Remotely Sensed Top-Down Derived Daily BC Over Rapidly Changing Western China shows Significant Decreases in Mass Due to Policy Action, Coupled with Inconsistent Changes in Number, Size, and Mixing Properties

Abstract Rapid economic and energy growth in Asia has led to change in the loadings of black carbon (BC) the last two decades. This work uses remotely sensed column aerosol absorption (AAOD) to quantify BC loading day-by-day and grid-to-grid. An unbiased set of observations of remotely sensed NOx and variance maximization uniquely quantify the spatial and temporal boundaries of urban, suburban, and rural sources, including those most rapidly increasing energy source regions. An inversely applied probabilistic MIE algorithm constrains the size, mixing state, and number of BC using four spectral observations. Significant differences in the particle size and mixing state are identified and attributed including: a drop around 2012 when China’s national air pollution policies were adopted, a drop ahead of the 2008 Olympics, and flattening from 2015 onward. A case of simultaneous reduction in BC mass and increase in BC number demonstrate PM2.5 controls may not fully control BC.