Siwen Wang

Global Chemical Composition of Ambient Fine Particulate Matter for Exposure Assessment

Epidemiologic and health impact studies are inhibited by the paucity of global, long-term measurements of the chemical composition of fine particulate matter. We inferred PM2.5 chemical composition at 0.1° × 0.1° spatial resolution for 2004–2008 by combining aerosol optical depth retrieved from the MODIS and MISR satellite instruments, with coincident profile and composition information from the GEOS-Chem global chemical transport model. Evaluation of the satellite-model PM2.5 composition data set with North American in situ measurements indicated significant spatial agreement for secondary inorganic aerosol, particulate organic mass, black carbon, mineral dust, and sea salt. We found that global population-weighted PM2.5 concentrations were dominated by particulate organic mass (11.9 ± 7.3 μg/m3), secondary inorganic aerosol (11.1 ± 5.0 μg/m3), and mineral dust (11.1 ± 7.9 μg/m3). Secondary inorganic PM2.5 concentrations exceeded 30 μg/m3 over East China. Sensitivity simulations suggested that population-weighted ambient PM2.5 from biofuel burning (11 μg/m3) could be almost as large as from fossil fuel combustion sources (17 μg/m3). These estimates offer information about global population exposure to the chemical components and sources of PM2.5.

Satellite measurements oversee China’s sulfur dioxide emission reductions from coal-fired power plants

To evaluate the real reductions in sulfur dioxide (SO2) emissions from coal-fired power plants in China, Ozone Monitoring Instrument (OMI) remote sensing SO2 columns were used to inversely model the SO2 emission burdens surrounding 26 isolated power plants before and after the effective operation of their flue gas desulfurization (FGD) facilities. An improved two-dimensional Gaussian fitting method was developed to estimate SO2 burdens under complex background conditions, by using the accurate local background columns and the customized fitting domains for each target source. The OMI-derived SO2 burdens before effective FGD operation were correlated well with the bottom-up emission estimates (R = 0.92), showing the reliability of the OMI-derived SO2 burdens as a linear indicator of the associated source strength. OMI observations indicated that the average lag time period between installation and effective operation of FGD facilities at these 26 power plants was around 2 years, and no FGD facilities have actually operated before the year 2008. The OMI estimated average SO2 removal equivalence (56.0%) was substantially lower than the official report (74.6%) for these 26 power plants. Therefore, it has been concluded that the real reductions of SO2 emissions in China associated with the FGD facilities at coal-fired power plants were considerably diminished in the context of the current weak supervision measures.

Global dry deposition of nitrogen dioxide and sulfur dioxide inferred from space‐based measurements

A method is developed to estimate global NO2 and SO2 dry deposition fluxes at high spatial resolution (0.1°×0.1°) using satellite measurements from the Ozone Monitoring Instrument (OMI) on the Aura satellite, in combination with simulations from the Goddard Earth Observing System chemical transport model (GEOS-Chem). These global maps for 2005–2007 provide a data set for use in examining global and regional budgets of deposition. In order to properly assess SO2 on a global scale, a method is developed to account for the geospatial character of background offsets in retrieved satellite columns. Globally, annual dry deposition to land estimated from OMI as NO2 contributes 1.5 ± 0.5 Tg of nitrogen and as SO2 contributes 13.7 ± 4.0 Tg of sulfur. Differences between OMI-inferred NO2 dry deposition fluxes and those of other models and observations vary from excellent agreement to an order of magnitude difference, with OMI typically on the low end of estimates. SO2 dry deposition fluxes compare well with in situ Clear Air Status and Trends Network-inferred flux over North America (slope = 0.98, r = 0.71). The most significant NO2 dry deposition flux to land per area occurs in the Pearl River Delta, China, at 13.9 kg N ha−1 yr−1, while SO2 dry deposition has a global maximum rate of 72.0 kg S ha−1 yr−1 to the east of Jinan in Chinas Shandong province. Dry deposition fluxes are explored in several urban areas, where NO2 contributes on average 9–36% and as much as 85% of total NOy dry deposition.

Satellite detection and model verification of NO x emissions from power plants in Northern China

We evaluate the recently increasing tropospheric NO2 columns in Northern China measured by the Ozone Monitoring Instrument (OMI) with an advanced power-plant NOx emission inventory and the NASA INTEX-B emission inventory, using a global chemical transport model (GEOS-Chem). In areas with newly built power plants the modeled and OMI-retrieved summertime average tropospheric NO2 columns increased by 55% and 47%, respectively, between 2005 and 2007. A monthly average increase of 1.79 Gg NOx emissions is calculated to lead to an increase of 1.0 × 10 15 molecules cm −2 in the modeled NO2 columns in the study areas. Good consistency (R 2 = 0.61, slope = 1.18, n = 14) between the increased modeled and OMI-retrieved summertime average NO2 columns is found. These results suggest that NOx emissions from large power plants in Northern China can be identified and quantified using OMI retrievals with confidence. The NASA INTEX-B emission inventory appears to underestimate the NOx emissions from the industry and transportation sectors, making it more difficult to quantify power-plant emissions when they are co-located with large cities.

Chemical composition of ambient PM 2. 5 over China and relationship to precursor emissions during 2005–2012

In this work, we presented the characteristics of PM2.5 chemical composition over China for the period of 2005–2012 by synthesis of in situ measurement data collected from literatures and satellite-based estimates using aerosol optical depth (AOD) data and the GEOS-Chem chemical transport model. We revealed the spatiotemporal variations in PM2.5 composition during 2005–2012 and investigated the driving forces behind the variations by examining the changes in precursor emissions using a bottom-up emission inventory. Both in situ observations and satellitebased estimates identified that secondary inorganic aerosols (i.e., sulfate, nitrate, and ammonium; SNA) ranked as the highest fraction of dust-free PM2.5 concentrations, followed by organic matter (OM) and black carbon (BC). For instance, satellite-based estimates found that SNA, OM, and BC contributed to 59, 33, and 8 %, respectively, of national population-weighted mean dust-free PM2.5 concentrations during 2005–2012. National population-weighted mean PM2.5 concentration increased from 63.9 μg m−3 in 2005 to 75.2 μg m−3 in 2007 and subsequently decreased to 66.9 μg m−3 from 2007 to 2012. Variations in PM2.5 concentrations are mainly driven by the decrease in sulfate and the increase in nitrate. Population-weighted mean sulfate concentration decreased by 2.4 % yr−1 during 2005–2012 (from 14.4 to 12.9 μg m−3), while population-weighted mean nitrate concentration increased by 3.4 % yr−1 during 2005– 2012 (from 9.8 to 12.2 μg m−3), largely offsetting the decrease in sulfate concentrations. By examining the emission data from the Multi-resolution Emission Inventory for China (MEIC), we found that the changes in sulfate and nitrate concentrations were in line with the decrease in SO2 emissions and the increase in NOx emissions during the same period. The desulfurization regulation in power plants enforced around 2005 has been the primary contributor to the SO2 emission reduction since 2006. In contrast, growth of energy consumption and lack of control measures for NOx resulted in a persistent increase in NOx emissions until the installation of denitrification devices on power plants late in 2011, which began to take effect in 2012. The results of this work indicate that the synchronized abatement of emissions for multipollutants is necessary for reducing ambient PM2.5 concentrations over China.

Analysis of the origins of black carbon and carbon monoxide transported to Beijing, Tianjin, and Hebei in China

A novel back-trajectory approach was adopted to determine the origins of black carbon (BC) and carbon monoxide (CO) transported to Beijing, Tianjin and Hebei. Results showed that the transport efficiency was controlled mainly by mid-latitude westerlies in winter, the South Asian monsoon in summer and prevailing westerly and northwesterly winds in spring and autumn. Hebei was identified as the most important source region of both BC (respectively accounting for 55% and 49%) and CO (39% and 38%) transported to Beijing and Tianjin. Inner Mongolia contributed more to the effective emission intensity (EEI) in winter than in summer for both BC and CO transported to Beijing and Tianjin. Shandong was responsible for higher EEI in summer than in winter. The six provinces making the greatest contributions to BC transported to Hebei were Shandong (19%), Shanxi (19%), Inner Mongolia (17%), Beijing (11%), Henan (11%), and Tianjin (10%), whereas those making the greatest contributions to CO transported to Hebei were Shandong (20%), Inner Mongolia (10%), Tianjin (9%), Henan (9%), Shanxi (9%), and Beijing (8%). In summary, Hebei, Inner Mongolia, Shandong, Tianjin and Shanxi were determined as the dominant source regions of not only BC but also CO transported to Beijing. Hebei, Shandong, Beijing, Inner Mongolia, Henan, Liaoning and Shanxi were relatively important source regions for Tianjin. Shandong, Shanxi, Inner Mongolia, Beijing, Henan, Tianjin, Liaoning, Jiangsu and Anhui were the main source regions for Hebei. Residential and industrial sectors were the dominant sectors for BC and CO transported to the receptors, respectively. These results are consistent with the results of previous studies. Finally, comparing the observed ΔBC/ΔCO ratio with the enhancement ratio of the EEI of BC with that of CO (ΔEEIBC/ΔEEICO) at Miyun site, we further confirmed that the EEI can be used to represent the amounts of BC and CO reaching receptors.