Assessment of carbonaceous aerosols in Shanghai, China – Part 1: long-term evolution, seasonal variations, and meteorological effects

Abstract

Carbonaceous aerosols are major chemical components of fine particulate matter (PM2. 5) with major impacts on air quality, climate change, and human health. Gateway to fast-rising China and home of over twenty million people, Shanghai throbs as the nation's largest mega city and the biggest industrial hub. From July 2010 to December 2014, hourly mass concentrations of ambient organic carbon (OC) and elemental carbon (EC) in the PM2. 5 fraction were quasi-continuously measured in Shanghai's urban center. The annual OC and EC concentrations (mean ±1σ) in 2013 (8.9 ± 6.2 and 2.6 ± 2.1 µg m−3, n = 5547) and 2014 (7.8 ± 4.6 and 2.1 ± 1.6 µg m−3, n = 6914) were higher than those of 2011 (6.3 ± 4.2 and 2.4 ± 1.8 µg m−3, n = 8039) and 2012 (5.7 ± 3.8 and 2.0 ± 1.6 µg m−3, n = 4459). We integrated the results from historical field measurements (1999–2012) and satellite observations (2003–2013), concluding that carbonaceous aerosol pollution in Shanghai has gradually reduced since 2006. In terms of monthly variations, average OC and EC concentrations ranged from 4.0 to 15.5 and from 1.4 to 4.7 µg m−3, accounting for 13.2–24.6 and 3.9–6.6 % of the seasonal PM2. 5 mass (38.8–94.1 µg m−3), respectively. The concentrations of EC (2.4, 2.0, 2.2, and 3.0 µg m−3 in spring, summer, fall, and winter, respectively) showed little seasonal variation (except in winter) and weekend–weekday dependence, indicating EC is a relatively stable constituent of PM2. 5 in the Shanghai urban atmosphere. In contrast to OC (7.3, 6.8, 6.7, and 8.1 µg m−3 in spring, summer, fall, and winter, respectively), EC showed marked diurnal cycles and correlated strongly with CO across all seasons, confirming vehicular emissions as the dominant source of EC at the targeted site. Our data also reveal that both OC and EC showed concentration gradients as a function of wind direction (WD) and wind speed (WS), generally with higher values associated with winds from the southwest, west, and northwest. This was consistent with their higher potential as source areas, as determined by the potential source contribution function (PSCF) analysis. A common high-potential source area, located along the middle and lower reaches of the Yangtze River instead of northern China, was pinpointed during all seasons. These results demonstrate that the measured carbonaceous aerosols were driven by the interplay of local emissions and regional transport.