Yuxuan
Wang
CURRENT
AND RECENT RESEARCH PROJECTS
Space-based NO2 measurements as constraints
on NOx emissions over east China
Space-based observations
of NO2 provide an important new source of data offering both global
and year-around coverage. We conducted a reanalysis of observations of NO2
columns inferred from the Global Ozone Monitoring Experiment (GOME) instrument,
developing ¡®top-down¡¯ estimates for the seasonal variation of NOx emissions over east China (100oE
-- 123oE and 20oN -- 42oN) where the
tropospheric contribution dominates in the total NO2 column. The
retrieval allows for zonal variations in the contribution of the stratosphere
to the NO2 column and removes a bias of ¡À10% on the seasonality of
retrieved columns introduced by cloud screening. We found that tropospheric NO2
columns derived from GOME were significantly higher over east China than
results obtained using a global three-dimensional chemical transport model
(GEOS-Chem) with seasonal variations notably
different from these assumed in the model.
The top-down
inventory is constructed using an inversion approach with a global 3-D chemical
transport model (GEOS-Chem) and combined subsequently
with the a priori inventory to
develop an a posteriori inventory.
The contribution of background NO2 arising
from non-surface sources (lightning) and long-range transport of emissions
originating outside of east China
is accounted for in the inversion. The regional total of the a posteriori inventory for east China is
4.66 Tg N/yr, 33% higher than the a priori value, and is shown to improve
agreement with surface measurements of nitrate wet deposition and
concentrations of NOy observed in China.
The uncertainty in the a posteriori
inventory is estimated at 30% for east China, lower than the uncertainty
in either the top-down or a priori
inventory.
Based on
multiple constraints on the spatial and seasonal variations of combustion and
microbial processes, we developed a simple, empirical, method to partition
composite emissions of NOx derived from
GOME among biomass burning, fuel combustion and microbial emissions over east China. Adopting
a priori knowledge of the dependence
of emission factors for NOx on
temperature and on the efficiency of combustion, we concluded that sources of NOx emitted from open-fire burning of crop
residues in China
were unlikely to play a significant role in the overall budget of NOx. The a
posteriori estimate for this source (0.08 TgN/yr
¡À 50%) is about a factor of 2 higher than the a priori estimate. Specific regions requiring significant
adjustments in the magnitude of the microbial source were identified as a
function of the uncertainty in the a priori
combustion inventory by comparing differences between the a posteriori estimate of composite emissions (after subtracting the
contribution from biomass burning) and the a
priori estimate of combustion sources. The resulting a posteriori inventory of 3.72 TgN/yr ¡À
32% from fuel combustion for east China is about 15% higher than the a priori value assumed in the GEOS-Chem model. The combustion source of NOx
derived here displays a maximum in winter with a minimum in summer, in contrast
to the weak seasonality indicated by the bottom-up approach. The a posteriori inventory of microbial
sources implies a source of 0.85 TgN/yr ¡À 40% for
east China,
a factor of 3 higher than the a priori
inventory, amounting to 23% of the fuel combustion source for the region. The
microbial source is unimportant in winter. It peaks in summer, accounting for
as much as 43% of the combustion source for that season, and is significant
also in spring and fall. The seasonality of microbial sources derived here is
attributed to the timing of fertilizer application and to the influence of
seasonally variable environmental factors, notably temperature and
precipitation. Our analysis, based on multiple-year results of GOME
observations, suggests that microbially mediated
decomposition of nitrogen processed through the human/animal/agriculture food
chain (including notably the response to application of chemical fertilizer) is
a significant source of NOx for east
China, lending support to conclusions derived earlier by Wang et al. [2004] and McElroy
and Wang [2005].
Reference:
Wang,Y.X., M. B. McElroy, R. V. Martin, D. G. Streets, Q. Zhang, and T.
M. Fu (2007), Seasonal variability of NOx
emissions over east China
constrained by satellite observations: Implications for combustion and microbial
sources, J. Geophys. Res., 112, D06301, doi:10.1029/2006JD007538 [pdf]
Top-down analysis of food-chain related emissions of N2O
and NOx
We examined the
influence of the agriculture/animal/human food chain on atmospheric
concentrations of N2O and NOx
from a global perspective. More than 220 Tg
N are processed annually through the global agriculture/animal/human food
chain, with over 50% contributed by domestic animals (left figure). Review of
the microbial processes governing emissions of N2O and NOx suggests that aerobic denitrification,
reduction of nitrite (formed in the first stage of nitrification) by nitrifying
bacteria under low oxygen conditions, is an important source not only of global
N2O but also of NOx. Yields of
both species are enhanced at conditions of low oxygen, with the ratio of NO to
N2O (mole N / mole N) greater than 1 and increasing with decreasing
levels of O2. A simple top-down method indicates a globally averaged
yield of 2% for N2O converted from nitrogen involved in this chain.
This yield can account not only for the contemporary budget of atmospheric N2O
but also for trends observed over the past 1000 years (right figure).
The associated
microbial source of NOx is estimated
assuming a NOx /N2O ratio of 3,
consistent with results from a variety of laboratory and field studies. The
yield for NOx is large enough to resolve
the discrepancy between bottom-up estimates for emissions of NOx from Central China
reported by Streets et al [2003] and values we derived using inverse,
top-down, methods. This source of NOx is
significant, particularly for large developing countries such as China and India for which its contribution is
comparable to that from fossil fuel.
Comparison
between observed (red symbols) and simulated (black line) atmospheric N2O.
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N Processed through the
Agriculture/Animal/Human Food Chain
(2001)
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Reference:
McElroy, M.B.
and Wang, Y.X. (2005), Human and Animal Wastes: Implications
for Atmospheric N2O and NOx, Global
Biogeochem. Cycles, 19, GB2008, doi:10.1029/2004GB002429 [pdf]
Inverse analysis of aircraft and surface observations
to constrain Chinese emissions of CO and NOx
Rapid
industrial development over the past 20 years in East Asia and specifically in China has
resulted in unprecedented growth in emission of important gaseous and
particulate components of the atmosphere with implications for both the global
and regional environment. An important objective of this thesis is to define
the nature and extent of these emissions. Three-dimensional (3-D) chemical
transport models in combination with high quality atmospheric measurements can
play a critical role in addressing this challenge. Uncertainty in our
understanding of relevant emissions poses a particular problem, however, in
realizing the potential of this approach.
We conducted a
reanalysis of the TRACE-P aircraft observations complemented by a study of data
from two ground stations in China.
The Chinese observations were taken at a remote coastal site (Hok Tsui) in Hong Kong (22º13'N,
114º15'E) and at a comparably remote rural site (Lin An) in east coastal China
(30º25'N, 119º44'E), overlapping in time with the aircraft measurements [ Wang et al., 2003, 2004b]. Comparisons
of model results for CO and NO y using the inventory of Streets et al. [2003]
suggested that emissions of both CO and NO x were underrepresented in the
bottom-up inventory.
We employed an
optimal estimation inverse model [Palmer et al. , 2003] to improve estimates of
Asian emissions of CO and NOx as
constrained by the integration of the aircraft and Chinese station data. The
inversion analysis requires 43% and 47% increases in Chinese emissions of CO
and NOx respectively, distributed
heterogeneously, with the largest adjustments required for central China . An analysis of emission ratios for CO relative to NOx for different industrial sectors indicates
that emissions of CO and NOx attributed to
industry and transportation may be underestimated in the bottom-up inventory
for Central China, while emissions from the domestic sector may be
underestimated for South China. Our inversion
results for CO are consistent with the conclusions independently reached in an
ongoing revision of ¡°bottom-up¡± emissions for CO over China. However,
the increase in NOx emissions inferred for
Central China is too large to be accommodated
by any reasonable adjustment in sources contributed by combustion of either
fossil or biofuel. We proposed that the missing source of NOx
may be associated significantly with microbially
mediated decomposition of human and animal wastes associated with the
agriculture/animal/human food chain and extensive applications of chemical
fertilizer.
Sensitivity
of the calculated a posteriori sources of CO (left) and NOy
(right) to
the error estimates in the inverse model.
Reference:
Wang,Y.X , M.B. McElroy, T. Wang, and P.I. Palmer (2004), Asian
emissions of CO and NOx : constraints from
aircraft and Chinese station data. J. Geophys. Res. ,109,
D24304, doi:10.1029/2004JD005250 [pdf]
Chemical transport of CO over Asia
in spring
The nested-grid
model is shown to provide good agreement with measurements of CO made during
the NASA GTE TRACE-P aircraft campaign in
spring 2001, with a typical correlation coefficient between model and
observation of greater than 0.7. Budget analysis suggests that horizontal
advection, mainly through the north boundary with European origin, contributes
a net source of CO to the study domain despite the polluted nature of the East Asia region. The model suggests that CO originated
outside Asia contributes more than 30% of the atmospheric burden below 2km for
Chinese regions north of 30oN and over the ocean. The major sink for
CO in the lower atmosphere is upward transfer to higher altitudes through large
scale advection and convection, with the latter significant mainly over
portions of India and Southeast Asia. Lifting ahead of southeastward-moving
cold fronts is responsible for an important pathway transporting CO emitted
from southern China
to the free troposphere. CO emitted from northern China is confined mainly to
the boundary layer due to the large scale subsidence associated with the
semi-permanent Siberia High and is occasionally lifted to the free troposphere
by eastward-moving, transient, middle latitude cyclones.
Latitudinal
distribution of measured and modeled CO during TRACE-P.
Reference:
Wang,Y.X ., M.B. McElroy, D. J. Jacob, and R.M. Yantosca
(2004). A nested-grid formation of chemical transport over Asia: applications
to CO. J. Geophy.
Res ., 109 , D22307,
doi:10.1029/2004JD005237 [pdf]
A nested grid formulation for chemical transport over Asia
This is the
focus of my Ph.D thesis research, under the guidance
of Prof. Michael McElroy at Harvard University.
It is motivated by an ambition to develop a credible 3-D chemical transport
model for air quality over East Asia, specifically over China. It is
part of a larger study at the China Project of the Harvard University Center for the
Environment seeking to estimate costs to the Chinese economy
associated with the complex impacts of air pollution on public health. To that
end, a model with moderately high spatial resolution over China and neighboring regions of East and Southeast Asia is required, while lower resolution is
acceptable elsewhere. This requirement is realized by developing a nested grid
capability in a global 3-D chemical transport model (GEOS-CHEM). The nested approach allows for
simulation of chemistry and transport of a particular region with relatively
high spatial resolution (e.g., East Asia) while at the same time preserving the
ability of the model to simulate the interaction of this region with the larger
global environment.
We examined
differences between results obtained using the nested grid (resolution 1o
x1 o), the coarse global model (4 o x5 o), and
the intermediate global model (2 o x2.5 o). Mixing ratios
of CO predicted by the coarse global model are significantly higher than those
for the high-resolution nested model at lower altitudes. The difference mainly
reflects the significance of localized regions of intense upward advection
responsible for removal of chemicals from the near surface environment, which
are not resolved in a coarser-resolution simulation.
Upper figure: Comparisons of CO mixing ratios averaged between 0 and 2 km
for March 2001, simulated by models with different horizontal resolution.
Left
figure: Schematic
representation of the nested window.
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Reference:
Wang,Y.X ., M.B. McElroy, D. J. Jacob, and R.M. Yantosca
(2004). A nested-grid formation of chemical transport over Asia: applications
to CO. J. Geophy.
Res ., 109 , D22307,
doi:10.1029/2004JD005237 [pdf]