Sources and Processes Affecting Fine Particulate Matter Pollution over North China: An Adjoint Analysis of the Beijing APEC Period

The stringent emission controls during the APEC 2014 (the Asia-Pacific Economic Cooperation Summit; November 5–11, 2014) offer a unique opportunity to quantify factors affecting fine particulate matter (PM<sub>2.5</sub>) pollution over North China. Here we apply a four-dimensional variational data assimilation system using the adjoint model of GEOS-Chem to address this issue. Hourly surface measurements of PM<sub>2.5</sub> and SO<sub>2</sub> for October 15-November 14, 2014 are assimilated into the model to optimize daily aerosol primary and precursor emissions over North China. Measured PM<sub>2.5</sub> concentrations in Beijing average 50.3 μg m<sup>–3</sup> during APEC, 43% lower than the mean concentration (88.2 μg m<sup>–3</sup>) for the whole period including APEC. Model results attribute about half of the reduction to meteorology due to active cold surge occurrences during APEC. Assimilation of surface measurements largely reduces the model biases and estimates 6%–30% lower aerosol emissions in the Beijing–Tianjin–Hebei region during APEC than in late October. We further demonstrate that high PM<sub>2.5</sub> events in Beijing during this period can be occasionally contributed by natural mineral dust, but more events show large sensitivities to inorganic aerosol sources, particularly emissions of ammonia (NH<sub>3</sub>) and nitrogen oxides (NO<sub><i>x</i></sub>) reflecting strong formation of aerosol nitrate in the fall season.