Explosive PM_2.5 growth and health effects during extreme haze in Shanghai, China: Insights from surface and vertical observations

Vertical observations were used to explore the explosive growth (EG) of PM_2.5 in Shanghai in 2018 to investigate the rapid evolutionary mechanisms and health effects of a highly severe haze episode. The EG is defined as the net increase in PM_2.5 concentration by more than 100 µg/m³ within 6 or 9 h. The average PM_2.5 concentration during EG was 118.1 µg/m³, dominated by NO₃^-, NH₄⁺, SO₄^2- (SNA), and organic matter (OM), with a proportion of 31.1 %, 15.1 %, 14.8 %, and 18.8 %, respectively. In addition to being affected by chemical reactions, regional transport was the main influencing factor for OM, but local emissions were the main factor for SNA. Vertical observation results showed that EG events were influenced commonly by local emission, regional transport, and chemical reactions with a different proportion. Moreover, the heterogeneous reaction could possibly cause more severe pollution. The daily average nonaccidental mortality for EG [13.9 (95 % CI: 5.5, 25.0)] was about 4.0 times higher than that in Clean Days (CDs) [3.5 (1.4, 6.3)], and EG episodes caused a more severe effect on cardiovascular disease compared with respiratory disease. Moreover, local emissions and secondary gas-phase oxidation exhibited crucial factors for human health during EG, but regional transport during CDs. To sum up, ground-level observation could not fully explain the process of atmospheric pollution, but the vertical measurements help to understand the regional transport influence on the EG process. In the future, it is recommended that vertical observation should play a vital role in investigating regional heavy pollution episodes.