Measurements of ozone and its precursors in Beijing during summertime: impact of urban plumes on ozone pollution in downwind rural areas

摘要

Sea-land and mount-valley circulations are the dominant mesoscale synopticsystems affecting the Beijing area during summertime. Under the influence ofthese two circulations, the prevailing wind is southwesterly from afternoonto midnight, and then changes to northeasterly till forenoon. In this study,surface ozone (O), carbon monoxide (CO), nitric oxide (NO), nitrogendioxide (NO), nitrogen oxide (NO) and non-methane hydrocarbons(NMHCs) were measured at four sites located along the route of prevailingwind, including two upwind urban sites (Fengtai "FT" and Baolian "BL"), anupwind suburban site (Shunyi "SY") and a downwind rural site (Shangdianzi"SDZ") during 20 June–16 September 2007. The purpose is to improve ourunderstanding of ozone photochemistry in urban and rural areas of Beijingand the influence of urban plumes on ozone pollution in downwind ruralareas. It is found that ozone pollution was synchronism in the urban andrural areas of Beijing, coinciding with the regional-scale synopticprocesses. Due to the high traffic density and local emissions, the averagelevels of reactive gases NO and NMHCs at the non-rural sites were muchhigher than those at SDZ. The level of long-lived gas CO at SDZ wascomparable to, though slightly lower than, at the urban sites. We estimatethe photochemical reactivity ( and the ozone formation potential(OFP) in the urban (BL) and rural (SDZ) areas using measured CO and NMHCs.The OH loss rate coefficient ( by total NMHCs at the BL and SDZsites are estimated to be 50.7 s and 15.8 s, respectively.While alkenes make a major contribution to the , aromatics dominateOFP at both urban and rural sites. With respect to the individual species, COhas the largest ozone formation potential at the rural site, and at theurban site aromatic species are the leading contributors. While the Odiurnal variations at the four sites are typical for polluted areas, theozone peak values are found to lag behind one site after another along theroute of prevailing wind from SW to NE. Intersection analyses of trace gasesreveal that polluted air masses arriving at SDZ were more aged with bothhigher O and O concentrations than those at BL. The resultsindicate that urban plume can transport not only O but its precursors,the latter leading more photochemical O production when being mixedwith background atmosphere in the downwind rural area.