Introduction
Volatile organic compounds (VOCs) emitted by vegetation can lead to the
formation of secondary organic aerosol (SOA) through atmospheric oxidation
and further chemical processes e.g.,. One important VOC is isoprene
(C5H8, 2-methyl-1,3-butadiene), the most abundant non-methane
hydrocarbon with a global emission rate of ∼500 Tg yr-1, with a
large contribution coming from the Amazon rainforest .
Also the conditions for photooxidative reactions are favored in this
tropical region. Isoprene is a short-lived atmospheric gas, which is oxidized
in the atmosphere by reactions with the hydroxyl radical (OH), nitrate
radical (NO3), or ozone (O3). OH-initiated oxidation leads to
isoprene peroxide radicals (ISOPOO). Depending on the NOx
and HOx concentrations, ISOPOO can react further through
different pathways.
For HO2-dominant conditions, meaning conditions with low amounts (<1 ppb) of NO , ISOPOO will mainly react with
hydroperoxyl radicals (HO2) to form the intermediate oligomer
hydroxyhydroperoxides (ISOPOOH). Further oxidation of ISOPOOH may lead to
isoprene epoxydiols (IEPOX), which then can partition into the particle phase
by condensation or reactive uptake and, thus, SOA derived from IEPOX
(IEPOX-SOA) can be formed e.g.,. Laboratory studies demonstrate that
around 50 % of isoprene-derived particulate matter is associated with
IEPOX production and uptake through the HO2 pathway and in the
presence of acidic aerosol particles
. The existence of aerosol as
seed particles seems to be necessary , but also
the acidity of aerosol can influence the formation yield of IEPOX-SOA.
Laboratory and field studies found a correlation between IEPOX-SOA and
sulfate, which is related to the acidity of aerosol
. Although the IEPOX
pathway is considered as the dominant one, further laboratory studies show
that also other gas-phase reactions of ISOPOOH with multifunctional
hydroperoxides contribute to the formation of isoprene-derived SOA
e.g.,.
A different oxidation pathway occurs at increased NO concentrations (>1 ppb) in NO-dominant conditions . Reaction pathways
for ISOPOO change towards reactions with NO instead of HO2, leading
to hydroxynitrates and/or hydroxyalkoxy radicals, which decompose further to
methacrolein (MACR) and methyl vinyl ketone (MVK). Whereas MACR is a
precursor for isoprene-derived SOA, MVK has almost no SOA contribution
.
In the central Amazon region, both HO2- and NO-dominated conditions
were observed depending on the time and location of measurements
. Thus, isoprene can undergo
different gas-phase reactions depending on several conditions, leading either
to IEPOX-SOA or to other types of isoprene-derived SOA.
Several field studies for investigating IEPOX-SOA were conducted in different
regions. The extensive study by summarizes those with a focus
on IEPOX-SOA measured by aerosol mass spectrometry. The tracer ion at mass-to-charge
ratio (m/z) 82 (C5H6O) was identified to be related to
IEPOX-SOA in combination with the ion at m/z 53 (C4H5)
. In order to quantify the
amount of IEPOX-SOA measured by aerosol mass spectrometry, the ratio of the
signal at m/z 82 to the whole organic signal was introduced and is defined
as f82 e.g.,. Background values of f82 were calculated on the basis of
different field studies worldwide. In areas with predominant monoterpene
emissions, the background value is 3.1±0.6 ‰. In areas with a
strong biomass burning and urban influence, a background value of 1.7±0.1 ‰ is reported, whereas ambient organic aerosol with a strong
isoprene influence under low-NO conditions shows an increased value of 6.5±2.2 ‰ .
The AMAZE-08 campaign focused on SOA production mechanisms at a pristine
continental site in the Amazon basin during the wet season
. The submicron aerosol particles were found to be
dominated by secondary organic material . From positive
matrix factorization (PMF) analysis one factor was associated with the
reactive uptake of IEPOX to acidic haze, fog, or cloud droplets
. Another field campaign that was conducted in the Amazon
basin is the GoAmazon2014/5 field campaign . One focus of
GoAmazon2014/5 is the study of aerosol sources and SOA formation and aging
comparing the dry and wet season and the influence of urban pollution
. Another focus is the investigation of parameters
influencing the pathways for isoprene oxidation. Shifts in the prevailing
regime of NO or HO2 pathways for isoprene photooxidation in the
central region of Amazonia were studied . Also,
shifts in the production of IEPOX-SOA with changing concentrations of sulfate
and NOx in the boundary layer of the central Amazon region
were studied as part of GoAmazon2014/5 . Increased NO
concentrations suppress IEPOX-SOA production. Despite the enhancing effect of
increased sulfate concentrations for IEPOX-SOA production, the NO effect is
dominating . During another airborne measurement campaign in
the Amazon rainforest (SAMBBA), the highest f82 values were around
9 ‰, measured at the top of the boundary layer with a maximum flight
altitude of 5 km . An upcoming paper by
will compare aircraft measurements from the GoAmazon2014/5 campaign with data collected during the
ACRIDICON-CHUVA campaign and with measurements taken at a ground station in
the Amazon (T3). The results show good agreement for many measured
atmospheric parameters and gives the opportunity to validate the data quality
for several measurements conducted on different sampling platforms
.
Very few measurements at altitudes higher than the boundary layer were
reported. From single particle mass spectrometric measurements IEPOX sulfate
esters were identified in 80 % and 50 % of the analyzed particles
measured at altitudes of 5 and 10 km in the tropical free
troposphere, respectively . In the boundary layer, IEPOX
sulfate ester in aerosol particles did not occur. This is explained by a low
abundance of acidic aerosol particles acting as seed particles and with a
relatively short time since the emission of isoprene .
However, with lofting of isoprene and its derivatives above the boundary
layer, it is suggested that IEPOX can partition more efficiently to acidic
aerosol particles. The reactive uptake of IEPOX contributed 1 %–20 %
of the tropospheric aerosol mass in the tropics where continental convection
was active .
The presence of SOA in the tropical upper troposphere (UT) can have different
effects. For example, it can be entrained into the tropical transition layer
(TTL) from where further slow, radiatively driven lifting could transport
them into the lower stratosphere . There, and in the
TTL region of enhanced new particle formation (NPF) near the tropopause, the
SOA could become part of the global tropical layer of elevated submicron
particle abundances . It has also been
suggested that aerosol formation in the upper troposphere can provide a
source for cloud condensation nuclei for lower altitudes in the Amazon region
.
Another interesting aspect is the presence of organic nitrates. Many studies
reported on the difficulties to measure organic nitrates quantitatively, but
suggested also a possibility to estimate the amount or at least the presence
of organic nitrates based on the measured ion ratios of NO+ to
NO2+ using aerosol mass spectrometry e.g.,. Oxidation of VOCs with the highly reactive nitrate radical
NO3 can lead to different nitrogen-containing oxidation products that
can partition to the aerosol phase e.g.,. The nitrate radical oxidation of VOCs can contribute up to
20 % of the global VOC oxidation and is supposed to increase the aerosol
mass significantly .
Field measurements have shown that the major aerosol-phase product of
monoterpene oxidation with nitrate radical is likely a hydroperoxy nitrate
(C10H17NO5), whereas the analogous isoprene oxidation product
has a contribution of less than 1 % of the total organic nitrate and
occurs more in the gas phase . Laboratory studies suggested that
isoprene-derived organic nitrates are formed from SOA reactions but undergo
substitution reactions in which nitrate is substituted by sulfate
. Studies from the southeastern US showed that organic
nitrate aerosol particles from monoterpenes are strongly influenced by
anthropogenic pollutants and may contribute 19 %–34 % of the
total organic aerosol content . Polluted urban regions are
often dominated by inorganic nitrates . However, in rural
forested areas a dominance of particulate organic nitrates formed from
oxidation of monoterpenes was reported . A recent study from
the Amazon showed with measurements at ground level that up to 87 % of
the total nitrate can be attributed to organic nitrate .
This study presents submicron aerosol chemical composition measurements and
focuses on the presence of IEPOX-SOA at different altitudes above the Amazon.
The analysis herein uses data from the ACRIDICON-CHUVA campaign, which was
conducted in September 2014 . The study gives insights
into the photooxidative state of organic aerosols, the presence of IEPOX-SOA,
and also the presence of particulate organic nitrates. A comparison of these
parameters for different altitudes is presented.
Data analysis
General C-ToF-AMS data approaches
The C-ToF-AMS was calibrated with monodisperse ammonium nitrate and sulfate
before, during, and after the campaign in order to estimate relative
ionization efficiencies (RIE) for nitrate and sulfate. The resulting
calibration values can be found in Table .
Relative ionization efficiencies (RIEs) used for data analysis.
RIENH4 and RIESO4 are determined from calibrations
with ammonium nitrate and sulfate before, during, and after the campaign.
RIENO3 and RIEOrg are literature values
.
RIENO3
RIENH4
RIESO4
RIEOrg
1.1
3.77
0.89
1.4
A value of 0.5 for the collection efficiency (CE) was used for all flights
. The main fraction of the measured
aerosol mass consists of organic matter (see Sect. ), which
has no clear effect on the CE. Furthermore, large amounts of nitrate that
would lead to a higher CE were not encountered during the campaign
.
Detection limits (DLs) were derived as 3 times the standard deviation from
the background signal according to . Here, a
time-dependent cubic spline function was used to determine a detection limit
for each data point. This function was developed at the MPIC and is introduced and
explained in more detail in a PhD thesis . For every
data point of the background signal a third-order polynomial (cubic) function
is calculated through the four neighboring points (two before and two after)
while omitting the actual point. Applying this method, all trends from the
background signal are excluded and just the short-term noise remains. A
quantity R is introduced in the algorithm and characterizes the statistical
spread of the noise. R is defined by the squares of the deviation between
the omitted center point and the cubic function along a moving window. To
relate this R to the standard deviation (σ) a proportionality factor
of 18/35 is needed . The exact derivation of
the proportionality factor will not be explained here, but qualitatively it
accounts for the fact that not only are the points affected by noise but also
the cubic function itself. Thus, R is larger than the standard deviation
(σ). This calculation also provides a continuous σ from which
the DL can be derived using Eq. ().
DL=3⋅2⋅σ
Averaged over a single flight, the detection limit is around
0.11 µg m-3 for organics, 0.02 µg m-3 for
nitrate, 0.03 µg m-3 for sulfate, and
0.09 µg m-3 for ammonium. It should be noted here that
ammonium can experience additional uncertainty due to interferences with water in
the fragmentation table . A vertical profile of the
averaged detection limits for all species can be found in the Supplement (see
Fig. S1).
Oxidation state of the organic aerosol
The oxidation state of the organic aerosol indicates the degree of
photochemical aging. It can be determined using the correlation between the
ratio of the signal at m/z 44 (mostly CO2+) to the total organic
signal, f44, and the ratio of the signal at m/z 43 (mostly
C2H3O+) to the total organic signal, f43 .
These two ions are important tools to identify the photochemical aging of
organic aerosol components in the atmosphere. Organic aerosol can be grouped
into oxygenated organic aerosol (OOA) and hydrocarbon-like organic aerosol
(HOA), whereby OOA is further classified into low-volatility OOA (LV-OOA) and
semi-volatile OOA (SV-OOA) e.g.,. The difference
between these two subcomponents is represented by the two ions at m/z 43
and m/z 44. These two ions, or more specifically f43 and f44,
change during photochemical aging of organic aerosol, leading to higher
f44 and lower f43 values for LV-OOA than SV-OOA. With increasing
photochemical aging, organic aerosols of different origins become more similar
in terms of chemistry . We use the ratio R44/43
(f44 divided by f43) to show the effect of photochemical aging on
organic aerosol particles dependent on the altitude.
Tracer for isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA)
The tracer ion at m/z 82 (C5H6O+) is attributed to methylfuran
and was identified to be related to IEPOX-SOA . Due to
thermal vaporization and subsequent electron impact ionization in the
C-ToF-AMS, isoprene photooxidation products in the aerosol are decomposed and
lead to an increased signal at m/z 82 . A
strong correlation is found with the ion at m/z 53, which corresponds to
C4H5+ . Calculating the ratio of the
signal at m/z 82 to the whole organic signal leads to the fraction
f82. This fraction is identified as a tracer just for IEPOX-SOA, not for
other isoprene-derived SOA from different reaction pathways
e.g.,. A study by
found background values of fC5H6O+ (ratio of
C5H6O+ to the total organic signal) for different regions
worldwide. For urban and biomass-burning-influenced regions an averaged
background value of 1.7±0.1 ‰ is found. Areas strongly
impacted by monoterpene emissions show higher averaged background values of
3.1±0.6 ‰. For ambient organic aerosol particles that are
influenced by enhanced isoprene emissions and no extensive NO emissions with
the presence of HO2, values of 6.5±2.2 ‰ have been
reported . This enhanced value indicates a strong IEPOX-SOA
influence. All the mentioned values are derived from high-resolution ToF-AMS
(HR-ToF-AMS) data. For C-ToF-AMS data with unit mass resolution, the tracer
f82 can be used, although interferences from ions other than
C5H6O+ at m/z 82 are possible. Empirical parameterizations
relating f82 and f44 for unit mass resolution are given by
Appendix A, which will be used here for the further analysis.
Figure shows the normalized, measured mass spectrum for a period
with high f82 during flight AC13 (for the flight notation, see
) conducted on 19 September 2014 at an altitude of
12.6 km. The averaging time for this spectrum was 1 min
(15:54–15:55 UTC). The two peaks for determining IEPOX-SOA, m/z 82
and m/z 53, are labeled and clearly visible in the spectrum. For m/z 82
and m/z 53 the same RIE as for organics is applied
. The calculated f82 from this spectrum is
27.4 ‰.
Normalized mass spectrum of IEPOX-SOA during flight AC13 conducted
on 19 September 2014 at an altitude of 12.6 km. Averaging time for
this spectrum was 1 min (15:54–15:55 UTC). The two distinct
IEPOX-SOA tracers m/z 82 and m/z 53 are labeled. The calculated f82
from this mass spectrum is 27.4 ‰.
A method for the estimation of the mass concentration of IEPOX-SOA is
reported by ; see Eq. (). For this
calculation the mass concentration at m/z 82 [C(m/z82)], total organic
mass concentration [C(Org)], a reference f82 value for
IEPOX-SOA (f82IEPOXSOA), and a background value
(f82Bg) are taken into account. The reference value for
f82IEPOXSOA is set to 22 ‰ . The
background value f82Bg can be determined with an empirical
equation and depends on the influence of urban, biomass burning, or strong
monoterpene emissions . For our data set we assume strong
monoterpene emission influence and use Eq. () to
determine f82Bg according to Appendix A.
C(IEPOX-SOA)=C(m/z82)-C(Org)⋅f82Bgf82IEPOXSOA-f82Bgf82Bg=0.0077-0.019⋅f44
In Eq. (), C(m/z82) and C(Org) designate
mass concentrations in units of µg m-3.
Particulate organic nitrates
Several studies show that qualitative measurements and quantification of
organic nitrates are of major interest e.g.,. Organic nitrates are decomposed during the evaporation and/or
ionization processes in the C-ToF-AMS and, therefore, are divided into an
organic and a nitrate signal . The following use of
RONO2 refers to the nitrate content of organic nitrates, as the
organic content cannot be estimated with the described methods. Due to
observations during the measurement campaign, the analysis of organic
nitrates is described as one part of this study. Four methods are applied to
find out whether organic nitrates have been present during the measurements.
A first estimation of organic nitrates can be derived from the ratio of
the nitrate-related ions at m/z 30 (NO+) and m/z 46
(NO2+). The signal at m/z 30 is mostly from NO+, but also
the organic ion CH2O+ can contribute with a small amount
. Such interferences at m/z 30 with
CH2O+ are corrected in the evaluation software by the fragmentation
table , but it is not possible to distinguish
unambiguously between the NO+ and the CH2O+ ions with a
C-ToF-AMS. The signal at m/z 46 is usually dominated by NO2+ ions
. As organic interferences on the mass
spectral signals at m/z 30 (interference from CH2O+) and
m/z 46 (interference from CH2O2+) can occur in environments with high
biogenic contribution and/or small nitrate concentrations, a correction
according to was applied. The correction of both signals at
m/z 30 and 46 is achieved by using correlated organic signals at m/z 29,
42, 43, and/or 45 derived by high-resolution measurements. The organic
signals at m/z 29 (CHO+) and m/z 45 (CHO2+) are closest
to those affected by the interference and used for the correction here.
Equations () and () give the individual
correction for the nitrate signal at m/z 30 and 46, respectively. The
correction for NO+ includes the total signal at m/z 30, the default
fragmentation correction from the air signal , and a
correction coefficient that depends on the m/z used for the correction
(Ai). As for m/z 30 the correlated organic signal at m/z 29 is used
here, and the organic signal at m/z 29 (Org29) needs to be taken into account
as well as the contribution of the isotopes of organic CO. For the correction
of the nitrate fraction at m/z 46, a term which includes a correlation
coefficient Bi and the organic signal at m/z 45 is subtracted from the
signal at m/z 46. The correction coefficient Ai is in this case 0.215,
and Bi is 0.127 (see Supplement to ). In the organic signal
at m/z 28, 29, 30, and 45, the relative ionization efficiency
(RIEOrg) is already applied and needs to be reversed for the
correction of the nitrate signal.
Nitrate fraction atm/z30:NO+=m/z30-0.0000136⋅m/z28-Ai⋅(Org29-0.011⋅Org28)⋅RIEOrg-Org30⋅RIEOrg.Nitrate fraction atm/z46:NO2+=m/z46-Bi⋅Org45⋅RIEOrg.
The total nitrate signal is then calculated by adding both fractions. The
final nitrate mass concentrations were reduced by
0.045 µg m-3 (STP), on average corresponding to an averaged
reduction of 39 % of the initial nitrate mass concentrations. A
comparison of the initial and finalized nitrate mass concentrations can be
found in the supplement (see Fig. S2).
The ratio of NO+ to NO2+ is different for inorganic ammonium
nitrate and organic nitrate. The ratio for inorganic nitrate is known from
the ionization efficiency calibration with pure ammonium nitrate. For our
instrument its value lies between 1.49 and 1.56 with a mean and standard
deviation value of 1.52±0.03 and is derived from calibration
measurements during the campaign. For organic nitrates the literature
presents a range of ratios of NO+ to NO2+ that are higher
than the ratio for inorganic nitrates and lie between 5 and 12.5
e.g.,.
Second, a range of possible mass concentrations of particulate organic
nitrate (pRONO2) can be determined by using C-ToF-AMS data. This
range is defined with an upper and lower limit. We calculate the amount of
inorganic nitrate from neutralization with ammonium and subtract this value
from the measured nitrate. For the upper limit we assume full neutralization
of sulfate by ammonium and allow only the remaining excess ammonium to be
available for neutralization of nitrate (see Eq. ). Resulting
negative values in the first step (neutralization of sulfate) mean that not
enough ammonium for full neutralization of sulfate was available and were set
to zero. In this case, nitrate could exist as organic nitrate such that the
upper limit for organic nitrate equals total measured nitrate. For the lower
limit we assumed that nitrate was neutralized by the available amount of
ammonium (see Eq. ). Resulting negative values were taken as due to
statistical variation. The remaining nitrate is the lowest possible amount of
organic nitrate, assuming that particulate nitric acid is not present.
Upper limit:pRONO2up=CNO3-CNH4-3696×CSO4×6218.Lower limit:pRONO2low=CNO3-CNH4×6218.
In Eqs. () and () pRONO2up,
pRONO2low, CNO3, CNH4, and
CSO4 designate mass concentrations.
The third possibility to estimate the mass concentration of the nitrate
content of organic nitrates comes from recent studies (see Eq. ). Besides the measured ratio of the
mass concentrations for NO+ to NO2+ (Rambient),
the ratio of NO+ to NO2+ derived from calibrations with
ammonium nitrate (Rcal) and a fixed value for the ratio from
organic nitrates (RRONO2) are used. In our analyses a value of
10 for RRONO2 was taken as described in
. For a detection limit, took
0.1 µg m-3 for a conservative data evaluation of
pRONO2Farmer.
pRONO2Farmer=NO3×(1+RRONO2)×(Rambient-Rcal)(1+Rambient)×(RRONO2-Rcal)
The fourth estimation method is described by and links the
ratios determined for inorganic and organic nitrate. The ratios of
NO+ to NO2+ calculated for inorganic and organic nitrate are
instrument specific, but a proportional co-variation is observed. Thus, a
ratio of the ratios, χ, is proposed and a value for χ of 2.25±0.35 is reported.
χ=RRONO2Rcal
With this χ and our calibration value Rcal, a value for
RRONO2 is calculated using Eq. (). The next step puts
the derived ratio for organic nitrates into Eq. () to determine
pRONO2Fry.
It should be emphasized again that only the mass concentration of the
nitrate content of organic nitrate is determined by all of the described
methods.
Results and discussion
Aerosol data obtained during 13 flights of the ACRIDICON-CHUVA campaign were
evaluated. Data collected during take-off and landing were removed, and cloud
passages were not considered. In the following section, vertical profiles of
median and interquartile ranges of different parameters are shown. They are
calculated for 500 m altitude bins. Data below 100 m were not
considered to eliminate the influence of the airport at take-off and landing.
Data above 14 km were not used in the binned vertical profiles due to
the low amount of available data points. One flight does not provide any
aerosol data (AC10, conducted on 12 September 2014). Therefore, this flight
is not included in the analysis of the C-ToF-AMS data. All figures are valid
for 13 flights, except where otherwise noted.
Meteorological conditions – boundary layer
The meteorological situation during the ACRIDICON-CHUVA campaign was quite
similar for all days. Convection was dominating the daily weather and
affecting every flight. The invariance of the meteorological situation is
also visible in the temperature profile (see Fig. a), which barely
shows any deviation. An overview of some flight details is provided in
Table S1 in the Supplement.
The boundary layer (BL) height was determined with the help of the ambient
temperature (Tambient), the virtual potential temperature
(Θv), relative humidity with respect to water
RHw, and aerosol number concentration for particle diameters
larger than 20 nm (Nd>20nm) (Fig. ).
During the whole campaign the vertical profiles of Tambient and
Θv showed almost no deviation, indicating stable conditions
for the measurement period. The daily evolution of the BL height was
noticeable during the flights, which lasted typically 7 h. The maximum
heights of the BL varied between 1.2 and 2.3 km, depending on the
flight time. The highest BL heights were measured later in the afternoon.
Over the whole measurement period, a mean height of the BL was found to be at
1.7±0.2 km (Fig. , horizontal dashed line; see also
Fig. S3). For the binned vertical profile shown in Fig. the BL
height is not so obvious due to smoothing effects while averaging over all
flights. The observed BL height is consistent with previous studies in the
Amazonian dry season . Above the BL is the convective cloud
layer, which reached altitudes of about 4–5 km during the campaign
. The thermal tropopause was at an altitude of 16.9±0.6 km (mean and standard deviation) . Therefore,
all flights of the ACRIDICON-CHUVA campaign were performed in the
troposphere.
Vertical profiles of (a) ambient temperature
(Tambient), (b) virtual potential temperature
(Θv), (c) relative humidity with respect to water
(RHw), and (d) aerosol number concentration for particle
diameters larger than 20 nm (Nd>20nm) for all
flights during the ACRIDICON-CHUVA campaign. Here medians (connected dots)
with interquartile ranges (shaded area) are shown for each plot. The
horizontal dashed line shows the mean height of the top of the boundary
layer.
Vertical profiles of (a) organics (green),
(b) nitrate (blue), (c) sulfate (red),
(d) ammonium (yellow), (e) black carbon (grey), and
(f) total aerosol (black) median mass concentration and
interquartile ranges (in 500 m bins) for 13 flights of the
ACRIDICON-CHUVA campaign. Horizontal dashed lines indicate the division into
the lower, middle, and upper troposphere.
Figure shows the vertical profiles of median relative humidity
with respect to water (RHw) and median aerosol number
concentration for particle diameters larger than 20 nm
(Nd>20nm). RHw decreases above the BL,
showing a minimum with constant median values between 5 and 9 km. At
higher altitudes, RHw increases again with altitude. The median
values of aerosol number concentration are constant in the BL, decrease at
middle altitudes with a minimum at 4 km, and rise strongly at
altitudes above 7 km. This increase was interpreted as evidence for
new particle formation at altitudes above 7 km by .
Aerosol mass concentration
In Fig. , the vertical profiles of median mass concentrations of
organics, nitrate, sulfate, ammonium, and black carbon given in
µg m-3 (STP) are shown, with STP calculated for T=300 K and p=995 hPa from calibration measurements at the
ground.
At all altitudes, the main fraction of the submicron particulate mass
consists of organic matter. The highest aerosol mass concentration, in terms
of total aerosol mass as well as median values for all species, is observed
at lower altitudes between 0.1 and 4.5 km. This includes the BL (see
Sect. ). An exception from that is nitrate, showing maximum
median values at altitudes above 10 km. At middle altitudes (between
5 and 8 km) the mass concentrations of all shown species decrease
rapidly. A different behavior of the species is observed at high altitudes
between 8 and 14 km. The mass concentrations increase again with
increasing altitude for organics and nitrate. For this altitude range,
nitrate shows the highest median values. Although the median values decrease
between 13 and 14 km, the 75th percentile range still indicates that
high mass concentrations were encountered. In Sect. the
increase in nitrate mass concentration is discussed with respect to the
potential existence of organic nitrates. In contrast to this, the mass
concentrations of sulfate and black carbon are highest at lower altitudes,
decline above 4.5 km, and stay constant at middle and high altitudes.
Just between 13 and 14 km, sulfate median values show a slight
increase again. At lower altitudes, the mass concentrations of ammonium are
highest, above 4.5 km decreasing, and stay constantly low for the
rest of the altitude range.
The vertical distribution of aerosol mass concentration allows the
classification into three different regions. These are the lower troposphere
(LT), ranging from 0.1 to 4.5 km; the middle troposphere (MT), covering
altitudes between 4.5 and 8 km; and the upper troposphere (UT), which
includes altitudes between 8 and 14 km (see Fig. ).
Scatter (triangle) plot of f44 against f43 for
(a) the lower troposphere (dark green, LT) and upper troposphere
(light green, UT) for 2 min averaged data and (b) a comparison
between different field campaigns performed in the Amazon region. Dashed
lines indicate a triangular area according to the criteria introduced by
. Squared colored markers and boxes show median values
and interquartile ranges for LT and UT, respectively.
Oxidation state of the organic aerosol
As described in Sect. , we calculated the correlation
between f43 and f44. Figure a presents the data from all
flights for the two different altitude regimes, LT (0.1 to 4 km) and
UT (8 to 14.4 km), averaged over 2 min. The dashed lines
represent the region where previous boundary layer aerosol data lie
. The arrow illustrates the direction (upper left corner)
in which data points are “moving” when photochemical aging occurs. The two
different colors indicate the altitude dependency. The dark green markers
represent data sampled in the LT, whereas the light green markers show data
from the UT. Although there is an overlapping region of both, there is a
difference between LT and UT. Most of the data sampled in the LT are located
towards the upper left corner of the triangle, meaning that they are more
oxidized. In comparison to this, data from the UT show different properties.
Lower f44 and at the same time increased f43 values show a lower
oxidation level of the organic aerosol. Also presented in Fig. a
are the median values with the interquartile ranges for LT and UT,
respectively. The median value for the UT has a higher f43 and a lower
f44 than that for the LT. That means organic aerosol measured in the UT
is significantly less photooxidized than in the LT. Thus, the organic aerosol
particles in LT and UT are different from each other. Some of the LT organic
aerosol may be transported to higher altitudes, but most of the organic
aerosol measured in the UT must have a different source that is not located
in the BL or in the LT. The possibility that substantial amounts of aerosol
are transported from the BL into the UT has been ruled out by the study of
, based on the absence of detectable amounts of black carbon
in the UT (also see Fig. ) and other differences in the properties
of aerosol in the LT and UT.
Previous field measurements that have been performed in the Amazon allow a
comparison of the presented data set with measurements taken at the ground at
two different stations (T3 and T0t). Station T3 is an open field ca.
70 km west of Manaus and has frequent pollution influence from this
city, whereas T0t is in a near-pristine rainforest ca. 60 km
northwest of Manaus. Thus, they provide anthropogenically influenced or
natural measurement conditions, respectively. For further information on the
research stations see and .
The data were collected during the AMAZE-08 and the GoAmazon2014/5 campaigns
during the wet season (AMAZE-08: February–March 2008; GoAmazon2014/5:
February–March 2014) and represent ground measurements . Figure b illustrates the median and interquartile
ranges for the different data sets. Median values from the GoAmazon2014/5
campaign are similar to the median values derived from the ACRIDICON-CHUVA
campaign sampled in the LT, showing that the organic aerosol is oxidized. In
comparison, the AMAZE-08 campaign data differ, showing that the organic
aerosol during AMAZE-08 was less oxidized. Our data for the UT differ from
both these data sets, indicating again a different source for organic aerosol
than in the LT. It should be mentioned here that there is a significant
variability of m/z 44 (and f44) among different AMS instruments such
that no quantitative comparison can be done among the different data sets
shown in Fig. .
Figure shows the vertical profiles of median and interquartile
ranges for R44/43, f82, calculated IEPOX-SOA mass concentration,
and relative contribution of IEPOX-SOA to the organic mass concentration,
RIEPOX-SOA/Org, respectively. Panel (a) illustrates the changes
of R44/43 with altitude. In the BL, R44/43 is constant,
demonstrating that this layer is well mixed. With increasing altitudes this
ratio decreases significantly to much lower values than in the LT. The lowest
median values are observed in the UT.
This raises the question about the source of the observed organics in the UT.
There are three different possibilities. First, horizontal long-range
transport and a subsequent mixing of these air masses with convectively
lofted air could occur. Second, particles from the near or distant BL might be transported aloft. A
third possibility would be in situ secondary organic aerosol formation in the
UT.
A horizontal long-range transport of air masses can be excluded due to the
less photochemically aged organics in the UT. The organic aerosol particles
in the UT show a lower R44/43 ratio, meaning that they did not experience
much photooxidation, as it would be expected from aerosol influenced by
long-range transport.
The second possibility, representing the fast convective vertical transport
of boundary layer particles, can also be ruled out. The aerosol number
concentrations differ considerably (see Fig. d) between the LT and
UT, with strongly increased values in the UT. Furthermore, the sulfate and
black carbon aerosol mass concentrations show the highest values in the LT
and decrease at higher altitudes (see Fig. c and d). In the case of a
fast convective vertical transport of boundary layer particles, the sulfate
and black carbon aerosol mass concentrations would show similar values in the
LT and UT. The decrease at altitudes above 4.5 km indicates that the
aerosol particles have been efficiently removed (e.g., scavenging) during
vertical transport .
Vertical profiles with medians and interquartile ranges of
(a) R44/43, (b) f82,
(c) C(IEPOX-SOA), and
(d) RIEPOX-SOA/Org. Horizontal dashed lines indicate
divisions into the LT with BL, MT, and UT. The vertical dashed line
in (b) presents the calculated median background values
f82Bg with interquartile ranges using
Eq. (). This equation is valid for areas with strong
monoterpene influence Appendix A.
Air mass trajectories were calculated using the FLEXPART model. The
trajectories are calculated along the flight tracks starting every minute and
calculated backwards for 10 days, providing hourly information on the location
of each trajectory. The FLEXPART model is not able to resolve convective
transport (see Fig. S4) for the ACRIDICON-CHUVA campaign. Nevertheless, the
origin of the trajectories that are released in the LT (<4 km)
differs from the origin of the trajectories released in the UT (>8 km) (see Fig. S5). The trajectories released in the LT have their
origin also in the LT and show almost no interaction with higher air masses.
Most of the trajectories come from the Atlantic Ocean and the southern part
of South America. In contrast to this, the trajectories released above
8 km have their origin mainly above the Pacific Ocean and circulate
at high altitudes above South America. Just a minor part of the trajectories
originate from the eastern direction, coming from the Atlantic Ocean and/or
Africa. Interactions with air masses at lower altitudes are rare; most
prominent is the lifting at the Andes mountains.
This leads to the conclusion that the third possibility, in situ SOA
formation with subsequent growth of the aerosol particles to large enough
sizes that they can be detected by the C-ToF-AMS, is the dominant process in
the UT.
Another indication supporting this is the size information of aerosol
particles with diameters between 90 and 600 nm. Figure
shows the vertical profile of the median and the mode of the binned size
distributions measured with the UHSAS-A (panel a). It should be noted here
that the lowest cutoff of the considered size range of the UHSAS-A is at
90 nm. Accordingly, the displayed mode diameters are confined by this
lower limit. Also, the displayed size distribution medians are affected by
the size range limits and should only be interpreted in this context. Whereas
in the LT the median and the mode are at diameters around 150 nm
(median) and 130 nm (mode), respectively, both the median and the
mode are shifted towards smaller diameters with increasing altitude. The
lowest value of the median is reached at altitudes above 4 km and
(apparently) remains constant. The color code in the vertical profile refers
to the size distributions for the three different altitude regions in
panel (b) of Fig. . Shown are the median and interquartile range
of the size distributions. The size distribution in the LT shows a maximum at
130 nm. The size distributions in the MT and UT are shifted towards
smaller diameters, and it is clearly seen that the highest concentrations of
small particles (around 90 nm) are found in the UT.
In the following we present evidence that the formation of IEPOX-SOA in the
UT can partly explain this observation.
Vertical profile of the medians (black dots) and the mode
(triangles) of the binned size distributions (a), and median and
interquartile size distributions of particles between 90 and 600 nm
in the UT (pink), in the MT (yellow), and in the LT (blue) (b). The
grey area in (a) gives the interquartile range. The dotted line
in (a) indicates the lower cutoff of the considered size range of
the UHSAS-A. The statistics shown in both (a) and (b) are
calculated from all valid UHSAS-A data from 10 flights (AC07-AC10,
AC15-AC20). Data are calculated for STP conditions.
Observations of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA)
Figure presents the vertical profile of the IEPOX-SOA tracer
f82 (panel b) and the calculated median background values f82Bg
(see Eq. ). The median background values are quite
constant between 4 ‰ and 5 ‰ over the whole altitude range
with small interquartile ranges (see Fig. b, vertical dashed line
with interquartile ranges in grey). This indicates that continuous emissions
and processing of isoprene tend to build an ubiquitous background level up to
14 km. In the LT, f82 shows constant median values around 5 ‰,
slightly increasing in the interquartile ranges in the upper part of the LT.
These values are similar to or higher than the calculated median background
values f82Bg, suggesting a strong IEPOX-SOA influence (see
Sect. ). In the UT, the values of f82 are again quite
constant over the altitude range, but with increased median values around
8 ‰. The values in the UT lie above the background values and are
even higher than in the LT, where an influence of IEPOX-SOA is observed. This
indicates that IEPOX-SOA can have an important impact also in the UT.
Scatter plot of f82 against R44/43 for the lower
troposphere (dark green, LT) and upper troposphere (light green, UT). Data
are averaged over 2 min. Red markers and boxes show median values and
interquartile ranges for LT and UT, respectively.
Although the characteristics of the MT are not the focus here due to the
overall low organic mass concentrations, it should be mentioned that the
highest median values of f82 were observed in the MT between 4.5 and
8 km. The interquartile ranges are extended here, but the median
values reach up to 12 ‰.
To the best of our knowledge, f82 data from the tropical upper
troposphere have not yet been reported in the literature. Aircraft
measurements above the Amazon rainforest were reported by
, but these data are restricted to altitudes
below 5 km, corresponding to the definition of the LT in this study.
In , two cases are presented and show background
(4 ‰) and increased values (9 ‰) of f82. The highest
values of f82 were found on top of the boundary layer, decreasing with
increasing altitudes. These values are similar to the data from the LT
presented here.
Scatter plot of C(IEPOX-SOA) against nitrate (a)
and sulfate (b) mass concentrations for the lower troposphere (dark
green, LT) and upper troposphere (light green, UT). Data are averaged over
2 min and presented together with the values for Pearson's r2 for the
correlation between C(IEPOX-SOA) and nitrate (a) and
sulfate (b) mass concentration for LT and UT, respectively.
In (a) linear regression for the correlation between
C(IEPOX-SOA) and nitrate mass concentration in the UT is presented.
Figure c depicts the vertical profile of the median IEPOX-SOA mass
concentrations, which are calculated using Eq. (). The
lowest values can be found in the LT and MT. However, in the UT, especially
at altitudes above 10 km, a strong increase in IEPOX-SOA is observed.
Here IEPOX-SOA contributes up to 20 % of the organic mass concentration
in the UT (see Fig. d). The highest contribution of IEPOX-SOA to
the organic aerosol mass is observed in the MT, whereas in the LT up to
10 % can be attributed to IEPOX-SOA. This suggests that in the MT as well
as in the UT IEPOX-SOA formation can occur.
The correlation of f82 with R44/43 is presented in Fig.
using data averaged over 2 min. Again, the two different green colors
refer to the LT (dark green) and the UT (light green). In the LT, high
R44/43 but low f82 values are observed. The f82 values are
similar to reported ones that describe a strong IEPOX-SOA influence (see
Sect. ). Interestingly, the values with low R44/43,
indicating less photooxidized organic aerosol, are correlated with even
higher f82.
This suggests that IEPOX-SOA is contributing to in situ SOA formation in the
UT. In general, SOA formation can occur either through new particle formation with
subsequent growth or by condensation or reactive uptake on
preexisting particles without NPF. However, at the time that NPF occurs, the
aerosol particles would be too small to be measurable with the C-ToF-AMS,
implying that growth of these newly formed particles is necessary.
The observed enhanced NO mixing ratios in the UT would likely change the
reaction pathway of isoprene to a non-IEPOX route and subsequently no
IEPOX-SOA formation would occur. Based on the observed IEPOX-SOA in the UT,
the oxidation of isoprene to IEPOX must occur before reaching these high
altitudes.
In previous laboratory studies it was found that acidic aerosol is needed for
reactive uptake or condensation of gaseous IEPOX onto particles. In the
laboratory, the acidic conditions are often realized by using sulfate seed
particles e.g.,. Also in
field studies a correlation between IEPOX-SOA and sulfate aerosol was
proposed e.g.,.
Figure shows the scatter plot of IEPOX-SOA against nitrate
(panel a) and sulfate (panel b) mass concentrations for LT and UT,
respectively. The data are averaged over 2 min. In the LT, no
correlation between IEPOX-SOA and nitrate is found. However, in the UT a
stronger correlation between IEPOX-SOA and nitrate can be seen (see
Fig. a). For sulfate, the correlation is very low both in the LT
and UT (see Fig. b). This indicates that sulfate might not be
necessary for the formation of IEPOX-SOA, but nitrate could be an important,
possibly sufficient component in the UT.
Although the correlation between IEPOX-SOA and nitrate is weak, it may
indicate that not only sulfate, but also nitrate can provide the acidic
conditions for the partitioning of IEPOX to IEPOX-SOA. For our data, taking
only the inorganic species (nitrate and sulfate) into account for acidity
calculations, the aerosol is mainly neutralized (see Fig. S6). Although there
is a tendency in the UT above 10 km that the measured ammonium is not
sufficient to neutralize the inorganic species, a quantitative statement
cannot be made as the values fall below or close to the DL. The presence of
organosulfates and organonitrates could also affect the acidity calculations. As
for the organonitrates, the data are already corrected; for organosulfates
such a similar correction is not possible with data from a C-ToF-AMS as there
are no different fragmentation patterns between inorganic and organic
sulfates . The partitioning of IEPOX on nitrate aerosol
results in organic nitrates. The formation and the estimated amount of
organic nitrates are discussed in Sect. .
Vertical profiles of (a) NO mixing ratio,
(b) reactive nitrogen NOy mixing ratio,
(c) NO+ (m/z 30, light blue) and NO2+ (m/z 46,
dark blue) mass signal, and (d) ratio of NO+ (m/z 30) to
NO2+ (m/z 46). Horizontal dashed lines indicate divisions into the
LT with BL, MT, and UT. The vertical dashed line in (d) presents the
ratio of NO+ to NO2+ derived during calibration
measurements.
Particulate organic nitrate
The vertical profiles of NO and NOy mixing ratios are shown
in Fig. a and b. The mixing ratios of both species are highest in
the LT and decrease towards the MT before reaching increased values in the UT
again. In the LT, increased NOx mixing ratios arise from
anthropogenic emissions coming from Manaus and other
pollution sources. A likely source for NOx in the UT is the
production by lightning . Due to the
relatively low O3 values in the UT, NOx exists
mainly in the form of NO. After conversion to NOy (mainly
HNO3), NOx can act here also as a source for
(organic) nitrate aerosol and explain the increase in nitrate aerosol mass
concentration (see Sect. ).
As already mentioned in Sect. , the detected ammonium
in the presented data is mainly sufficient enough to neutralize the aerosol.
There is a tendency that the aerosol is not fully neutralized above
10 km. However, organics can also react with inorganic species
forming organic nitrates and sulfates. In the following, the four approaches
to estimate the presence of organic nitrates as described in
Sect. are discussed.
Scatter plot of NO+ and NO2+ for the lower
troposphere (LT, dark blue) and the upper troposphere (UT, light blue). The
nitrate signals have been corrected for organic interference according to
. Linear fit curves are shown in red; the ratio of NO+
and NO2+ derived from calibrations with ammonium nitrate is
presented by the red dashed line.
Vertical profiles of (a) calculated lower and upper limits
of organic nitrate mass concentration (approach Sect. ,
this study), (b) measured nitrate mass concentration and organic
nitrate mass concentration calculated according to , and
(c) measured nitrate mass concentration and organic nitrate mass
concentration calculated according to . Horizontal dashed
lines indicate the divisions into the LT with BL, MT, and UT. The vertical
dashed line in panel (b) shows the conservative detection limit of
0.1 µg m-3 according to .
A first estimation of the particulate nitrate content from organic nitrates
is the ratio of NO+ to NO2+. From calibration measurements
with ammonium nitrate during the campaign this ratio is known and was found
to be in the range between 1.49 and 1.56 with a mean and standard deviation
value of 1.52±0.03.
Figure shows the scatter plot of the corrected NO+ and
NO2+ for the LT (dark blue) and the UT (light blue). The linear fit
curves for the LT and UT have an intercept of 0, proving that the applied
correction is essential. Nevertheless, it has to be noted that the correction
is based on correlations between different m/z signals derived from
measurements at low altitudes ; thus, the application to UT
data bears uncertainties, because the conditions (especially temperature) are
different. However, high-resolution AMS measurements at these altitudes are
currently not available.
The linear fit for the LT data shows a higher slope
than that derived from calibrations with ammonium nitrate. The linear fit for
the UT shows an even higher slope and some data points are still
significantly above the fitted ratio between NO+ and NO2+.
This can be seen as a first hint that organic nitrates might be observed,
especially in the UT.
Vertical profiles of median values of m/z 30 and m/z 46 and of the ratio
NO+ to NO2+ are depicted in Fig. c and d,
respectively. During the whole vertical profile the two lines in panel (c) behave
similarly, except for the altitude
range between 2 and 6 km, where the distance between them becomes
smaller. Compared to the values derived from calibrations with ammonium
nitrate, the measured ratios of NO+ to NO2+ during the
flights are much higher and the median values range between 2 and
5 (panel d). As described in
Sect. , higher NO+ to NO2+ ratios are
linked to organic nitrates and the observed ratio profile can be seen as a
first evidence for the presence of organic nitrates.
The second estimation provides a range with a lower and upper limit of
nitrate mass concentration of organic nitrates according to Eqs. ()
and () (Sect. ). Figure a shows the
estimated lower and upper limits as a vertical profile. In the LT and MT the
derived values are below or around zero such that a presence of organic
nitrates is unlikely. However, with increasing altitude, the lower and upper
limits are also increasing. Especially at altitudes higher than 10 km,
both parameters are above zero and the presence of organic nitrates becomes
likely. The weakness of this method lies in its dependence on ammonium
measurements (see Eqs. and ). Interferences of water in
the fragmentation table can lead to a biased estimation of ammonium
concentrations . Biased ammonium concentrations can
be one reason for the derived negative values for upper and lower limits of
pRONO2.
Scatter plots of measured nitrate mass concentration against organic
nitrate mass concentration calculated according to
(a) and and
(b) for the lower troposphere (dark blue, LT) and
upper troposphere (light blue, UT), respectively. Data are averaged over
2 min. The horizontal dashed line shows the conservative detection limit of
0.1 µg m-3 according to . The 1 : 1 line
indicates that all particulate nitrate is present as organic nitrate. Also
shown are the two fit functions (red) for UT and LT in both plots,
respectively.
The third approach uses Eq. () to estimate the amount of nitrate
mass concentration of organic nitrates. As described in
, a fixed value of 10 for RRONO2 is
used here (see Sect. ). In Fig. b the
calculated particulate nitrate mass concentration of organic nitrates
(pRONO2Farmer) is presented. The measured nitrate mass
concentration is also shown. The detection limit is set to
0.1 µg m-3, which was reported by as a
conservative approach. Calculated pRONO2Farmer values are quite similar
to the measured nitrate in the vertical profile, although they are slightly
smaller, especially in the UT. Values for pRONO2Farmer in the LT
and MT are below the detection limit. Interestingly, at altitudes above
10 km the derived values are partly above the detection limit. This
points again to the presence of organic nitrates, especially in the UT at
altitudes above 10 km, but the presence of organic nitrates at
altitudes below 10 km cannot be excluded.
The fourth approach is based on Eq. () and addresses an estimation
of the ratio of organic nitrates from the ratio for inorganic nitrates
determined from the usual calibration measurements (see
Sect. ). Using our value of Rcal=1.53±0.03 results in RRONO2=3.44±0.05 (mean and standard
deviation). This value is lower than the ratios reported above from standards
and also lower than the ratio that is used for the estimation of
and . The vertical profile of pRONO2Fry is
shown in Fig. c, again with the measured nitrate mass
concentration in the back. The vertical profile of pRONO2Fry shows
similar values to the measured nitrate. At altitudes above 10 km the highest values are
reached, suggesting the likely presence of organic nitrates. Some median
values of pRONO2Fry are slightly higher than the measured nitrate
concentration, but this lies in the range of uncertainty of the data.
In Fig. , a comparison between the measured nitrate mass
concentration and the calculated pRONO2 according to
and (panel a) and (panel b) is presented. The data are
divided into LT (dark blue) and UT (light blue) and averaged over 2 min. The
1 : 1 line implies that all nitrate is present as organic nitrate with a
relative fraction of 100 %. The slope of the linear regression that lies
below the 1 : 1 line describes the relative content of nitrate that is
present as organic nitrate less than 100 %.
As a result of the scatter plot shown in Fig. , the estimation
according to and could explain
around 65 % of the measured nitrate in the UT and 35 % in the LT with
organic nitrates (see Fig. a). For the comparison with
, the whole amount of measured nitrate in the UT and 62 %
of the measured nitrate in the LT could be explained by organic nitrates (see
Fig. b). The slight overestimation for the UT lies in the range of
uncertainty of the data. As a qualitative result both comparisons show that
in the UT a higher organic nitrate fraction is observed than in the LT.
The quantification of the nitrate content of organic nitrates remains
difficult, but a qualitative statement can be made. Methods one, three, and
four agree with each other that at all altitudes organic nitrates are likely
present. In the LT and MT, the calculated values lie below the detection
limit introduced by . However, in the UT above 10 km
the presence of organic nitrates is supported by the results of all four
methods.
Organic sulfates can also be formed in the absence of sufficient ammonium.
reported that organic sulfates cannot be quantified using
C-ToF-AMS data due to a missing tracer ion. Organic sulfates would lead to
similar fragmentation patterns as inorganic sulfates .
Facing the similar mass concentration of nitrate and sulfate, and especially
the increase in nitrate in the UT, the presence of organic nitrates likely
plays a similar or even larger role than organic sulfates.
The study by about formation and stability of organic
nitrates and sulfates reports that nitrate and sulfates have similar kinetic
properties regarding the reaction with tertiary epoxides. According to their
study, organic nitrates have shorter lifetimes than organic sulfates and are
stable only for short time periods (a few days) before they undergo
substitution reactions of nitrate by sulfate or water. Unfortunately, no
statement of the temperature dependency of the reactions is given
. The temperatures measured in the UT are low
(210–240 K) and could slow down chemical reaction processes and
possibly shift the reactions in favor of organic nitrates for a longer time
period. In field experiments it was shown that low temperatures shift the
chemistry to the formation of organic nitrates . The
condensation of organic nitrates may be important for the growth of newly
formed particles in the atmosphere . Also the phase
state of the particles may be important for condensational growth. Over the
Amazon basin, SOA particles are predicted to be solid at altitudes above
5 km .
Summary and conclusion
We presented results from airborne aerosol measurements with a C-ToF-AMS
conducted during the ACRIDICON-CHUVA campaign in September and October 2014
in the tropical lower, middle, and upper troposphere over the Amazon region.
Vertical profiles of the aerosol mass concentrations for organics, nitrate,
sulfate, ammonium, and black carbon show an overall decrease in the mass
concentrations above the lower troposphere. For organics and nitrate the mass
concentrations increase again with increasing altitude in the upper
troposphere. The characteristics of the organic aerosol were analyzed. The
photooxidation state of the organics shows a well-mixed lower troposphere
with mainly oxidized organics, whereas in the upper troposphere less oxidized
organics were observed. Fast vertical transport from the boundary layer or
horizontal long-range transport can be excluded as an explanation for this
feature. Thus, SOA formation in the upper troposphere is proposed as the most
likely process explaining the less oxidized organics. Furthermore, IEPOX-SOA
was identified at all altitudes, indicating a strong influence on the organic
aerosol composition. Previous measurements that reported the enhanced
IEPOX-SOA influence in the boundary layer were confirmed
e.g.,. In the upper
troposphere, the IEPOX-SOA mass concentration increases and is associated
with less oxidized organics. This suggests that, after emission of isoprene by
vegetation, oxidation of isoprene by HO2 proceeds at low altitudes
and/or during vertical transport to higher altitudes. The oxidation product,
IEPOX, must have been formed before reaching the upper troposphere, where
different conditions for the oxidation pathway of isoprene are observed
(increased NO and NOy mixing ratios). In the upper
troposphere IEPOX can then partition on preexisting aerosol particles and
thus IEPOX-SOA is formed.
Furthermore, the increase in IEPOX-SOA in the upper troposphere is most
likely associated with organic nitrate formation. An increase in nitrate mass
concentration was observed in the upper troposphere. Four different methods
to estimate the presence and the nitrate mass of organic nitrates were
applied and the results support the fact that organic nitrates are present at
altitudes above 10 km.
These findings suggest that the formation of IEPOX-SOA and organic nitrates
are combined with each other. Two processes could explain the partitioning of
IEPOX to the aerosol phase. The first possibility is that nitrate provides
the required acidic conditions and IEPOX is taken up into the preexisting
aerosol containing organic nitrates. The second possibility is that IEPOX
partitions on already neutralized organic nitrates. In this case, the
preexisting aerosol would not need to be acidic.
The formation of IEPOX-SOA in the upper troposphere is an important source
for organic aerosol particles at these high altitudes, contributing about
20 % to the total organic aerosol. Further vertical transport of the
aerosol particles may lead to entrainment of upper-tropospheric aerosol into
the tropical tropopause layer (TTL), the “gate to the stratosphere”
. By this process, organic aerosol particles of
tropospheric origin could enter the stratosphere. Also the downward transport
of these particles could lead to a regular influence on the aerosol
composition at lower altitudes and especially in the boundary layer
. This would have effects on cloud
condensation nuclei production and thus on cloud properties, as well as also on the
radiative budget.
Emissions from the rainforest influence the tropical atmosphere in several
ways up to high altitudes. The processes described in this study may
provide further understanding of the mechanisms that occurred in the
preindustrial tropical atmosphere.