Atmospheric Deposition of Reactive Nitrogen in India

Abstract

The increasing demand of food and energy of the global population is contributing excess reactive nitrogen (N_r) in the atmosphere primarily in the form of ammonium and nitrate compounds. Subsequently, through wet and dry deposition processes, these compounds are deposited onto the ground, vegetation, soils, water bodies etc. enriching these systems with excess nitrogen (N). Knowledge about N deposition in North America and Europe is quite advanced because of systematic studies. But the present knowledge about N deposition in India and the south Asian region is very limited due to a lack of systematic measurements dedicated to nitrogenous compounds. Though a number of wet deposition studies have been reported by various groups for different sites and years in India, only a few of these are considered as having good quality data. This chapter reports quality controlled wet deposition fluxes of N_r at rural (2006–2008) and urban (2005–2008) sites in south India as part of the RAPIDC-CAD programme. The rural site Hudegadde is located in the reserve forest area of Western Ghats by the south-west coast of India while the urban site Hyderabad, the capital of Andhra Pradesh State is located in south-central India. In general, at both the sites, wet deposition of N through NH₄ ⁺ was observed to be higher than NO₃ ⁻. Fluxes of NH₄ ⁺ showed an increasing pattern at Hudegadde, while those of NO₃ ⁻ showed an increasing pattern at Hyderabad. The possible reason for increasing NH₄ ⁺  at Hudegadde may be an increase in biomass burning and vegetation decay in the forest areas, which contribute higher ammonia to the atmosphere, together with transboundary pollution due to air-masses from nearby continental areas. The increasing pattern of NO₃ ⁻ fluxes at Hyderabad might be due to an increase in population, vehicular density and other urban activities. Projections using the MATCH model coupled with rain chemistry measurements showed that the Indo-Gangetic region experiences very high wet deposition of NH₄ ⁺ which might be due to a prevailing higher density of ammonia sources in the region. This chapter also highlights the importance of dry deposition of N_r species for the Indian region.