Satellite-derived spatio-temporal variations of aerosol properties over China: competing anthropogenic and meteorological effects

Satellite observations showed the increase of the aerosol optical depth (AOD, a measure for the aerosol burden), over China at the end of the previous century, continuing until about 2007 and decreasing after 2011/2014. The initial increase, in response to economic growth and urbanization, was mitigated by the successive implementation of a series of emission reduction policy measures that resulted in strong AOD variations between 2007 and 2014, followed by a substantial AOD decrease after the implementation of the 2013 - 2017 Clean Air Action Plan. However, AOD time series show that the reductions were cancelled or even reversed over extended periods of time when the AOD increased. Model simulations show that these variations can be attributed to influences of unfavorable meteorological effects on the AOD which become stronger as AOD decreases. Further analysis shows the different effects of the occurrences of El Niño and La Niña on the AOD, in addition to economic effects. Furthermore, the emission reductions result not only in the decrease of aerosols but also affect the concentrations of precursor gases, both direct and through the chemical balance which effects the oxidative capacity of the atmosphere. As a result, aerosol composition changes occur which in turn affect aerosol optical properties. Changes in both concentrations and optical properties provide a plausible explanation for satellite observations of changes in AOD patterns. The reduction of aerosol concentrations reduces both the direct effect of aerosols and indirect effects on the Earth radiative balance.