Distribution of reactive trace gases over South Asia: Observations and modeling

Reactive trace gases (O<inf>3</inf>, CO, NO<inf>x</inf>, SO<inf>2</inf>, VOCs) play vital roles in air pollution and climate change. In this chapter, the distributions of trace gases and governing processes over the South Asian region are discussed. Primary trace gases show enhanced levels over Indo-Gangetic Plain (IGP) particularly during winter, whereas O<inf>3</inf> over IGP is highest in spring due to favorable meteorology and regional pollution including biomass burning emissions. Trace gases exhibit lower levels during monsoon mainly due to inflow of marine air, washout effect, and suppressed photochemistry. While the diurnal characteristics of O<inf>3</inf> are distinct and depend on local environments (urban, rural, clean), the seasonality is similar over subregions. O<inf>3</inf> pollution causes significant crop losses over South Asia, besides impacting human health. Models reproduce the observed variations of trace gases but have a bias in capturing magnitude. More detailed measurements and improvements in models are important future perspectives for the South Asian region.