Regional discrepancies in spatiotemporal variations and driving forces of open crop residue burning emissions in China.

As a major source of secondary organic aerosol, open crop residue burning (OCB) emits a large number of trace gases and particulates to the atmosphere. Extensive OCB is detected during harvest seasons in most regions of China. Emissions from OCB have been widely investigated in China, but few studies have focused on regional discrepancies in spatiotemporal variations with a long timescale. In this study, emissions from OCB in three typical regions of China, including Northeast China, Chengdu-Chongqing and Guangdong, from 2003 to 2016, were estimated using a combination of statistical data and the Moderate Resolution Imaging Spectroradiometer (MODIS) observations, and regional discrepancies in spatiotemporal variations and driving forces were analyzed in detail. The results showed that OCB emissions of CO and PM2.5 in 2016 were 3.63 × 106 and 6.96 × 105 tons in Northeast China, 9.77 × 105 and 1.36 × 105 tons in Chengdu-Chongqing, and 1.24 × 105 and 1.19 × 104 tons in Guangdong, respectively. The OCB emissions in Northeast China maintained an overall increasing trend, which were mainly dominated by rural economic development and agricultural modernization, with great potential for reduction. In Chengdu-Chongqing, emissions increased until 2013 and then decreased, due to the impact of policies prohibiting OCB. In Guangdong, emissions showed annual fluctuations with a slightly decreasing trend influenced by metrological conditions and related policies. The analysis of the driving forces behind the regional discrepancies among these typical regions indicated that OCB emissions in China were characterized by uneven rural economic development, control policies, and natural conditions, suggesting that region-dependent control measures are needed for reducing OCB emissions. This study provided a better understanding of OCB emission characteristics in the three typical regions and revealed the diversity of OCB emission characteristics in China.

Assessment of satellite-estimated near-surface sulfate and nitrate concentrations and their precursor emissions over China from 2006 to 2014.

China is the largest anthropogenic aerosol-generating country worldwide; however, few studies have analyzed the PM2.5 chemical components and their underlying precursor emissions over long periods and across the national domain. First, global 3-D tropospheric chemistry and transport model (GEOS-Chem)-integrated satellite-retrieved aerosol optical depth (AOD) and vertical profiles were used to estimate near-surface sulfate and nitrate levels at 10-km resolution over China from 2006 to 2014. Ground measurement validation of our satellite model yielded correlation coefficients (r) of 0.7 and 0.73 and normalized mean bias (NMB) values of -37.96% and - 32.73% for sulfate and nitrate, respectively. Second, analyses of the spatiotemporal distributions of sulfate and nitrate as well as the vertical density Ozone Monitoring Instrument (OMI)-measured SO2 (PBL_SO2) and NO2 (TVCD_NO2) indicated that the highest nitrate and sulfate levels occurred in the North China Plain (~25 µg/m3) and Sichuan Basin (SCB) (~30 µg/m3), respectively. The long-term variations in the estimated components and precursor gases indicated that the large sulfate decline was positively correlated with the SO2 emission reduction due to the mandatory desulfurization implemented in 2007. The annual growth rate of sulfate relative to the national mean was -6.19%/yr, and the concentration decreased by 17.10% from 2011 to 2014. Energy consumption increases and a lack of control measures for NO2 resulted in persistent increases in NO2 emissions and nitrate concentrations from 2006 to 2010, particularly in the SCB. With energy consumption structure advancements, reductions in NO2 emissions and corresponding nitrate levels over three typical regions were prominent after 2012. Third, the estimated national-scale uncertainties of satellite datasets at 0.1°â€¯× 0.1° were 26.88% for sulfate and 25.55% for nitrate. Differences in the spatial distributions and temporal trends between our estimated components and precursor gases were mainly attributed to the dataset accuracy, the data pre-processing strategy, inconsistent column density and near-surface mass concentration, meteorological variables and complex chemical reactions.