School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India.
Centre for Atmospheric Sciences, IIT Delhi, New Delhi 110016, India.
Sci Total Environ. 2016 Apr 15;550:994-1000. doi: 10.1016/j.scitotenv.2016.01.191. Epub 2016 Feb 6.
Changes in aerosol characteristics (spectral aerosol optical depth, AOD and composition) are examined during the transition from 'relatively clean' to 'extreme' aerosol days in the summer of 2012 at Delhi National Capital Region (NCR), India. AOD smaller than 0.54 (i.e. 12-year mean AOD-1σ) represents 'relatively clean' days in Delhi during the summer. 'Extreme' days are defined by the condition when AOD0.5 exceeds 12-year mean AOD+1 standard deviation (σ). Mean (±1σ) AOD increases to 1.2±0.12 along with a decrease of Angstrom Exponent from 0.54±0.09 to 0.22±0.12 during the 'extreme' days. Aerosol composition is inferred by fixing the number concentrations of various individual species through iterative tweaking when simulated (following Mie theory) AOD spectrum matches with the measured one. Contribution of coarse mode dust to aerosol mass increased from 76.8% (relatively clean) to 96.8% (extreme events), while the corresponding contributions to AOD0.5 increased from 35.0% to 70.8%. Spectrally increasing single scattering albedo (SSA) and CALIPSO aerosol sub-type information support the dominant presence of dust during the 'extreme' aerosol days. Aerosol direct radiative forcing (ADRF) at the top-of-the-atmosphere increases from 21.2Wm(-2) (relatively clean) to 56.6Wm(-2) (extreme), while the corresponding change in surface ADRF is from -99.5Wm(-2) to -153.5Wm(-2). Coarse mode dust contributes 60.3% of the observed surface ADRF during the 'extreme' days. On the contrary, 0.4% mass fraction of black carbon (BC) translates into 13.1% contribution to AOD0.5 and 33.5% to surface ADRF during the 'extreme' days. The atmospheric heating rate increased by 75.1% from 1.7K/day to 2.96K/day during the 'extreme' days.
在 2012 年夏季，印度德里国家首都地区（NCR）经历了从“相对清洁”到“极端”气溶胶日的转变，研究了气溶胶特性（谱气溶胶光学深度、AOD 和成分）的变化。AOD 小于 0.54（即 12 年平均 AOD-1σ）表示德里夏季的“相对清洁”日。“极端”日的定义是 AOD0.5 超过 12 年平均 AOD+1 标准差（σ）的条件。在“极端”日期间，平均（±1σ）AOD 增加到 1.2±0.12，同时 Angstrom 指数从 0.54±0.09 减小到 0.22±0.12。通过迭代调整各种单个物种的数浓度来推断气溶胶成分，当模拟（根据 Mie 理论）的 AOD 光谱与测量的光谱匹配时。粗模态尘埃对气溶胶质量的贡献从 76.8%（相对清洁）增加到 96.8%（极端事件），而对 AOD0.5 的相应贡献从 35.0%增加到 70.8%。谱增加的单次散射反照率（SSA）和 CALIPSO 气溶胶亚型信息支持在“极端”气溶胶日期间尘埃的主要存在。在大气层顶部的气溶胶直接辐射强迫（ADRF）从 21.2Wm(-2)（相对清洁）增加到 56.6Wm(-2)（极端），而相应的表面 ADRF 的变化从-99.5Wm(-2)到-153.5Wm(-2)。在“极端”日期间，粗模态尘埃对观测到的表面 ADRF 的贡献为 60.3%。相反，黑碳（BC）的 0.4%质量分数转化为 AOD0.5 的 13.1%贡献和“极端”日期间表面 ADRF 的 33.5%贡献。在“极端”日期间，大气加热率从 1.7K/天增加到 2.96K/天，增加了 75.1%。
