Assessment of carbonaceous aerosols in Shanghai, China: Long-term evolution, seasonal variations and meteorological effects
Authors: Chang Yunhua , Deng Congrui , Cao Fang , Cao Chang , Zou Zhong , Liu Shoudong , Lee Xuhui , Li Jun , Zhang Gan , Zhang Yanlin 
Published: April 2017

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. In a continuing effort to pursue economic growth, haze-plagued Shanghai also spearheaded China’s air-cleaning moves through a portfolio of measures such as greater adoption of renewable energy and raising standards for vehicle emissions since 2000s. This article presents a detailed analysis of the high temporal resolution and long-term measurement of carbonaceous aerosols in Shanghai, allowing the assessment of the effectiveness of policy in terms of reducing the health and environmental impacts of particulate matter. 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 that 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). After compiling historical (1999–2012) filter-based measurements of carbonaceous aerosols and satellite-based observations of Aerosol Optical Depth (AOD; 2003–2013), we infer that our on-line monitoring data set may not necessarily reflect an increasing trend of carbonaceous aerosol loading in Shanghai but is more likely due to the short-term strong influence of the unprecedented severe haze which occurred during winter 2013. Although we failed to quantify the decrease of carbonaceous aerosols, the longest time-series changes of SO2 concentrations (2000–2015) and emissions (2000–2013) data presented in this study provide compelling evidence that coal-related emissions in Shanghai had been effectively controlled since 2006 after replacement of coal with cleaner energy (e.g., natural gas) as a fuel source. This was confirmed to co-benefit the reduction of carbonaceous aerosols emissions. Over the duration of the campaign, monthly 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, 3.0 µg m−3 in spring, summer, fall, and winter, respectively) showed little seasonal variation (excepting winter) and weekend-weekday dependence, indicating EC are a relatively stable constitute 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 high concentrations of OC are usually associated with high temperatures (T > 30 ℃) that favor the formation of secondary organic aerosols, and there is a clear pattern of RH/T dependence for EC. Moreover, EC show more evident concentration gradients as a function of wind speed, reflecting the local nature of EC. Conversely, OC frequently peak under high wind speed conditions, highlighting the importance of regional transport contributing to OC. The bivariate (wind speed and wind direction) polar plots demonstrate that both OC and EC generally have higher values associated with winds from the southwest and west. This was consistent with their higher potential as source areas, as determined by the potential source contribution function 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. Besides, the declining carbonaceous aerosols in Shanghai during the last decade demonstrate the effectiveness of adopting clean energy resources and reducing vehicle emissions in China.