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Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

机译:2009年季风前季节期间,尼泊尔喜马拉雅山中山脉的化学成分和气溶胶粒径分布

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摘要

Aerosol particle number size distribution and chemical composition weremeasured at two low altitude sites, one urban and one relatively pristinevalley, in Central Nepal during the 2009 pre-monsoon season (May–June). Thisis the first time that aerosol size distribution and chemical compositionwere measured simultaneously at lower elevations in the middle Himalayanregion in Nepal. The aerosol size distribution was measured using a ScanningMobility Particle Sizer (SMPS, 14–340 nm), and the chemicalcomposition of the filter samples collected during the field campaign wasanalyzed in the laboratory. Teflon membrane filters were used for ionchromatography (IC) and water-soluble organic carbon and nitrogen analysis.Quartz fiber filters were used for organic carbon and elemental carbonanalysis. Multi-lognormal fits to the measured aerosol size distributionindicated a consistent larger mode around 100 nm which is usually the oldest,most processed background aerosol. The smaller mode was located around 20 nm,which is indicative of fresh but not necessarily local aerosol. The diurnalcycle of the aerosol number concentration showed the presence of two peaks(early morning and evening), during the transitional periods of boundarylayer growth and collapse. The increase in number concentration during thepeak periods was observed for the entire size distribution. Although thepossible contribution of local emissions in size ranges similar to thelarger mode cannot be completely ruled out, another plausible explanation isthe mixing of aged elevated aerosol in the residual layer during the morningperiod as suggested by previous studies. Similarly, the evening timeconcentration peaks when the boundary layer becomes shallow concurrent withincrease in local activity. A decrease in aerosol number concentration wasobserved during the nighttime with the development of cold (downslope)mountain winds that force the low level warmer air in the valley to rise.The mountain valley wind mechanisms induced by the topography along with thevalley geometry appear to have a strong control in the diurnal cycle of theaerosol size distribution. During the sampling period, the chemicalcomposition of PM was dominated by organic matter at both sites. Organiccarbon (OC) comprised the major fraction (64–68%) of the aerosolconcentration followed by ionic species (24–26%, mainly SO and NH). Elemental Carbon (EC) compromised 7–10% of thetotal composition and 27% of OC was found to be water soluble at bothsites. The day-to-day variability observed in the time series of aerosolcomposition could be explained by the synoptic scale haze that extended tothe sampling region from the Indian Gangetic Plain (IGP), and rainfalloccurrence. In the presence of regional scale haze during dry periods, themean volume aerosol concentration was found to increase and so did theaerosol mass concentrations.
机译:在2009年季风前季节(5月至6月),在尼泊尔中部的两个低海拔站点(一个城市和一个相对原始的谷地)测量了气溶胶颗粒的粒径分布和化学成分。这是首次在尼泊尔喜马拉雅中部中部较低高度同时测量气溶胶粒径分布和化学成分。使用ScanningMobility粒度仪(SMPS,14–340 nm)测量了气溶胶的大小分布,并在实验室中分析了野战期间收集的过滤器样品的化学成分。聚四氟乙烯膜过滤器用于离子色谱(IC)和水溶性有机碳和氮分析,石英纤维滤器用于有机碳和元素碳分析。多对数正态拟合与测得的气溶胶尺寸分布相吻合,表明在约100 nm处有一个较大的一致模式,该模式通常是最古老,处理最多的背景气溶胶。较小的模式位于20 nm左右,这表示新鲜但不一定是局部气溶胶。在边界层生长和塌陷的过渡时期,气溶胶浓度浓度的昼夜周期显示存在两个峰值(清晨和傍晚)。对于整个尺寸分布,在高峰期观察到数字浓度的增加。尽管不能完全排除局部排放在较大范围内相似的可能贡献,但另一个合理的解释是,早先的研究表明,在早晨期间,残留层中的老化气溶胶会混合在一起。同样,当边界层在局部活动增加的同时变浅并发时,晚上的时间浓度达到峰值。夜间观测到的气溶胶浓度浓度下降,这是由于寒冷(下坡)山风的形成,迫使山谷中的低层暖空气上升。地形和山谷的几何形状所引起的山谷风机制似乎具有强烈控制气溶胶粒径分布的昼夜周期。在采样期间,两个站点的PM的化学成分均以有机物为主。有机碳(OC)占气溶胶浓度的主要部分(64–68%),其次是离子种类(24–26%,主要是SO和NH)。元素碳(EC)损害了总成分的7-10%,并且发现27%的OC在两个站点上都是水溶性的。气溶胶组成时间序列中观察到的日常变化可以用天气尺度的薄雾来解释,该天气的薄雾从印度恒河平原(IGP)延伸到采样区域,并且出现降雨。在干旱期间存在区域规模的雾霾时,发现单质体体积气溶胶浓度增加,而气溶胶质量浓度也增加。

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