Exploring the inconsistent variations in atmospheric primary and secondary pollutants during the G20 2016 Summit in Hangzhou, China: implications from observation and model
Authors: Zhang Gen , Xu Honghui , Wang Hongli , Xue Likun , He Jianjun , Xu Wanyun , Qi Bing , Du Rongguang , Liu Chang , Li Zeyuan , Gui Ke , Jiang Wanting , Liang Linlin , Yan Yan , Meng Xiaoyan 
Published: January 2020

Abstract. Complex aerosol and photochemical pollution (ozone and peroxyacetyl nitrate (PAN)) frequently occur in eastern China and mitigation strategies to effectively alleviate both kinds of pollution are urgently needed. Although the effectiveness of powerful control measures implemented by the Chinese State Council has been comprehensively evaluated on reducing atmospheric primary pollutants, the effectiveness on mitigating photochemical pollution is less assessed and therein the underlying mechanisms are still poorly understood. The stringent emission controls implemented from 24 August to 6 September 2016 during the summit for Group of Twenty Finance Ministers and Central Bank Governors (G20) provides us a unique opportunity to address this issue. Surface concentrations of atmospheric O3, PAN, and their precursors including volatile organic compounds (VOCs) and nitrogen dioxides (NOx), in addition to the other trace gases and particulate matter were measured at the National Reference Climatological Station (NRCS) (30.22° N, 120.17° E, 41.7 m a.s.l.) in urban Hangzhou. We found significant decreases in atmospheric PAN, NOx, the total VOCs, PM2.5, and sulfur dioxide (SO2) under the unfavorable meteorological condition during G20 (DG20) relative to the adjacent period before and after G20 (BG20 and AG20), indicating that the powerful control measures have taken into effect on reducing the pollutants emissions in Hangzhou. Unlike with the other pollutants, daily maximum average-8 h (DMA8) O3 exhibited a slight increase and then decrease from BG20 to AG20, which was mainly attributed to the variation in the solar irradiation intensity and regional transport besides the contribution from the implement of stringent control measures. Results from observation-based chemical model (OBM) indicated that acetaldehyde and methyl glyoxal (MGLY) were the most important second-generation precursors of PAN, accounting for 37.3–51.6 % and 22.8 %–29.5 % of the total production rates including the reactions of OVOCs, propagation of other radicals, and the other minor sources. Moreover, we confirmed the productions of PAN and O3 were both sensitive to VOCs throughout the whole period, specifically dominated by aromatics in BG20 and DG20 but alkenes in AG20. These findings suggested that reducing emissions of aromatics, alkenes, and alkanes would mitigate photochemical pollution including PAN and O3. Source appointment results attribute the reductions of VOCs source and ozone formation potentials (OFP) during G20 to the effective emission controls on traffic (vehicle exhaust) and industrial processes (solvent utilization and industrial manufacturing). However, fuel combustion and biogenic emission both weakened such effect with sizeable contribution on the VOCs mixing ratios (18.8 % and 20.9 %) and OFPs (25.6 % and 17.8 %), especially during the latter part of G20 (G20 II) when anthropogenic VOCs were substantially reduced. This study highlights the effectiveness of stringent emission controls in relation to traffic and industrial sources, but a coordinated program related with controlling fuel combustion and biogenic emissions is also required on addressing secondary pollution.