Comparison of aerosol properties from the Indian Himalayas and the Indo-Gangetic plains

Abstract. Gual Pahari is a polluted semi-urban background measurement site at the Indo-Gangetic plains close to New Delhi and Mukteshwar is a relatively clean background measurement site at the foothills of the Himalayas about 270 km NE from Gual Pahari and about 2 km above the nearby plains. Two years long data sets including aerosol and meteorological parameters as well as modeled backward trajectories and boundary layer heights were compared. The purpose was to see how aerosol concentrations vary between clean and polluted sites not very far from each other. Specifically, we were exploring the effect of boundary layer evolution on aerosol concentrations. The measurements showed that especially during the coldest winter months, aerosol concentrations are significantly lower in Mukteshwar. On the other hand, the difference is smaller and also the concentration trends are quite similar from April to October. With the exception of the monsoon season, when rains are affecting on aerosol concentrations, clear but practically opposite diurnal cycles are observed. When the lowest daily aerosol concentrations are seen during afternoon hours in Gual Pahari, there is a peak in Mukteshwar aerosol concentrations. In addition to local sources and long-range transport of dust, boundary layer dynamics can explain the observed differences and similarities. When mixing of air masses is limited during the relatively cool winter months, aerosol pollutions are accumulated to the plains, but Mukteshwar is above the pollution layer. When mixing increases in the spring, aerosol concentrations are increased in Mukteshwar and decreased in Gual Pahari. The effect of mixing is also clear in the diurnal concentration cycles. When daytime mixing decreases aerosol concentrations in Gual Pahari, those are increased in Mukteshwar.