Comparison of Black Carbon Aerosols in Urban and Suburban Areas of Shanghai

Black carbon (BC) aerosols influence climate by absorbing incoming solar radiation resulting in a warming effect with large uncertainties. To better understand the anthropogenic BC pollution and its transport in the region of Shanghai, the BC mass concentrations are measured at Pudong (the urban site) and Dongtan (the suburban wetland site), during a one-year period from December of 2007 to November of 2008. The diurnal and seasonal variations of BC mass concentration and the wind-concentration relationships at two sites are compared. According to the Aethalometers, the hourly BC mass concentration of Pudong is 1.2—7.6 μg·m^-3 (10%—90% percentiles), with an average value of 3.8 μg·m^-3, which is significantly higher than that of Dongtan (0.1—4.3 μg·m^-3, with an average of 1.7 μg·m^-3). The BC concentrations show similar variation patterns at both sites in seasonal scale, with the highest concentrations usually observed in winter and the lowest concentrations in summer. However, different diurnal patterns are usually observed at different sites. The average diurnal variation measured at the urban (Pudong) site revealed a dual-peak pattern, with peaks between 09:00 and 10:00 and between 20:00 and 21:00, while the minimum values generally appear in the afternoon and midnight. The morning peaking is attributed to enhanced traffic during rush hour, while the evening rush hour combined with the collapse of convective boundary layer result in the evening peak. Additionally, pronounced "weekend effect" is observed at the urban (Pudong) site, when the average BC concentration of non-workdays is about 13% lower than that of workdays. These results indicate that the BC concentration at Pudong site is mainly influenced by the local anthropogenic emissions. In contrast, neither significant diurnal variation nor significant "weekend effect" is observed at the suburban (Dongtan) site, indicating that the local anthropogenic emissions at Dongtan are relatively weaker. Surface wind direction and wind speed has substantial effects on the BC concentrations. Wind could either dilute BC aerosols or transport them from other places and contribute to the measured concentration, depending on different wind directions and speeds. Different characteristics of wind-concentration relationships are observed for urban and suburban areas in which the BC concentrations are mainly influenced by the local emissions and by the regional pollutant transport, respectively.