Optical, Radiative and Chemical Characteristics of Aerosol in Changsha City, Central China

Abstract

Industrial pollution has a significant effect on aerosol properties in Changsha City, a typical city of central China. Therefore, year-round measurements of aerosol optical, radiative and chemical properties from 2012 to 2014 at an urban site in Changsha were analyzed. During the observation period, the energy structure was continuously optimized, which was characterized by the reduction of coal combustion. The aerosol properties have obvious seasonal variations. The seasonal average aerosol optical depth (AOD) at 500 nm ranged from 0.49 to 1.00, single scattering albedo (SSA) ranged from 0.93 to 0.97, and aerosol radiative forcing at the top of the atmosphere (TOA) ranged from −24.0 to 3.8 W m⁻². The chemical components also showed seasonal variations. Meanwhile, the scattering aerosol, such as organic carbon, SO₄²⁻, NO₃⁻, and NH₄⁺ showed a decrease, and elemental carbon increased. Compared with observation in winter 2012, AOD and TOA decreased by 0.14 and −1.49 W m⁻² in winter 2014. The scattering components, SO₄²⁻, NO₃⁻ and NH₄⁺, decreased by 12.8 µg m⁻³ (56.8%), 9.2 µg m⁻³ (48.8%) and 6.4 µg m⁻³ (45.2%), respectively. The atmospheric visibility and pollution diffusion conditions improved. The extinction and radiative forcing of aerosol were significantly controlled by the scattering aerosol. The results indicate that Changsha is an industrial city with strong scattering aerosol. The energy structure optimization had a marked effect on controlling pollution, especially in winter (strong scattering aerosol).

摘要

工业污染对中国中部典型城市长沙市的气溶胶特性具有很大的影响。2012−14年观测表明, 长沙的气溶胶特性具有明显的季节性变化。冬季各参数明显高于其他季节, 夏季表现出低值。500 nm波段气溶胶光学厚度(AOD)冬季均值为0.90, 夏季为0.75, 大气层顶气溶胶辐射强迫(TOA)冬季的冷却效应比夏季强−18.3 W m⁻², PM_2.5冬季比夏季高43.5 μg m⁻³。化学成分也表现出相应的季节性变化。PM_2.5中有机碳(OC)、SO₄²⁻、NO₃⁻和NH₄⁺散射性气溶胶冬季均值分别比夏季高9.1 μg m⁻³、6.1 μg m⁻³、15.8 μg m⁻³、8.9 μg m⁻³。不同来向气团气溶胶特性也有较大差异。来源于南方湿润气团的AOD较小(0.54−0.64), PM_2.5浓度也较低(32.0−36.1 μg m⁻³), 对应的TOA呈现加热效应(4.3−4.5 W m⁻²)。来源于西北方的干燥气团带来粗模态粒子, PM_2.5浓度仍相对较低(43.6−79.4 μg m⁻³), 对应的AOD与TOA分别为0.65−0.84和−16.8至−2.46 W m⁻², 表现为弱冷却效应。局地传输气团气溶胶污染最严重, AOD、TOA 和PM2.5可达1.56、−52.9 W m⁻²和114 μg m⁻³。随着国家大气污染防治行动计划执行, 2012−14年该地区能源结构不断优化, 燃煤比重显著减小。OC、SO₄²⁻、NO₃⁻和NH₄⁺等散射性气溶胶减少。与2012年冬季的观测值相比, 2014年冬季的AOD和TOA分别下降了0.14和−1.49 W m⁻²。SO₄²⁻, NO₃⁻和NH₄⁺这三种散射性成分分别下降了12.8 μg m⁻³(56.8%), 9.2 μg m⁻³(48.8%)和6.4 μg m⁻³(45.2%)。该地区气溶胶的消光和辐射强迫主要受散射性气溶胶的控制, 随着能源结构调整, 散射性气溶胶大幅度减少, 大气能见度和污染扩散条件得到了改善。