SO₂ measurements at a high altitude site in the central Himalayas: Role of regional transport

Abstract

Continuous measurements of a climatically important acidic gas, SO2, were made over Nainital (29.37ºN, 79.45ºE; 1958 m amsl), a regionally representative site in the central Himalayas, for the first time during 2009-2011. Unlike many other sites, the SO₂ levels over Nainital are higher during pre-monsoon (345 pptv) compared to winter (71 pptv). High values during pre-monsoon are attributed to the transport of air masses from regions viz. Indo-Gangetic Plain (IGP), northern India and north-East Pakistan, which are dotted with numerous industries and power plants, where coal burning occurs. Transport from the polluted regions is evinced from good correlations of SO₂ with wind speed, NOy and UV aerosol index during these periods. Daytime elevations in SO₂ levels, influenced by ‘valley winds’ and boundary layer evolution, is a persistent feature at Nainital. SO₂ levels are very much lower during monsoon compared to pre-monsoon, due to oxidation losses and wet scavenging. Despite this, SO₂/NOy slopes are high (>0.4) both during pre-monsoon and monsoon, indicating impacts of point sources. The SO₂ levels during winter are lower as the measurement site is cut off from the plains due to boundary layer dynamics. Further, the SO₂ levels during winter nights are the lowest (lesser than 50 pptv) and resemble free tropospheric conditions.