The relative enrichment of SOA is also found during multi-year CNY holidays with dramatic reduction of anthropogenic emissions. We found a 17% increase of SOA proportion during the COVID lockdown.

Here we provide an observational analysis of secondary OA (SOA) variation properties in Yangtze River Delta (YRD) of eastern China in response to large scale of emission reduction during Chinese New Year (CNY) holidays from 2015 to 2020, and the COVID-19 pandemic period from January to March, 2020. Emission reduction can alter both precursors and oxidants which further affects secondary OA formation. Organic aerosol (OA) is a major component of atmospheric particulate matter (PM) with complex composition and formation processes influenced by various factors. Overall, the results from various approaches revealed that local polluting sources those contributed to PM2.5 mass were kept under significant control, indicating that the implementation of alleviation measures were helpful in improving the air quality of Hangzhou city. Although, secondary organic carbon (OCsec) exhibited more sensitively than primary organic carbon (OCpri) to control policies, the inclusive contribution to PM2.5 mass implicated regional transport of aged secondary aerosols to the study area. Because of higher prevailing wind speed in the study area during Control period, the contributions from sea salt to total PM2.5 mass were observed to be more than Pre-control. However, significant contributions from sources such as biomass burning and industry emissions during Control period indicated the role of long-range transport of PM2.5 from remote areas to this city. The emissions of PM2.5 from local pollution sources caused by fugitive dust and coal combustion were effectively reduced during Control period. The results showed that PM2.5 was alleviated through control policies due to its lowest mean mass concentrations (29.5 μg m-3) observed during Control period compared to Pre-control and Post-control periods. The experimental results of various chemical constituents were used for qualitative and quantitative assessment on trends in contribution of responsible sources to PM2.5 mass concentration under three different scenarios including Pre-control, Control and Post-control periods.

This comprehensive field campaign was conducted for chemical characterization of particulate matter of aerodynamic dimeter ≤ 2.5 μm (PM2.5) for elements, inorganic ions, carbon contents and priority organic markers to evaluate the efficacy of control policies implemented by Chinese government in improving air quality during G20 Summit in Hangzhou. 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. 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.

While the more oxidized SOA showed a large decrease by 75 %, the less oxidized SOA was comparable during (5.6 µg m−3) and after the control periods (6.5 µg m−3). However, we also observed significant changes in SOA composition.

Positive matrix factorization of OA further illustrated that primary OA (POA) showed similar decreases as secondary OA (SOA) at both ground level (40 % vs. Organic aerosols (OA) dominated PM1 during the control period at both ground level and 260 m (55 % and 53 %, respectively), while its contribution showed substantial decreases (~ 40 %) associated with an increase in secondary inorganic aerosols (SIA) after the parade indicating a larger impact of emission controls on SIA than OA. Our results showed that the average PM1 concentrations are 19.3 and 14.8 µg m−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). Here we conducted synchronous measurements of submicron aerosols (PM1) at ground level and 260 m 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. Strict emission controls were implemented in Beijing and adjacent provinces to ensure good air quality during the 2015 China victory day parade.

Chinese victory day