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. To evaluate the effect of temporary emission control measures on air qualityduring the 2016 G20 summit held in Hangzhou, China, an intensive fieldcampaign was conducted with a focus on aerosol chemistry and gaseousprecursors from 15 August to 12 September 2016. The concentrations of fineparticles were reduced during the intense emission control stages, with thereduction of carbonaceous matter being mostly responsible for this observeddecrease. This, in turn, was mainly ascribed to the decrease of secondaryorganic aerosols via the suppression of daytime peak secondary organic carbon(SOC)formation. Although the regional joint control was enacted extending to the Yangtze River Deltaregion, the effect of long-range transport on the air quality of Hangzhou wasubiquitous. Unexpectedly high NOx concentrations wereobserved during the control stage, when the strictest restriction on vehicleswas implemented, owing to contributions from upstream populous regions suchas Jiangsu and Shandong provinces. In addition, the continental outflowtraveling over the ocean triggered a short pollution episode on the first dayof the G20 summit, resulting in a significant enhancement of thenitrogen/sulfur oxidation rates. In the wake of the summit, all airpollutants evidently rebounded after the various control measures werelifted. Overall, the fraction of secondary inorganic aerosols (SIA; in thiscase sulfate, nitrate, and ammonium aerosols – SNA) in PM2.5 increased as relative humidity increased; however, the overall concentrationof PM2.5 did not increase. Aerosol components thathad distinctly different sources and formation mechanisms, e.g.,sulfate/nitrate and elemental carbon, exclusively showed strong correlationsduring the regional/long-range transport episodes. The sulfate, nitrate, andammonium to elemental carbon ( SNA∕EC ) ratio, which was used as a proxyfor assessing the extent of secondary inorganic aerosol formation, was foundto be significantly enhanced under transport conditions from northern China.This study highlighted that emission control strategies were beneficial forcurbing particulate pollution, in addition to the fact thatregional/long-range transport may offset local emission control effects tosome extent.

Counteractive effects of regional transport and emission control on the formation of fine particles: a case study during the Hangzhou G20 summit