Atmospheric Transport and Chemistry Modeling

Although China has surpassed the United States as the world’s largest carbon dioxide emitter, in situ measurements of atmospheric CO2 have been sparse in China.
This paper analyzes hourly CO2 and its correlation with CO at Miyun, a rural site near Beijing, over a period of 51 months (Dec 2004 through Feb 2009). The CO2-CO correlation analysis evaluated separately for each hour of the day provides useful information with statistical significance even in the growing season. We found that the intercept, representing the initial condition imposed by global distribution of CO2 with influence of photosynthesis and respiration, exhibits diurnal cycles differing by season. The background CO2 (CO2,b) derived from Miyun observations is comparable to CO2 observed at a Mongolian background station to the northwest.  Annual growth of overall mean CO2 at Miyun is estimated at 2.7 ppm yr−1 while that of CO2,b is only 1.7 ppm yr−1 similar to the mean growth rate at northern mid-latitude background stations. This suggests a relatively faster increase in the regional CO2 sources in China than the global average, consistent with bottom-up studies of CO2 emissions. For air masses with trajectories through the northern China boundary layer, mean winter CO2/CO correlation slopes (dCO2/dCO) increased by 2.8±0.9 ppmv/ppmv or 11% from 2005–2006 to 2007–2008, with CO2 increasing by 1.8 ppmv. The increase in dCO2/dCO indicates improvement in overall combustion efficiency over northern China after winter 2007, attributed to pollution reduction measures associated with the 2008 Beijing Olympics. The observed CO2/CO ratio at Miyun is
25% higher than the bottom-up CO2/CO emission ratio, suggesting a contribution of respired CO2 from urban residents as well as agricultural soils and livestock in the observations and uncertainty in the emission estimates. 

A series of aggressive measures was launched by
the Chinese government to reduce pollutant emissions from
Beijing and surrounding areas during the Olympic Games.
Observations at Miyun, a rural site 100 km downwind of the
Beijing urban center, show significant decreases in concentrations
of O3, CO, NOy, and SO2 during August 2008, relative
to August 2006–2007. The mean daytime mixing ratio
of O3 was lower by about 15 ppbv, reduced to 50 ppbv, in
August 2008. The relative reductions in daytime SO2, CO,
and NOy were 61%, 25%, and 21%, respectively. Changes in
SO2 and in species correlations from 2007 to 2008 indicate
that emissions of SO2, CO, and NOx were reduced at least
by 60%, 32%, and 36%, respectively, during the Olympics.
Analysis of meteorological conditions and interpretation of
observations using a chemical transport model suggest that
although the day-to-day variability in ozone is driven mostly
by meteorology, the reduction in emissions of ozone precursors
associated with the Olympic Games had a significant
contribution to the observed decrease in O3 during August
2008, accounting for 80% of the O3 reduction for the
month as a whole and 45% during the Olympics Period (8–
24 August). The model predicts that emission restrictions
such as those implemented during the Olympics can affect
O3 far beyond the Beijing urban area, resulting in reductions
in boundary layer O3 of 2–10 ppbv over a large region of the
North China Plain and Northeastern China.

Rapid economic and industrial development in
China and relatively weak emission controls have resulted in
significant increases in emissions of nitrogen oxides (NOx)
in recent years, with the exception of late 2008 to mid 2009
when the economic downturn led to emission reductions detectable
from space. Here vertical column densities (VCDs)
of tropospheric NO2 retrieved from satellite observations by
SCIAMACHY, GOME-2 and OMI (both by KNMI and by
NASA) are used to evaluate changes in emissions of NOx
from October 2004 to February 2010 identifying impacts of
the economic downturn. Data over polluted regions of Northern
East China suggest an increase of 27–33% in 12-month
mean VCD of NO2 prior to the downturn, consistent with an
increase of 49% in thermal power generation (TPG) reflecting
the economic growth. More detailed analysis is used to
quantify changes in emissions of NOx in January over the
period 2005–2010 when the effect of the downturn was most
evident. The GEOS-Chem model is employed to evaluate
the effect of changes in chemistry and meteorology on VCD
of NO2. This analysis indicates that emissions decreased by
20% from January 2008 to January 2009, close to the reduction
of 18% in TPG that occurred over the same interval. A
combination of three independent approaches indicates that
the economic downturn was responsible for a reduction in
emissions by 9–11% in January 2009 with an additional decrease
of 10%attributed to the slow-down in industrial activity
associated with the coincident celebration of the Chinese
New Year; errors in the estimate are most likely less than
3.4 %.

Simulation of summertime U.S. surface ozone diurnal cycle is influenced by the model representation of planetary boundary layer (PBL) mixing, spatial resolution, and precursor emissions. These factors are investigated here for five major regions (Northeast, Midwest, Southeast, California, and Southwest) by using the Model for Ozone And Related chemical Tracers version 2.4 (MOZART-2.4), with important modifications, to conduct sensitivity experiments for summer 1999 with three PBL mixing schemes, two horizontal resolutions and two emissions datasets. Among these factors, the PBL mixing is dominant. The default non-local scheme well reproduces the observed ozone diurnal variation, where the timing for the afternoon maximum and the morning minimum is within 1 h of the observed; biases for the minimum are less than 5 ppb except over the Southeast; and the ozone maximum–minimum contrast (OMMC) is within 10 ppb of observations except for the overprediction by 18.9 ppb over the Northeast. In contrast, the local scheme significantly overestimates the OMMC by 10–34 ppb over all regions as ozone and precursors are trapped too close to the ground. On the other hand, the full-mixing assumption underestimates the OMMC by 0–25 ppb, except over the Northeast, as the nighttime ozone decline is greatly underpredicted. As compared to PBL mixing, the effects of horizontal resolutions and precursor emissions being used are smaller but non-negligible. Overall, with the non-local mixing scheme, relatively high horizontal resolution (∼1.1°) and updated emissions data, the modified MOZART is capable of simulating the main features of the observed ozone diurnal cycle.

A new methodology is developed to constrain
Chinese anthropogenic emissions of nitrogen oxides (NOx)
from four major sectors (industry, power plants, mobile and
residential) in July 2008. It combines tropospheric NO2 column
retrievals from GOME-2 and OMI, taking advantage
of their different passing time over China (10:00 a.m. LT
(local time) versus 02:00 p.m.) and consistent retrieval algorithms.
The approach is based on the difference of NOx
columns at the overpass times of the two instruments; it thus
is less susceptible to the likely systematic errors embedded
in individual retrievals that are consistent with each other.
Also, it explicitly accounts for diurnal variations and uncertainties
of NOx emissions for individual sources. Our best
top-down estimate suggests a national budget of 6.8 TgN/yr
(5.5 TgN/yr for East China), close to the a priori bottom-up
emission estimate from the INTEX-B mission for the year of
2006. The top-down emissions are lower than the a priori
near Beijing, in the northeastern provinces and along the east
coast; yet they exceed the a priori over many inland regions.
Systematic errors in satellite retrievals are estimated to lead
to underestimation of top-down emissions by at most 17%
(most likely 10%). Effects of other factors on the top-down
estimate are typically less than 15% each, including lightning,
soil emissions, mixing in planetary boundary layer, anthropogenic
emissions of carbon monoxide and volatile organic
compounds, magnitude of a priori emissions, assumptions
on emission diurnal variations, and uncertainties in the
four sectors. The a posteriori emission budget is 5.7 TgN/yr
for East China.

The Chinese government has moved aggressively since 2005 to reduce emissions of a number of pollutants including primary particulate matter (PM) and sulfur dioxide (SO₂), efforts inadvertently aided since late 2008 by economic recession. Satellite observations of aerosol optical depth (AOD) and column nitrogen dioxide (NO₂) provide independent indicators of emission trends, clearly reflecting the sharp onset of the recession in the fall of 2008 and rebound of the economy in the latter half of 2009. Comparison of AOD with ground-based observations of PM over a longer period indicate that emission-control policies have not been successful in reducing concentrations of aerosol pollutants at smaller size range over industrialized regions of China. The lack of success is attributed to the increasing importance of anthropogenic secondary aerosols formed from precursor species including nitrogen oxides (NO_x), non-methane volatile organic compounds (NMVOC), and ammonia (NH₃).

An updated version of the nested-grid GEOSChem
model is developed allowing for higher horizontal
(0.5×0.667) resolution as compared to global models. CO
transport over a heavily polluted region, the Beijing-Tianjin-
Hebei (BTH) city cluster in China, and the pattern of outflow
from East China in summertime are investigated. Comparison
of the nested-grid with global models indicates that the
fine-resolution nested-grid model is capable of resolving individual
cities with high associated emission intensities. The
nested-grid model indicates the presence of a high CO column
density over the Sichuan Basin in summer, attributable
to the low-level stationary vortex associated with the Basin’s
topographical features. The nested-grid model provides good
agreement also with measurements from a suburban monitoring
site in Beijing during summer 2005. Tagged CO simulation
results suggest that regional emissions make significant
contributions to elevated CO levels over Beijing on polluted
days and that the southeastward moving cyclones bringing
northwest winds to Beijing are the key meteorological mechanisms
responsible for dispersion of pollution over Beijing in
summer. Overall CO fluxes to the NW Pacific from Asia are
found to decrease by a factor of 3–4 from spring to summer.
Much of the seasonal change is driven by decreasing
fluxes from India and Southeast Asia in summer, while
fluxes from East China are only 30% lower in summer than in
spring. Compared to spring, summertime outflow from Chinese
source regions is strongest at higher latitudes (north of
35 N). The deeper convection in summer transporting CO
to higher altitudes where export is more efficient is largely
responsible for enhanced export in summer.

Observations of CO₂ and O₂ are interpreted to develop an understanding of the changes in the abundance of atmospheric CO₂ that have arisen over the period 1995–2007. Fossil fuels accounted for an addition of 89.3 Gt of carbon to the atmosphere over this time period, 29% of which was transferred to the ocean, 15% to the global biosphere, with the balance (57%) retained by the atmosphere. Analysis of historical data for CO₂ derived from studies of gases trapped in ice at Law Dome in Antarctica indicate that the biosphere represented a net source of atmospheric CO₂ prior to 1940, switching subsequently to a net sink.

The concept of Global Warming Potentials
(GWPs) has been extensively used in policy consideration
as a relative index for comparing the climate impact of an
emitted greenhouse gas (GHG), relative to carbon dioxide
with equal mass emissions. Ozone depletion due to emission
of chlorinated or brominated halocarbons leads to cooling
of the climate system in the opposite direction to the direct
warming contribution by halocarbons as GHGs. This
cooling is a key indirect effect of the halocarbons on climatic
radiative forcing, which is accounted for by indirect GWPs.
With respect to climate, it is critical to understand net influences
considering direct warming and indirect cooling effects
especially for Halons due to the greater ozone-depleting efficiency
of bromine over chlorine. Until now, the indirect
GWPs have been calculated using a parameterized approach
based on the concept of Equivalent Effective Stratospheric
Chlorine (EESC) and the observed ozone depletion over the
last few decades. As a step towards obtaining indirect GWPs
through a more robust approach, we use atmospheric models
to explicitly calculate the indirect GWPs of Halon-1211
and Halon-1301 for a 100-year time horizon. State-of-theart
global chemistry-transport models (CTMs) were used as
the computational tools to derive more realistic ozone depletion
changes caused by an added pulse emission of the
two major Halons at the surface. The radiative forcings on
climate from the ozone changes have been calculated for indirect
GWPs using an atmospheric radiative transfer model
(RTM). The simulated temporal variations of global average
total column Halons after a pulse perturbation follow an exponential
decay with an e-folding time which is consistent
with the expected chemical lifetimes of the Halons. Our cal-
culated indirect GWPs for the two Halons are much smaller
than those from past studies but are within a single standard
deviation of WMO (2007) values and the direct GWP values
derived agree with the published values. Our model-based
assessment of the Halon indirect GWPs thus confirms the
significant importance of indirect effects on climate.

The mass concentrations of black carbon (BC) were measured continuously at Miyun, a rural site near Beijing, concurrently with some trace gases (CO, CO₂, NO_y, SO₂) during the nonheating seasons of 2010 (April to October). The average concentration of BC was 2.26 ± 2.33 μg m⁻³. About 70%–100% of the air masses arriving at the site from June to September were from the source region of Beijing and the North China Plain (NCP), while in the spring, 40% were of continental background origin. BC had moderate to strong positive correlations with CO (R² = 0.51), NO_y (R² = 0.58), and CO₂ (nonsummer, R² = 0.54), but not with SO₂ (R² < 0.1). The observed ΔBC/ΔCO ratio was 0.0050 ± 0.0001 μg m⁻³/ppbv for the regional air masses (excluding the influence of biomass burning). This ratio increased by 68% to 0.0084 ± 0.0004 μg m⁻³/ppbv after excluding the influence of wet deposition. Accounting further for the impact of atmospheric processes on the observation, we derived an average top‐down BC/CO emission ratio of 0.0095 ± 0.002 μg m⁻³/ppbv for the source region of Beijing and NCP that is 18%–21% lower than the average emission ratio from the bottom‐up inventory of Zhang et al. (2009), whereas the difference is substantially lower than the uncertainty of emissions for either species. The difference between the mean bottom‐up and top‐down emission ratios is most likely to be attributed to the residential sector, which needs to have a lower share in the total emissions of BC or a much lower BC/CO emission ratio. The industry and transportation sectors are found to be dominant sources of BC from Beijing and the NCP rather than from the residential sector as suggested by the bottom‐up inventory.

Mixing in the planetary boundary layer (PBL) affects vertical distributions of air tracers in the lower troposphere. An accurate representation of PBL mixing is critical for chemical-transport models (CTMs) for applications sensitive to simulations of the vertical profiles of tracers. The full mixing assumption in the widely used global CTM GEOS-Chem has recently been supplemented with a non-local PBL scheme. This study analyzes the impact of the non-local scheme on model representation of PBL mixing, consequences for simulations of vertical profiles of air tracers and surface air pollution, and implications for model applications to the interpretation of data retrieved from satellite remote sensing. The non-local scheme significantly improves simulations of the vertical distributions for NO₂ and O₃, as evaluated using aircraft measurements in summer 2004. It also reduces model biases over the U.S. by more than 10 ppb for surface ozone concentrations at night and by 2–5 ppb for peak ozone in the afternoon, as evaluated using ground observations. The application to inverse modeling of anthropogenic NO_x emissions for East China using satellite retrievals of NO₂ from OMI and GOME-2 suggests that the full mixing assumption results in 3–14% differences in top–down emission budgets as compared to the non-local scheme. The top–down estimate combining the non-local scheme and the Lin et al. inverse modeling approach suggests a magnitude of 6.6 TgN yr⁻¹ for emissions of NO_x over East China in July 2008 and 8.0 TgN yr⁻¹ for January 2009, with the magnitude and seasonality in good agreement with bottom–up estimates.

More than 220 Tg N are processed annually through the global agriculture/animal/human food chain. It is suggested that aerobic denitrification, reduction of nitrite formed in the first stage of nitrification, is an important source not only of global N₂O but also of NO_x. A simple top‐down method indicates a globally averaged yield of 2% for N₂O emitted as a consequence of human disturbances to the global nitrogen cycle. This yield can account not only for the contemporary budget of atmospheric N₂O but also for trends observed over the past 1000 years. The associated microbial source of NO_x is estimated assuming a NO_x/N₂O ratio of 3, consistent with results from a variety of laboratory and field studies. This source is significant, particularly for large developing countries such as China and India for which its contribution is comparable to that from fossil fuel.

An interdisciplinary, quantitative assessment of the health and economic costs of air pollution in China, and of market-based policies to build environmental protection into economic development.