind reversal we instead applied an estimated
〈[OH]〉 of 1.44×106moleculescm-3. Emission ratios
of 0.29 and 0.15 for propane/ethane and n-butane / ethane, respectively,
were assumed. Figure shows a clear
clustering of the NMHC ages, with photochemically younger air in the south
(Δlat<∼3∘) which had recent contact with fresh
emissions and older air in the north, with more aged air having already
experienced several days of photochemical processing during transport in the
UTAC. The NMHC-based average ages in the two clusters are 4.5 days (south) compared to 11 days (north).
Photochemical age of
pollutants in the air samples estimated from NMHC ratios (see
Sect. and Baker et al., 2011) versus
relative latitude for the flights in July and the first two flights in
August. Note the clustering of the chemically young samples close to and
south of the wind reversal (Δlat<∼3∘) and the older
more chemically processed samples further north. Shown is the mean
chemical age together with the minimum and maximum age determined from the
propane/ethane, n-butane/ethane, and n-butane/propane ratios.
The comparison of the mean chemical age of the air pollutants with the
transport times estimated from the FLEXPART trajectory calculations is shown
in Fig. . For July, the best fit was found
when comparing the time since the air had last been east of 95∘ E.
For August, the correlations do not change much for a source region between
85 and 95∘ E. Table lists the slopes
and squared Pearson correlation coefficients R2 for source regions at
80 to 100∘ E. For 95∘ E, the linear
least-squares regression lines have slopes of 0.97 ± 0.19
and 0.93 ± 0.20 and squared Pearson correlation coefficients R2
of 0.87 and 0.68 for the flights in July and August, respectively.
Considering that the NMHC age estimates use surface emission ratios based on
ground-based data as the starting point for the age calculations and that
convection (which is partly parameterised in the FLEXPART model) is most
frequent over the Bay of Bengal, a source region east of 90–95∘ E
seems reasonable and fits to the source regions discussed in
Sect. . Although the degree of significance of the
correlations is largely due to the existence of two clusters of data, the
approximate overall agreement between the two methods provides confidence
that our interpretation of pollutant distributions and transport within the
UTAC is realistic. In most cases where there is a large disagreement between
NMHC chemical age and FLEXPART transport age, the NMHC ages show a large
spread. Reasons for this may be mixing with background air, different source
ratios than assumed in the analysis, or situations where the OH concentrations
deviated from the assumed climatological averages used in the NMHC age
calculations. Uncertainties in 〈[OH]〉 of ±25% correspond to uncertainties of the derived NMHC ages of
20–33 %see.
Comparison of the photochemical
age of air estimated from NMHC ratios and the time the air had last been
east of 95∘ E according to the FLEXPART trajectory
calculations. Shown are the mean chemical age together with the minimum
and maximum age determined from the propane / ethane, n-butane / ethane, and
n-butane / propane ratios. The colours indicate different flights (see
legend). The black line indicates perfect agreement of both age estimates,
while the dashed lines indicate linear least-squares regression lines of
transport time and chemical ages derived from the different NMHC ratios
(magenta regression lines for July, light blue for August flights).
Slopes of the correlation of NMHC
photochemical ages and FLEXPART trajectory transport times for different
assumed source regions. The squared Pearson correlation coefficients (R2) are
shown in parentheses (see Sect. for
details).
The South Asian summer monsoon UTAC is often described as a processing
reactor for the pollution emitted at the surface which is rapidly transported
upward by convection to become trapped in the UTAC e.g.and
references therein. However,
the horizontal wind fields shown in Fig. and
the forward trajectories in Fig. indicate that
the trapping is temporary, i.e. the trajectories do not stay in the monsoon
UTAC forever. As soon as 6 days after the flight, around 50 % of the
trajectories have left the UTAC (see Fig. described
below). At the south-western part of the UTAC, air escapes to central Africa,
part of which may return via the Mediterranean and the Middle East to become
again entrained in the UTAC in the northwest. In the northeastern region, air
is leaving the UTAC with the eastward jet stream and is transported over
China towards the Pacific and finally North America. Quantifying these losses
is the goal of the leak rate analysis described below.
The calculated trajectories have been used to estimate the residence time of
the observed air inside the monsoon ellipse defined above. For this analysis,
only the trajectories along the flight track that started inside the ellipse
are used. The number of trajectories remaining within the ellipse was
determined for the preceding and following 10 days at time steps
of 6 h centred around the middle of the time period the flight track
spent inside the ellipse, i.e. midway (in time) between when the aircraft
entered the monsoon ellipse and when it landed in Chennai and vice versa for
southward and northbound flights, respectively. This analysis is similar to
the leak rate calculations by their Fig. 14.
Monsoon leak rates for all flights between
June and September 2008. Shown is the fraction of the backward and forward
trajectories which started inside the monsoon ellipse (t=0) and are inside
the monsoon ellipse at the given time in days before and after the aircraft
was inside the monsoon ellipse (see Sect. ).
The solid coloured lines are monthly means with the results from the single
flights indicated by dotted lines. Colours indicate the months. The dashed
black line shows the mean leak rate averaged over all flights.
The results, colour-coded for flight month, are shown in
Fig. . The solid coloured lines show the monthly means,
while the dashed black line indicates the average over all months. On
average, 65 % of the trajectories had resided inside the monsoon
ellipse for at least 10 days, while 30 % of the air had become
entrained within the last 3 days prior to measurement. Following the air
masses after their measurement shows a different picture: after 6 days
only 50 % of the trajectories, on average, had remained within the
ellipse. However, while fewer trajectories were within the ellipse before the
measurement in August, they tended to remain inside longer after the
measurement than in the other months. Especially in June and September, the
exit from the ellipse was more rapid than in July and August, related to the
weaker monsoon circulation during these months. The calculated leak rates are
higher, i.e. the trajectories leave the UTAC region faster, than those
calculated by , which is consistent with the much larger
monsoon anticyclone area definition used by these authors.
An explanation for the asymmetric shape of the calculated leak rates is
related to the position of the aircraft within the UTAC and hence within the
monsoon ellipse defined above. The flight tracks cross the western part of
the ellipse. The UTAC has two main points of air discharge: one in the
south-west, where air is transported westwards towards Africa, and one in the
north-east, where it is transported eastwards towards China and the Pacific
Ocean. In the northern part of our flight track inside the UTAC, the air
masses observed had not left towards Africa but stayed inside the UTAC. In
the forward direction, the air masses have a chance of leaving the UTAC at
its north-eastern part, i.e. towards China and the Pacific Ocean. Having
traversed to the southern part, the air masses observed have not left towards
the Pacific but stayed inside the UTAC. But looking forward again, the air
masses have a chance of leaving the UTAC towards Africa. Since the calculated
leak rates are an average over all trajectories started inside the UTAC, it
is consistent with our understanding of the monsoon circulation that looking
backwards, there is a tendency that the air masses have already been for
quite some time inside the UTAC (at least the majority of the trajectories)
while looking forwards, the air masses have a higher chance of leaving the
UTAC quickly (or at least faster than they have entered the UTAC). In August,
which from all CARIBIC data seems to be the most representative monsoon month
(red lines in Fig. ), the UTAC contains the air masses
more strongly than in the other months. In addition, Figs. S30 and S31 in the
Supplement show that the trapping efficiency changes little even when
considering altitudes 2 km above the actual flight level. This is
somewhat unexpected. As the UTAC has its maximum some kilometres above the
actual flight altitude of the CARIBIC aircraft of ∼11–12 km, a stronger confinement at altitudes
above the CARIBIC flight altitudes is expected.
Conclusions
The data from the CARIBIC passenger aircraft observatory discussed in this
study cover the South Asian summer monsoon season 2008 from June to
September. Even though the flights crossed the western part of the monsoon
UTAC and not its centre and the cruise altitude of ∼11km is
at the lower boundary of the UTAC e.g.,
Figs. and show
systematic profiles and trace gas correlations over a distance of
3500 km (approximately from Teheran, Iran, to Chennai, India), as
previously reported by and . This
matches earlier observations from CARIBIC phase 1 during flights from Germany
to Colombo, Sri Lanka, and Malé, Maldives (not shown), in the summers
1998–2000, despite consisting of a more limited chemical data set
(, and unpublished data).
In accordance with our current understanding of the South Asian summer
monsoon and in particular its role trapping surface air that was rapidly
transported upwards by convective activity, all the CARIBIC observations
during the monsoon months in this region, although they were made in
different years, fit the same overall picture of pollution build-up in the
UTAC. Starting from this consistent data set, of particular interest is, on the
one hand, the understanding of the chemical composition of the air in the UTAC
and, on the other hand, the export of this air to other regions, for which the
CARIBIC flight to Toronto in September 2007 (see Sect. S1 in the Supplement)
gives a good example.
Based on the measured reversal of the zonal wind at flight altitude across
the centre of the UTAC located at 27∘ N in June to 23∘ N in
September (Table , cf. New Delhi 28.4∘ N,
Calcutta 22.3∘ N), the chemical and aerosol particle data are
presented using relative latitude with respect to the wind reversal and by
doing so provide a very consistent picture of humid, recently polluted,
low-O3, medium-CO air and low-NOy air with a high burden of
aerosol particles in the south and a strengthening tendency (increasing
CO and O3, declining humidity) towards the centre of the UTAC.
Northwards from there dry air is observed, with higher NOy but strongly
declining CO, moderately declining O3 (up to a relative
latitude Δlat of around 12∘), and low aerosol particle
number concentrations.
The vertical profiles of O3 and CO over Chennai are very
distinct compared to other profile observations over India
(; Sahu et al. 2014). They are likely influenced by the
local emissions in the Chennai area together with the advection of clean
maritime air and polluted air from other parts of India and the surrounding
region to form the characteristic “C shape” observed for CO in all
months except July. In that month a strong free-tropospheric CO
enhancement of up to 116 ppb was observed, which cannot be explained
solely by local convection of polluted surface air but is instead probably
due to long-range transport of polluted air masses. Detailed modelling
studies with the WRF-Chem online regional chemistry transport model to
understand these vertical profiles have been published
separately .
Hydrocarbon ratios used in photochemical age of air calculations show that
the air masses in the southern part of the UTAC are younger (chemical ages
between 3.5 and 6.5 days) than in the north (chemical ages between 10
and 13.5 days). The clustering of the chemical ages compares well
with that based on the transport age determined from the FLEXPART backward
trajectories, namely 1–7 days compared to 7–14 days for
transport from 95∘ E to the CARIBIC flight track. The spread in the
FLEXPART transport ages is larger and they are in general higher by
0.5–1.5 days. Given the uncertainties of air mass trajectory
calculations, the problems in how to determine its starting point in time
over polluting sources, and the uncertainty in photochemical age that
may be affected by dilution with background air, the correspondence is
encouraging.
Air sampled by CARIBIC originated from a large region. Although the
convection over the Bay of Bengal is greatest, for a quantification of its
effect on pollution the vertical transport has to be combined with the
emission source distributions, which is beyond the scope of this study. For the vertical
profiles, such a study has been done by . Instead, we have
focused on the FLEXPART backward-trajectory-based footprint (air at cruise
level that had come from below 5 km) for air in the southern and
northern part of the UTAC. For the southern part, the footprint is situated
somewhat more to the south and west. For August the surface area of the
footprint is largest (see Figs. S26–S29 in the Supplement). When the UTAC
wanes in September, and its centre moves south to reach 23∘ N north,
the footprint likewise shifts southward and is more concentrated over the
Indian subcontinent than in the other months. However, no major differences
in footprint for air in the southern compared to the northern part of the
UTAC exist; the age difference seems to be the fundamental difference. In
other words, the difference between the two regimes is due to the time
elapsed since fresh pollution was injected into the UTAC.
The receptor region analyses and the “leak” rates show that air in the
southern part of the UTAC is mostly in an ozone-forming chemical regime
(Fig. ) and part of it is exported towards central
Africa and the Mediterranean. In contrast, the air in the northern part,
which features an ozone neutral or partially even ozone destroying chemical
regime, is exported towards China and the Pacific Ocean and in rare cases
even up to the east coast of North America (Sect. S1 in the Supplement).
Export from the UTAC into the stratosphere peaked in June and July
(Fig. ). With CO values declining
from June onward, this implies an “optimal” input of CO into the
lowermost stratosphere. However, approximately two-thirds of the air is
dispersed within the troposphere. Such transport to the stratosphere has been
observed in a similar way by the German HALO (High Altitude and LOng range)
research aircraft, which detected monsoon pollution transported to the
lowermost stratosphere over Europe in late summer 2012 during the TACTS
(Transport And Composition in the upper troposphere/lowermost stratosphere)
campaign (A. Roiger, private communication, 2014).
The large-scale movement in the UTAC as in a large-scale atmospheric
“merry-go-round” is not without losses (Fig. ). While
surface air very high in pollutants reaches the UTAC from below and becomes
trapped, similar amounts of air leak out of the system, mostly at its
northeastern and southwestern edges. Still, the trapping process is
impressive. Even going back in time 10 days prior to sampling by
CARIBIC, on average 65 % of the back trajectories had resided within
the UTAC defined for this calculation as an ellipse. After having been probed
by CARIBIC, only 40 % of the trajectories stay in the UTAC ellipse
for more than 10 days. This asymmetry is an artifact of the
geographical skew in the sampling by CARIBIC. Interestingly, calculations
show that the leakage rates 2 km above cruise altitude of CARIBIC are
not significantly lower (Sect. S8 in the Supplement). A mass balance of the
UTAC is beyond the scope of this work. Undoubtedly, air from outside the
UTAC is also being entrained, leading to dilution that intensifies towards
its borders.
The population of the monsoon source region of South Asia was about
1.4 billion people in 2008 and the population is still growing. Together with
the economic development in this region, this means that the pollutant
emissions today are larger than in 2008 and will further increase in the
future. This also encompasses export of even more polluted air as shown in
this study with implications for the atmospheric chemistry and air quality in
the receiving regions.
The Supplement related to this article is available online at doi:10.5194/acp-16-3609-2016-supplement.
Acknowledgements
We wish to thank all CARIBIC partners as well as Lufthansa, especially
T. Dauer and A. Waibel, and Lufthansa Technik for their ongoing support for
many years. We especially acknowledge D. Scharffe, C. Koeppel, and S. Weber
for the operation of this complex platform. The FLEXPART model was provided
by A. Stohl at the Norwegian Institute for Air Research (NILU) and was used
with meteorological data from the European Centre for Medium-Range Weather
Forecasts (ECMWF). The article processing charges for this open-access
publication were covered by the Max Planck Society.
Edited by: E. Harris
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