Long-Term Trends in Visibility and at Chengdu, China

Long-term (1973 to 2010) trends in visibility at Chengdu, China were investigated using meteorological data from the U.S. National Climatic Data Center. The visual range exhibited a declining trend before 1982, a slight increase between 1983 and 1995, a sharp decrease between 1996 and 2005, and some improvements after 2006. The trends in visibility were generally consistent with the economic development and implementation of pollution controls in China. Intensive PM2.5 measurements were conducted from 2009 to 2010 to determine the causes of visibility degradation. An analysis based on a modification of the IMPROVE approach indicated that PM2.5 ammonium bisulfate contributed 27.7% to the light extinction coefficient (bext); this was followed by organic mass (21.7%), moisture (20.6%), and ammonium nitrate (16.3%). Contributions from elemental carbon (9.4%) and soil dust (4.3%) were relatively minor. Anthropogenic aerosol components (sulfate, nitrate, and elemental carbon) and moisture at the surface also were important determinants of the aerosol optical depth (AOD) at 550 nm, and the spatial distributions of both bext and AOD were strongly affected by regional topography. A Positive Matrix Factorization receptor model suggested that coal combustion was the largest contributor to PM2.5 mass (42.3%) and the dry-air light-scattering coefficient (47.7%); this was followed by vehicular emissions (23.4% and 20.5%, respectively), industrial emissions (14.9% and 18.8%), biomass burning (12.8% and 11.9%), and fugitive dust (6.6% and 1.1%). Our observations provide a scientific basis for improving visibility in this area.

本研究采用美国国家气候数据中心（U.S. National Climatic Data Center）的气象资料，探究了中国成都1973年至2010年的能见度长期变化趋势。研究时段内，能见度整体呈现以下特征：1982年前呈下降趋势，1983—1995年间小幅回升，1996—2005年间显著下降，2006年后则有所改善。上述能见度变化趋势与中国的经济发展进程及污染管控措施实施情况总体契合。为探明能见度恶化的成因，研究团队于2009—2010年间开展了高密度细颗粒物（PM2.5）观测。基于改进型IMPROVE法的分析结果显示，PM2.5中的硫酸氢铵对消光系数（bext）的贡献率达27.7%，其次分别为有机颗粒物（21.7%）、地表湿度（20.6%）以及硝酸铵（16.3%）；元素碳与土壤扬尘的贡献率相对偏低，分别为9.4%与4.3%。人为源气溶胶组分（硫酸盐、硝酸盐与元素碳）以及地表湿度同样是550 nm处气溶胶光学厚度（AOD）的关键影响因子；同时，消光系数与气溶胶光学厚度的空间分布均受到区域地形的显著调控。本研究采用正矩阵因子分解（Positive Matrix Factorization）受体模型分析显示，燃煤是PM2.5质量浓度（42.3%）与干空气光散射系数（47.7%）的最大贡献源；其次分别为机动车尾气排放（占比分别为23.4%与20.5%）、工业排放（14.9%与18.8%）、生物质燃烧（12.8%与11.9%）以及扬尘（6.6%与1.1%）。本研究观测结果可为该区域能见度的改善提供科学依据。