In winter, the surface temperature in the plains plummets, and the air is dry. The southern edge of the Tibetan Plateau at upper levels is also battered by the sub-tropical jetstream, with weak westerly wind below it. The low surface temperature in the plains confines the pollution to a shallow boundary layer that is lower than the Siwalik Range. Up-valley winds along the river valleys then transport the haze into the Inner Himalaya.
The river valley-plains temperature contrast produces a pressure gradient that drives this flow up the valley during the day and down the valley during the night. Besides, the topography, vegetation cover and groundwater level in the slopes can also contribute to warming of valley air which could further amplify the up-valley wind.
In addition, the daytime up-slope and night-time down-slope winds developing in the river valleys also play and important role in ventilating the air pollution from the valley surface.
Measurements of the nature and size of fine aerosols were conducted for the first time in the middle Himalayan region in the pre-monsoon. In general, the aerosol size distribution peaks around 100nm, which is usually the oldest, most processed background aerosol and about the size of the Covid-19 virus.
Another smaller peak around 20 nm, is indicative of fresh but not necessarily local aerosols. The chemical composition of PM2.5 (particulate matter below 2.5 microns) is dominated by organic matter. Organic carbon (OC) comprises the major fraction (64–68%) of the aerosol concentration followed by ionic species (24–26%), Elemental Carbon (EC) compromises 7–10% of the total composition and 27% of OC is water soluble.
Elemental Carbon and Organic Carbon together are aerosols contributed by burning of wood fuels, crop-residue, coal including vehicular emission and others. The light absorbing properties of elemental carbon play a crucial role in atmospheric warming and surface cooling due to their interaction with the daytime solar heating.
Industrial and vehicular emissions, biomass burning, soil dust and others including chemical transformations in the atmosphere contribute to the ionic species in aerosols. These are readily water-soluble parts of the aerosol which along with organic particles aid in their growth in presence of moisture, contributing to increase in optical thickness of haze.
The aerosol concentration also changes over the day. It increases in the morning (05:00-10:00) and in the evening (17:00-22:00). The lower values in the afternoon could be attributed partly to more mixing, and increase in the boundary layer height.
During the night, the downslope winds lift the warm air mass in the valley, resulting in the decrease of the aerosol concentration near the surface. On the other hand, aerosol loading can also be eventually washed away by local rainfall during the pre-monsoon and winter.