Abstract

Aerosol emissions in South Asia are large. The emitted aerosols can travel significant distances and, during the Asian southwest monsoon especially, are prone to modification through cloud processing and wet scavenging while being transported. The scale of emissions and transport means that the global climate impact of these aerosols are sensitive to modification en route, but the process-level understanding is still largely lacking. In this study, we analyse long-term aerosol data measured at an observatory established in Hanimaadhoo, Republic of Maldives, to investigate the long-term properties of aerosols over the Indian Ocean as well as to understand the effect of precipitation on the aerosol particle size distribution during long-range transport. The observatory location is ideal because it is a receptor site with little local influence, and, depending on the season, receives either polluted air masses coming from the Indian subcontinent or clean marine air masses from the Indian Ocean. We analysed the sub-micron particle number size distribution measured during the years 2004–2008, and 2014–2017, and this is the first inter-seasonal long-term study of the sub-micron aerosol features in the region. The aerosol origin and its relative exposure to wet scavenging during long-range transport were analysed using back-trajectory analysis from HYSPLIT. By comparing aerosol measurements to precipitation along its transport, this study shows that there is a substantial change in particle number size distributions and concentrations depending on the amount of rainfall during transport. During the southwest monsoon season, the aerosol size distribution was notably bi-modal and total particle concentrations clearly reduced in comparison with the prevailing aerosol size distribution during the northeast monsoon season. Precipitation during transport usually corresponded with a greater reduction in accumulation mode concentrations than for smaller sizes, and the shape of the median size distribution showed a clear dependence on the trajectory origin and route taken.

摘要

南亚的气溶胶排放量很大。所散发的气溶胶可以传播很远的距离，尤其是在亚洲西南季风期间，在运输过程中容易通过云处理和湿清除而发生变化。排放和运输的规模意味着这些气溶胶对全球气候的影响对途中的修改很敏感，但仍缺乏对过程水平的了解。在这项研究中，我们分析了在马尔代夫共和国哈尼马杜建立的观测站测得的长期气溶胶数据，以调查印度洋上气溶胶的长期特性，并了解降水对气溶胶粒径的影响。远程运输过程中的分布。理想的观测站位置是因为它是一个受局部影响很小的受体位置，并且 根据季节的不同，接收来自印度次大陆的受污染的气团或接收来自印度洋的清洁的海洋气团。我们分析了2004-2008年和2014-2017年期间测得的亚微米颗粒数大小分布，这是该地区亚微米气溶胶特征的第一个季节间长期研究。使用HYSPLIT的反向轨迹分析法分析了远程运输过程中气溶胶的起源及其相对于湿清除的相对暴露。通过将气溶胶测量结果与沿运输过程的降水进行比较，这项研究表明，颗粒大小分布和浓度存在很大的变化，这取决于运输过程中的降雨量。在西南季风季节，与东北季风季节的主要气溶胶粒径分布相比，气溶胶粒径分布明显呈双峰态，总颗粒浓度明显降低。运输过程中的降水通常比较小的体积对应更大的积累模式浓度降低，并且中值尺寸分布的形状显示出对轨迹起源和所采用路线的明显依赖。