Insights into aerosol chemistry during the 2015 China Victory Day parade: results from simultaneous measurements at ground level and 260 m in Beijing

Strict emission controls were implemented in Beijing and adjacent provinces to ensure good air quality during the 2015 China Victory Day parade. Here, we conducted synchronous measurements of submicron aerosols (PM1) at ground level and 260m on a meteorological tower by using a high-resolution aerosol mass spectrometer and an aerosol chemical speciation monitor, respectively, in Beijing from 22 August to 30 September. Our results showed that the average PM1 concentrations are 19.3 and 14.8 mu gm(-3) at ground level and 260 m, respectively, during the control period (20 August-3 September), which are 57 and 50% lower than those after the control period (4-30 September). Organic aerosols (OAs) dominated PM1 during the control period at both ground level and 260m (55 and 53 %, respectively), while their contribution showed substantial decreases (similar to 40 %) associated with an increase in secondary inorganic aerosols (SIAs) after the parade, indicating a larger impact of emission controls on SIA than OA. Positive matrix factorization of OA further illustrated that primary OA (POA) showed similar decreases as secondary OA (SOA) at both ground level (40% vs. 42 %) and 260 m (35% vs. 36 %). However, we also observed significant changes in SOA composition at ground level. While the more oxidized SOA showed a large decrease by 75 %, the less oxidized SOA was comparable during (5.6 mu g m(-3)) and after the control periods (6.5 mu g m(-3)). Our results demonstrated that the changes in meteorological conditions and PM loadings have affected SOA formation mechanisms, and the photochemical production of fresh SOA was more important during the control period. By isolating the influences of meteorological conditions and footprint regions in polluted episodes, we found that regional emission controls on average reduced PM levels by 44-45 %, and the reductions were close among SIA, SOA and POA at 260 m, whereas primary species showed relatively more reductions (55-67 %) than secondary aerosol species (33-44 %) at ground level.