Impact of precursor gases and meteorological variables on satellite-estimated near-surface sulfate and nitrate concentrations over the North China Plain

Previous studies exploring the impact of precursor emissions and meteorological variables on PM2.5 concentrations were typically conducted over a short period and in urban environments. In this analysis, near-surface sulfate and nitrate mass concentrations at a 10-km resolution over the North China Plain (NCP) from 2006 to 2014 were first obtained based on satellite aerosol optical depth (AOD) data and the global 3D chemical transport model (GEOS-Chem) in combination with vertical profiles from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Using ground-based PM2.5 component concentrations, our satellite-based model yielded correlation coefficients (r) of 0.72 for sulfate and 0.70 for nitrate. Second, the spatio-temporal patterns of the chemical composition and corresponding precursor gases collected from the Ozone Monitoring Instrument (OMI) during the study period were analyzed. The highest levels of nitrate (similar to 30 mu g/m(3)) and sulfate (similar to 25 mu g/m(3)) were concentrated in Hebei, Shandong, Henan and Beijing, and the monthly variations of nitrate and NO2 presented a strong correlation coefficient (0.67). Inter-annual changes of nitrate and NO(2 )exhibited increasing trends of 1.43%/yr and 1.60%/yr, respectively. In contrast, sulfate and SO2 decreased at rates of 3.5%/yr and 0.76%/yr, respectively, and presented a negative correlation (similar to 0.54). Third, the relationships between the components or precursor gases and relative humidity (RH), planetary boundary layer height (PBLH), temperature (TEM) and wind speed (WS) were compared. The results showed that high levels of NO2 were usually accompanied by stable atmospheric conditions (i.e., low TEM, shallow PBLH, low RH and mild WS). Moreover, the nitrate concentration was often dependent on NO2, and they showed similar characteristics with meteorological factors. PBLH and WS had the greatest influence on nitrate concentrations, whereas the effects of RH and WS varied seasonally. Under beneficial conditions, i.e., high TEM, RH and PBLH and low WS, SO(2 )rapidly converted to sulfate and fluctuated inversely with the sulfate concentration. Overall, high nitrate levels were primarily associated with anthropogenic causes (i.e., extensive emissions), whereas meteorological conditions have a greater effect on the formation of sulfate concentrations than emissions.