Tropospheric NO2 variability over the metropolitan cities and hill stations of India

Monthly and inter-annual variation in tropospheric nitrogen dioxide (NO₂) have been examined over metropolitan cities (New Delhi, Kolkata, Mumbai and Chennai) and hill stations (Mount Abu, Nainital, Srinagar, Kodaikanal, Dalhousie, Gulmarg, Shimla and Munnar) of India during the period 2004?C2010 using satellite-based SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). It is observed that the monthly variation in NO₂ over the metropolitan cities is higher during winter (November?CDecember?CJanuary?CFebruary) months and lower during summer monsoon (June?CJuly?CAugust?CSeptember) months. Lower NO₂ in summer monsoon leads to the presence of deep convection and higher in winter leads to calm winds and more residential time of gases. Moreover, rapid industrialization and traffic growth are also responsible for the higher NO₂. Mean values of NO₂ over New Delhi and Mumbai as well as hill stations, such as Mount Abu, Nainital and Shimla, have exhibited more pollution. Similarly, maximum NO₂ occurred over the hill stations during pre-monsoon months (April?CMay) and early part of summer monsoon (June). Higher NO₂ values are observed in November?CDecember months. All the hill stations also show increasing trend of NO₂ during the period 2004?C2010. Increasing pollution of NO₂ over the hill stations might also be due to forest fires, biomass burning and long-range transport. Back trajectory analysis shows that the observed peaks in NO₂ are a resultant of the long-range transported component amplified by the local environment. In the northern hill stations, pollution seems transported from west Asian and European countries while in the southern hill stations, pollution is originated from southern Indian Ocean and East Asian countries.