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Megacities have strong interactions with the surrounding regions throughtransport of air pollutants. It has been frequently addressed that the airquality of Beijing is influenced by the influx of air pollutants from theNorth China Plain (NCP). Estimations of air pollutant cross-boundarytransport between Beijing and the NCP are important for air qualitymanagement. However, evaluation of cross-boundary transport using long-termobservations is very limited. Using the observational results of the gaseouspollutants SO2, NO, NO2, O3, and CO from August 2006 toOctober 2008 at the Yufa site, a cross-boundary site between the megacityBeijing and the NCP, together with meteorological parameters, we explored amethod for evaluating the transport flux intensities at Yufa, as part of the“Campaign of Air Quality Research in Beijing and Surrounding Region2006–2008” (CAREBeijing 2006–2008). The hourly mean ± SD (median)concentration of SO2, NO, NO2, NOx, O3, Ox, and COwas 15 ± 16 (9) ppb, 12 ± 25 (3) ppb, 24 ± 19 (20) ppb,36 ± 39 (23) ppb, 28 ± 27 (21) ppb, 52 ± 24 (45) ppb,and 1.6 ± 1.4 (1.2) ppm during the observation period, respectively.The bivariate polar plots showed the dependence of pollutant concentrationson both wind speed and wind direction, and thus inferred their dominanttransport directions. Surface flux intensity calculations furtherdemonstrated the regional transport influence of Beijing and the NCP on Yufa.The net surface transport flux intensity (mean ± SD) of SO2, NO,NO2, NOx, O3, Ox, and CO was 6.2 ± 89.5,−4.3 ± 29.5, −0.6 ± 72.3, −4.9 ± 93.0,14.7 ± 187.8, 14.8 ± 234.9, and70 ± 2830 µg s−1 m−2 during the observationperiod, respectively. For SO2, CO, O3, and Ox the surface fluxintensities from the NCP to Yufa surpassed those from Beijing to Yufa in allseasons except winter, with the strongest net fluxes largely in summer, whichwere about 4–8 times those of other seasons. The surface transport fluxintensity of NOx from Beijing to Yufa was stronger than that from theNCP to Yufa except in summer, with the strongest net flux in winter, whichwas about 1.3–8 times that of other seasons. The flux intensities were thenassigned to the corresponding trajectories in the potential sourcecontribution function analysis (PSCF), which confirmed the results of fluxintensity calculations. Our study also suggested that various factors, suchas the wind field, emission inventory, and photochemical reactions, couldinfluence transport of air pollutants. The decrease of surface flux intensityduring the Olympic Games implied the role of both local emission reductionand regional cooperation in successful air quality management. Threedimensional observations are needed for further comprehensive discussion ofthe regional transport between Beijing and the NCP

Observation of regional air pollutant transport between the megacity Beijing and the North China Plain