We present a new top‐down approach that spatially constrains the amount of aerosol emissions using satellite (Moderate Resolution Imaging Spectroradiometer (MODIS)) observed radiances with the adjoint of a chemistry transport model (GEOS‐Chem). This paper aims to demonstrate the approach through applying it to a case study that yields the following emission estimates over China for April 2008: 1.73 Tg for SO₂, 0.72 Tg for NH₃, 1.38 Tg for NO_x, 0.10 Tg for black carbon, and 0.18 Tg for organic carbon from anthropogenic sources, which reflects, respectively, a reduction of 33.5%, 34.5%, 18.8%, 9.1%, and 15% in comparison to the prior bottom‐up inventories of INTEX‐B 2006. The mineral dust emission from the online dust entrainment and mobilization module is reduced by 56.4% of 19.02 to 8.30 Tg. Compared to the prior simulation, the posterior simulation shows a much better agreement with the following independent measurements: aerosol optical depth (AOD) measured by AERONET sun‐spectrophotometers and retrieved from Multi‐angle Imaging SpectroRadiometer (MISR), atmospheric NO₂ and SO₂ columnar amount retrieved from Ozone Monitoring Instrument (OMI), and in situ data of sulfate‐nitrate‐ammonium and PM₁₀ (particular matter with aerodynamic diameter less than 10 µm) mass concentrations over both anthropogenic pollution and dust source regions. Assuming the bottom‐up (prior) anthropogenic emissions are the best estimates for their base year of 2006, the overwhelming reduction in the posterior (top‐down) estimate indicates less emission in April 2008 especially for the SO₂ tracer in the central and eastern parts of China, and/or an overestimation in the prior emission. The former is supported by the AOD change detected by MODIS and MISR sensors, while the latter is likely the case for NO_x and NH₃ emissions because no evidence shows that their atmospheric concentration has declined over China. With the promising results shown in this study, continuous efforts are needed toward a holistic and comprehensive inversion of emission using multisensor remote sensing data (of trace gases and aerosols) for constraining aerosol primary and precursor emissions at various temporal and spatial scales.

Considerable concern has been raised on the severe wintertime haze episodes over eastern China (ECN) where visibility (Vis) decline in winter is identified from 1973 to 2012 (−0.68 km per 10 years or −26% in 40 years). Based upon the analysis of daily Vis and weather records, cold wave (CW) originating from high latitudes is found to increase Vis by 2.7 km on average because of its relatively stronger wind and drier, cleaner air mass compared with the typical, stable midlatitude air over ECN in winter. However, the lessening frequency of CW occurrence and cold air activity in recent years and the accompanied decrease of surface wind speed (−0.15 m/s per 10 years or −18% in the 40 years) may have amplified the effect of increased anthropogenic emissions on Vis and consequently resulted in more substantial Vis decline. A comparison of Vis trends on the “normal wind” days and on all days in winter implies that the emission increase has contributed to about 79% of the declining Vis trend, while the meteorology change contributed 21%. Furthermore, the diurnal cycle of the boundary layer height is found to have weakened or in some cases disappeared in the winters with less CW, which probably contributed to the long‐lasting characteristic of the wintertime low Vis events in this region. Hence, the effect of climate change, such as the decrease of CW occurrence, should be accounted as part of the interpretation for the steady decrease of winter Vis over ECN in the past four decades.

Daily visibility (Vis) data from 354 meteorological stations are used to evaluate the seasonal Vis trends over China from 1973 to 2009, which show that Vis decline is most distinct over eastern China in summer, with a trend of −1.4 km per decade or −34% in the 37 years. This rapid decline of summertime Vis is found to be partially associated with the intensification and westward extension of the western Pacific subtropical high (WPSH) that has occurred in recent years. Such a change in the strength and breadth of WPSH has resulted in more days with stable, hot, and humid weather, which is favorable for Vis decrease. Analysis through decomposing the trend of Vis into contributions respectively from days with and without WPSH‐type meteorological conditions further shows that the intensification of WPSH leads to more episodes of Vis impairment and amplifies the effect of increased aerosol and precursor emissions on the summertime Vis decline in eastern China. Consequently, the difference of Vis between summer and winter follows a declining trend, although the Vis level in winter is still lower than that in summer. The result of this study underscores the importance of considering not only the variation of anthropogenic emissions, but also the change of climate and synoptic systems in the prediction and regulation of air quality and visibility.