Volatile organic compounds at a rural site in Beijing: Influence of temporary emission control and wintertime heating

Abstract. Secondary organic aerosols (SOA) contribute substantially to PM2.5 during wintertime severe haze events in north China, yet ambient volatile organic compounds (VOCs) as SOA precursors are comparatively much less characterized in winter especially in rural areas. In this study, ambient air samples were collected in 2014 from 25 October to 31 December at a rural site inside the campus of University of Chinese Academy of Sciences (UCAS) in northeast Beijing for the analysis of VOCs. Since that temporary intervention measures were implemented during 3–12 November to improve air quality for the Asian-Pacific Economic Cooperation (APEC) summit held in 5–11 November in Beijing, and that wintertime central heating started since 15 November in Beijing after the APEC summit, it is a good opportunity to study the influence of the temporary control measures and the wintertime heating on the ambient VOCs. As a result of temporary intervention measures implemented during 3–12 November (period II), total mixing ratios of non-methane hydrocarbons averaged 11.25 ppb, about 50 % lower than that of 23.41 ppb before the APEC (25 October–2 November; Period I) or 21.71 ppb after the APEC (13 November–31 December; Period III). Their ozone and SOA formation potentials decreased by ∼50 % and ∼70 %, respectively, with the larger drop in SOA formation potentials attributed to more effective control of aromatic hydrocarbons mainly from solvent use. Back trajectory analysis revealed that the average mixing ratios of VOCs in the southerly air masses were 2.3, 2.3 and 2.9 times that in the northerly ones during period I, II and III, respectively; and all VOC episodes occurred under the influence of southerly winds, suggesting much stronger emissions in the south urbanized regions than in the northern rural areas. Based on the positive matrix factorization (PMF) receptor model, changed contributions from traffic emission and solvent use could explain 47.9 % and 37.6 % of the reduction in ambient VOCs, respectively, during the Period II relative to the Period I, indicating that the temporary control measures on vehicle emission and solvent use were effective in lowering ambient levels of VOCs. Coal/biomass burning, gasoline exhaust, and industrial emission were among the vital sources, and they altogether contributed 60.3 %, 78.6 % and 78.7 % of VOCs during the period I, II and III, respectively. Coal/biomass burning, mostly residential coal burning, became the dominant source which accounted for 45.1 % of the VOCs during the wintertime heating period, with a remarkably lower average contribution percentage (38.2 %) in the southerly air masses than that of 48.8 % in the northerly air masses.