Investigating the PM_2.5 mass concentration growth processes during 2013-2016 in Beijing and Shanghai

The North China Plain and the Yangtze River Delta are the two of the most heavily polluted regions in China. Observational studies revealed that 'explosive' PM_2.5 mass concentration growths frequently occurred in the two regions. This study analyzed all the PM_2.5 mass concentration growth processes from clean condition (i.e., <35 μg m^-3) to heavy pollution condition (i.e., >150 μg m^-3) in Beijing (BJ) and Shanghai (SH), two representative cities of the two regions, using hourly monitored PM_2.5 concentrations during 2013-2016. 173 and 76 growth processes were identified in BJ and SH, respectively. PM_2.5 rising rates (PMRR) and dynamic growth durations were calculated to illustrate the characteristics of the growth processes. Hourly particulate chemical composition data and meteorological data in BJ and SH were further analyzed. The 4-year averaged PMRR of PM_2.5 total mass were similarly of 7.11 ± 9.82 μg m^-3 h^-1 in BJ and 6.71 ± 6.89 μg m^-3 h^-1 in SH. A decreasing trend was found for the PM_2.5 growth processes in two cities from 2013 to 2016, reflecting the effectiveness of emission controls implemented in the past years. The contributions of particulate components to the PM_2.5 total mass growth were different in BJ and SH. Average PMRR value of PM₁ organic aerosols (OA), SO₄^2-, NO₃^-, and NH₄⁺ in BJ was 1.90, 0.95, 0.82, and 0.53 μg m^-3 h^-1, respectively. Average PMRR of PM_2.5 OA, SO₄^2-, NO₃^-, and NH₄⁺ in SH was 1.70, 1.18, 1.99 and 1.14 μg m^-3 h^-1, respectively. Based on the contributions of different components, the PM_2.5 mass concentration growth processes in BJ and SH were proposed to be classified into 'other components-dominant growth processes', 'all components-contributing growth processes', 'one or more explosive secondary components-dominant growth processes', and 'mixed-factor growth processes'. Potential source contribution function analysis and the meteorological condition analysis showed that source origins and prevailing wind for the two cities during different categories of growth processes had substantial difference. The important source areas included Hebei and Shandong for BJ, and Jiangsu and Anhui for SH. The dominant wind directions during growth processes were northeast, south and southwest in BJ, and were west to north in SH. The results suggested the contributing components, the prevailing wind conditions, and the formation processes were substantially different in the two cities, despite the similar PMRR of PM_2.5 total mass during the growth processes between BJ and SH. Future research is needed to study the detailed formation mechanisms of the different PM_2.5 mass concentration growth processes in the two cities.