Introduction
The Pearl River delta (PRD) is a metropolis of nine cities on the southern
coast of China with 57 million people as of 2013. Rapid economic growth over
the past 3 decades has created a serious air quality problem within the
region, with ozone (O3) and particulate matter (PM) air quality standards
frequently violated. Volatile organic compounds (VOCs) are important O3
and PM precursors. Our recent retrieval of atmospheric glyoxal (CHOCHO) from
the OMI (Ozone Monitoring Instrument) satellite instrument, including a
number of corrections to previous retrievals, finds the CHOCHO column
concentrations over the PRD to be the highest in the world
. Here we use the OMI satellite data for CHOCHO and
formaldehyde (HCHO) in the PRD to evaluate VOC emission inventories used by
atmospheric models and the related VOC chemistry.
The PRD has undergone rapid industrialization since 1980, when a series of
economic reforms reduced restrictions on foreign investment. The PRD is now
referred to as the “World Factory”, producing 25 % of China's exports
. Major industries include printing, oil refining, chemical
production, automobile assembly, and electronics manufacturing
.
This industrialization has led to worsening air quality throughout the
region. Surface O3 and PM are routinely in excess of Chinese national
ambient air quality standards . Ozone production in
the PRD is predominantly VOC-limited
(;
; ;
), and the aromatic species toluene and
xylene play a dominant role . Aromatics have also
been identified as an important regional source of secondary organic aerosol
via reactive uptake of their oxidation products , including
glyoxal .
CHOCHO is a high-yield product of aromatic oxidation
. Previous analyses of CHOCHO satellite
observations over China have suggested that inventories of aromatic emissions
are too low. used 2005 observations of CHOCHO and
HCHO from the SCIAMACHY satellite instrument and found the global RETRO VOC
inventory to be too low in the PRD by over a factor of 2.
used 2007 SCIAMACHY CHOCHO observations and found the
INTEX-B East Asian inventory to be too low in the PRD
by a factor of 10–20.
Our OMI CHOCHO retrieval is systematically lower than the older SCIAMACHY
data, with very different patterns, as a result of improved background
corrections and removal of NO2 interferences . An
independent OMI CHOCHO retrieval by is also
systematically lower than SCIAMACHY. This calls for revisiting the
interpretation of CHOCHO data from space. Focus on the PRD not only targets a
hotspot in the OMI data, but enables comparison to a highly detailed local
VOC inventory for the region .
Data and methods
The OMI was launched onboard the NASA Aura satellite in July 2004
. Aura is in sun-synchronous orbit with an equatorial crossing
time of 13:38 LT (local time). OMI measures backscattered solar radiation at
a nadir spatial resolution of 13 km × 24 km and achieves daily
global coverage by cross-track imaging. Spectral fitting yields slant columns
of CHOCHO, HCHO and NO2 along the optical path. These are converted to
vertical columns using air mass factors (AMFs) that combine scattering
weights and vertical concentration profiles . We use
CHOCHO data from , and HCHO and NO2 data from the
OMI Version 3 product release . Vertical
profiles for the AMF computation are from the GEOS-Chem chemical transport
model (v9-01-3; http://geos-chem.org). GEOS-Chem was originally
described by and the glyoxal simulation was first
introduced by . The chemical mechanism in v9-01-3 is
described in .
Observations are averaged on a 0.25∘×0.3125∘ grid
using an area-weighted tessellation algorithm .
We exclude observations from the first and last cross-track positions, those
that fail the retrieval algorithm statistical quality checks, and those
impacted by the row anomaly
(http://www.knmi.nl/omi/research/product/rowanomaly-background.php).
Validation with aircraft data indicates that the OMI HCHO and NO2
retrievals are accurate within 20 and 30 %, respectively
. CHOCHO/HCHO column ratios from OMI
are consistent with aircraft observations , whereas
previous SCIAMACHY retrievals showed large discrepancies
.
We relate the CHOCHO and HCHO satellite observations over the PRD to VOC
emissions using a 1-D advective–reactive plume model
, assuming a constant wind u, and
treating the PRD as a Gaussian-distributed source (N(x;σ)) orthogonal
to the wind with total emission rate Ei (e.g., mol s-1). Let li
represent the vertical column density of VOC species i integrated in the
horizontal orthogonally to the wind (molecules cm-1). The continuity
equation is written as
∂li(x,t)∂t+u∂li(x,t)∂x=Ei(t)N(x;σ)-ki[OH](t)li(x,t).
Here ki is the rate constant of the reaction of VOC i with the hydroxyl
radical OH (the main sink for the VOCs of interest). The local diurnally
varying concentration of OH is calculated from GEOS-Chem and peaks at
1.5×107 molecules cm-3 at local noon, close to observed
values in the PRD . Ei varies diurnally using
source scaling factors from GEOS-Chem . We use the
NO2 plume as a proxy to derive the along-trajectory width of the VOC
source region (σ), using the exponential decay model from
. The derived half-maximum width (∼85 km) is
reasonable given the observed extent of PRD urban land cover from MODIS.
CHOCHO is treated as a product of VOC oxidation with yield αi from
VOC i, and is lost by reaction with OH and photolysis (rate constants kg
and Jg, respectively). The CHOCHO vertical column density integrated in
the horizontal orthogonal to the wind (g(x,t)) is then given by
∂g(x,t)∂t+u∂g(x,t)∂x=∑iαiki[OH](t)li(x,t)-kg[OH](t)+Jg(t)g(x,t).
Annual mean vertical column densities of NO2, HCHO, and CHOCHO
for 2006–2007. Values are OMI observations from for
CHOCHO, for HCHO, and for
NO2.
Yearly VOC emissions (2006) in the PRD (21.5–24∘ N,
112–115.5∘ E) and corresponding yields and production rates of
CHOCHO and HCHO over 1 day of aging. VOC emissions are from
. Yields are computed using the MCMv3.2 chemical
mechanism .
A similar equation holds for HCHO. Jg is calculated using the Fast-JX
radiative transfer model . The yields
(αi) are calculated for a 1-day VOC aging time using the box model
simulation of with the MCMv3.2 chemical mechanism
, and assuming a
high-NOx regime where organic peroxy radical products of VOC oxidation
react mainly with NO.
We apply the plume model to VOC emissions from five different inventories –
RETRO , MACCity , REASv2
, INTEX-B , and the
local PRD inventory from .
Results and discussion
Figure shows the mean 2006–2007 vertical columns of CHOCHO,
HCHO, and tropospheric NO2 over China. OMI CHOCHO columns in the PRD
(23∘ N, 113∘ E) peak at
1.0×1015 molecules cm-2, the highest in the world on an
annual basis . HCHO in the PRD is also high but
comparable to values in the industrial Szechuan Basin to the northwest and in
the densely populated East China Plain. NO2 is high but less than in the
East China Plain. As pointed out previously by and
, the unusually high CHOCHO concentrations over the
PRD can be attributed to high emissions of aromatic VOCs.
The PRD emissions inventory includes detailed
VOC speciation profiles of local sources ,
resolving 91 individual VOCs, and adds biogenic VOC emissions from GloBEIS
. The inventory does not contain primary CHOCHO
emissions, and primary HCHO emissions are negligibly small.
Mean OMI vertical column densities of CHOCHO, HCHO, and NO2 over
the PRD for 2006 to 2007, segregated by wind direction. Wind vectors at 60 m
altitude are from the NASA GEOS-5 assimilated meteorology product. The
distribution of urban land cover from the MODIS type 5 land cover product is
shown in grey.
Figure shows the VOC emissions from
and the corresponding HCHO and CHOCHO
production rates. Aromatic VOCs have higher CHOCHO yields than other
precursors, and their emissions are high enough to dominate CHOCHO
production. Paints and solvents are the largest source of aromatics in the
inventory, responsible for over 50 % of benzene, toluene and xylene
emissions. Atmospheric VOC observations in the PRD are consistent with that
solvent/paint signature , in contrast to
other Chinese cities, where VOC emissions are predominantly from combustion
. Acetylene emitted from combustion has a 64 %
ultimate yield of CHOCHO , but its lifetime is too long
(about 10 days) to make a major contribution to the local CHOCHO budget.
HCHO is produced with a more consistent yield from different VOCs, as shown
in Fig. . VOCs emitted by vehicles including alkenes and ≥C4 alkanes play a dominant role in HCHO production, with biogenic
isoprene making an additional seasonal contribution. This explains why OMI
HCHO columns in the PRD are comparable to other Chinese urban areas
(Fig. ).
Figure shows mean 2006–2007 OMI columns over the PRD
segregated by northeasterly, easterly, and calm (<2 m s-1) wind
conditions. The segregation is based on GEOS-5 surface wind data at Shenzhen
(23.5∘ N, 114∘ E). The shape of the urban plume is
consistent with wind direction. Ninety percent of northeasterly conditions
are in fall and winter. Fifty percent of calm conditions are in summer, and
easterly conditions are evenly spread over the seasons. These seasonal
dependences explain the higher HCHO columns under calm conditions, as
biogenic VOCs make a larger contribution in summer . On
the other hand, NO2 is lower because of faster photochemical loss. CHOCHO
shows much less variability between wind sectors, consistent with a dominant
anthropogenic source and with photochemistry driving both production and
loss.
We select observations from the northeasterly sector for application of the
advective–reactive plume model to evaluate emission inventories. Wind under
these conditions is relatively steady, with low diurnal variability, and the
urban plume is transported over flat terrain. The prevailing fall/winter
conditions minimize the influence of biogenic VOCs.
Figure shows cross-wind integrals of CHOCHO and HCHO vertical
column densities as a function of transport time calculated using the
local inventory along the mean flow
trajectories, and initialized upwind of the PRD. A regional background has
been subtracted prior to integration using observations in a sector upwind of
the plume source (114–116∘ E, 22–23∘ N). We ascribe a
20 % relative error to the observations from systematic AMF uncertainties
and a spatially uniform error from uncertainty in
the background column value .
Mean CHOCHO and HCHO PRD plumes under northeasterly flow conditions.
Left: vertical column densities, overlaid with surface air (60 m)
trajectories for the mean wind field of Fig. . The trajectories
are initialized upwind of the PRD (t=0), and transport times in hours
along the trajectories are indicated. The grey hatched area indicates the
location of maximum emissions as diagnosed by the peak concentrations for the
calm wind conditions in Fig. (8×1014 and
1.25×1016 molecules cm-2 for CHOCHO and HCHO, respectively).
Right: CHOCHO and HCHO cross-wind integrals of vertical column density. The
OMI observations are line integrals across the trajectories in the left
panels, and vertical bars are retrieval uncertainties. The stacked contours
are results from the 1-D plume model showing the contributions from
individual VOCs as given by the PRD inventory,
combined with the CHOCHO and HCHO yields of Fig. . VOC emissions
in the plume model for CHOCHO and HCHO are centered at transport time t=6.5
and t=7.0 h, respectively, based on the plume location during calm wind
conditions.
Also shown in Fig. are the results from the
advective–reactive plume model using the PRD
emission inventory for individual VOCs, with MCMv3.2 yields for HCHO and
CHOCHO (Fig. ). The model does not include biogenic emissions
(isoprene, monoterpenes, and methanol), which are relatively weak in
fall/winter and would be included in the regional background. The
anthropogenic emissions are released at t=6.5 h for CHOCHO and t=7 h
for HCHO, based on the location of the observed maximum column of each
species during calm conditions (Fig. ).
Figure shows that the model can generally replicate the
observed concentrations (line densities) of CHOCHO and HCHO as a function of
transport time. We do not expect the model to perfectly replicate the shape
of the plume, due to its simplistic treatment of transport, spatiotemporal
allocation of emissions, and chemistry. Comparison of the integrated plume
totals of the model and OMI is more robust. Specification of OH
concentrations and photolysis rates is a source of uncertainty in the modeled
plume total. We estimate a 30 % uncertainty in OH concentrations, and a
20 % uncertainty for photolysis rates, with the latter driven by aerosol
scattering . Integrating the plume model results
between t=5 and t=20 h in Fig. , we find good agreement
with OMI for both CHOCHO (370±50 kmol modeled vs. 350±90 kmol
OMI) and HCHO (3.2±0.6 Mmol modeled vs. 2.6±0.7 Mmol OMI), and
conclude that the PRD inventory of is
consistent with observations.
We repeated the same plume model calculation with the INTEX-B, REASv2, RETRO,
and MACCity emission inventories for the PRD. All inventories are for 2006
except RETRO (2000). Figure shows the emissions from each
inventory, together with integrated CHOCHO and HCHO plume enhancements in the
PRD integrating the OMI observations and plume model results in
Fig. between t=5 and t=20 h. With the exception of RETRO,
all inventories have similar total VOC emissions on a per C basis, though
they differ in speciation, and they reproduce the observed CHOCHO and HCHO
plumes within 40 % for CHOCHO and 55 % for HCHO.
VOC emissions in the PRD from five different inventories (see text),
and corresponding plume amounts of CHOCHO and HCHO as computed from the plume
model discussed in the text and integrated from t=5 to t=20 h on the
trajectory time grid shown in Fig. . Model uncertainty bars are
from uncertainties in OH concentrations and photolysis rates (see text). OMI
observations integrated on the same trajectory grid are also shown.
Pathways to glyoxal formation from toluene oxidation by OH in
MCMv3.2. Only species relevant to CHOCHO formation are shown, and are labeled
by their MCMv3.2 name. Branching ratios (blue) and the share of glyoxal
formation from each boxed species (red) are from the 24 h box model
simulation described in the text. The high NO2 pathway (not in MCMv3.2 but
relevant in chamber studies) is indicated in pink.
The good agreement between VOC emission inventories and satellite
observations of CHOCHO and HCHO is in sharp disagreement with
, who inferred a 10–20-fold underestimation of PRD
aromatic emissions in the INTEX-B inventory using SCIAMACHY CHOCHO
observations. The same inventory in our plume model underestimates the OMI
CHOCHO concentration by only a factor of 2. Increasing aromatic VOC emissions
by a factor of 10 would also overestimate HCHO by more than a factor of 2.
Annually averaged SCIAMACHY CHOCHO columns are ∼60 % higher than
OMI in the PRD, but this is not enough to explain the difference. Different
aromatic CHOCHO yields likely play a larger role. Molar yields of CHOCHO in
were 25 % for benzene, 16 % for toluene, and
16 % for xylenes, based on a literature average of chamber experiments
compiled by . By contrast, the MCMv3.2 molar yields used
here are 75 % for benzene, 70 % for toluene, and 36 % for
xylenes.
Figure shows the pathways to CHOCHO formation from toluene in
MCMv3.2. Approximately half of CHOCHO formation in MCMv3.2 is produced as a
first-generation product via a bicyclic intermediate (TLBIPERO). The rest of
CHOCHO production involves intermediate products, implying delays and
additional uncertainties.
Studies reporting CHOCHO yields at the lower end of the range reported in
were conducted under very high NOx conditions, resulting
in OH-adduct reactions (pink pathway, Fig. ) that would suppress
CHOCHO formation . The highest yield of
39.0±10.2 % measured by was performed
under NOx levels closer to ambient conditions; however, it was later
revised to 30.6±6.0 % after CHOCHO measurements from the experiment
were revised downward based on more accurate CHOCHO absorption cross sections
. corrected for NO2
reactions in their kinetics analysis to determine a yield of
26.0±2.2 %, in close agreement with . In
both studies, CHOCHO production was solely from first-generation production.
This is very consistent with the 32 % first-generation CHOCHO yield from
MCMv3.2 via TLBIPERO (Fig. ). Thus the higher yield of CHOCHO
from toluene in the MCMv3.2 mechanism relative to the
compilation is due to the accounting of later-generation production.
experimentally observed CHOCHO production from
butenedial (MALDIAL), confirming the existence of later-generation CHOCHO
production from toluene. Other later-generation CHOCHO formation pathways in
MCMv3.2 still need to be experimentally confirmed. However, the combined data
on CHOCHO and HCHO from the satellite observations do provide additional
constraints. If the CHOCHO yield from aromatics were much lower than MCMv3.2,
then aromatic emissions would need to be increased in a way that would be
inconsistent with the HCHO data.
In conclusion, the CHOCHO hotspot over the Pearl River delta seen by the OMI
satellite instrument can be explained by a very large industrial source of
aromatic VOCs, consistent with current emission inventories used in
atmospheric models. There has been little confidence in the past in
interpreting CHOCHO data from space, in part because of inconsistency with
surface observations . This issue seems to be
resolved with the OMI observations , and we find
CHOCHO to be an excellent tracer of aromatic VOC emissions where these are
high. Further work will need to examine other sources of CHOCHO relevant to
interpreting satellite observations, in particular biogenic isoprene. The
multi-generation CHOCHO yields from the atmospheric oxidation of aromatic
VOCs also need to be better established.