Using a mobile laboratory to characterize the distribution and transport of sulfur dioxide in and around Beijing

Abstract. Megacities are places with intensive human activity and energy consumption. To reduce air pollution, many megacities have relocated energy supplies and polluted industries to their outer regions. However, regional transport then becomes an important source of air pollution in megacities. To improve air quality before and during the 2008 Beijing Olympics, a wide range of control strategies were implemented, including the relocation of polluting industries. High sulfur dioxide (SO₂) concentrations were occasionally observed during this period. Potential sources from southern regions of Beijing were indicated by backward trajectories model and urban/rural stationary measurements, but direct evidence was lacking. Here we used a mobile laboratory to characterize the spatial distribution and regional transport of SO₂ to Beijing during the Campaign for Air Quality Research in Beijing and the Surrounding Region (CAREBEIJING)-2008. Among the five days chosen for the case studies during the Olympic air pollution control period, four had high SO₂ concentrations (6, 20 August and 3, 4 September 2008) while one had low SO₂ concentration (11 September 2008). The average values of SO₂ during the low SO₂ concentration day were 3.9 ppb, much lower than during the high concentration days (7.8 ppb). This result implied an impact by regional transport from outside Beijing. During these days, we captured transport events of SO₂ from areas south of Beijing, with a clear decrease in SO₂ concentrations southeast of the 6th to 4th Ring Roads around Beijing and along the 140 km highway from Tianjin to Beijing. The influx of SO₂ through the 4th to 6th Ring Roads ranged from 2.1 to 4.6 kg s⁻¹ on 4 September and 0.2 to 1.6 kg s⁻¹ on 20 August 2008. The differences of influx in days were due to the variations of emission changes, transport directions and dilutions. Locally emitted SO₂ from a source located along Jingshi Highway outside the southwest section of the 5th Ring Road of Beijing was identified using wind field data generated by the Weather Research and Forecasting model and the measured particle size distribution, with an estimated flux of 0.1 kg s⁻¹ to Beijing. Estimated uncertainties for SO₂ influx were approximately 31%.