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. Complex aerosol and photochemical pollution (ozone andperoxyacetyl nitrate, PAN) frequently occur in eastern China, and mitigationstrategies to effectively alleviate both kinds of pollution are urgentlyneeded. Although the effectiveness of powerful control measures implementedby the Chinese State Council has been comprehensively evaluated in terms of reducingatmospheric primary pollutants, the effectiveness in mitigatingphotochemical pollution is less assessed and therefore the underlyingmechanisms are still poorly understood. The stringent emission controlsimplemented from 24 August to 6 September 2016 during the summit for the Groupof Twenty (G20) provide us aunique opportunity to address this issue. Surface concentrations ofatmospheric O3, PAN, and their precursors including volatile organiccompounds (VOCs) and nitrogen dioxides (NOx), in addition to the othertrace gases and particulate matter, were measured at the National ReferenceClimatological 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, total VOCs, PM2.5, and sulfur dioxide (SO2) underthe unfavorable meteorological conditions during G20 (DG20) relative to theadjacent period before and after G20 (BG20 and AG20), indicating that thepowerful control measures were effective in reducing the pollutantemissions in Hangzhou. Unlike with the other pollutants, daily maximum8 h average (DMA8) O3 exhibited a slight increase and then decreasefrom BG20 to AG20, which was mainly attributed to the variation in the solarirradiation intensity and regional transport in addition to the contribution fromthe implementation of stringent control measures. Results from an observation-basedchemical model (OBM) indicated that acetaldehyde and methylglyoxal (MGLY)were the most important second-generation precursors of PAN, accounting for37.3 %–51.6 % and 22.8 %–29.5 % of the total production rates includingthe reactions of OVOCs, propagation of other radicals, and other minorsources. Moreover, we confirmed the production of PAN and O3 wassensitive to VOCs throughout the whole period, specifically dominated byaromatics in BG20 and DG20 but by alkenes in AG20. These findings suggestedthat reducing emissions of aromatics, alkenes, and alkanes would mitigatephotochemical pollution including PAN and O3. Source appointmentresults attributed the reductions of VOC source and ozone formationpotentials (OFPs) during G20 to the effective emission controls on traffic(vehicle exhaust) and industrial processes (solvent utilization andindustrial manufacturing). However, fuel combustion and biogenic emissionsboth weakened such an effect with a sizable contribution to the VOC mixingratios (18.8 % and 20.9 %) and OFPs (25.6 % and 17.8 %), especiallyduring the latter part of G20 (G20 II) when anthropogenic VOCs weresubstantially reduced. This study highlights the effectiveness of stringentemission controls in relation to traffic and industrial sources, but acoordinated program related to controlling fuel combustion and biogenicemissions is also required to address secondary pollution.

atmospheric chemistry and physics

Exploring the inconsistent variations in atmospheric primary and secondary pollutants during the 2016 G20 summit in Hangzhou, China: implications from observations and models