Continental export efficiencies and delineation of sources for trace gases and black carbon in North-East India: Seasonal variability

• Export efficiency of BC, NOx and SO2 from the IGP to NER are estimated.
• BC and NOx export efficiency is highest in winter and lowest in monsoon.
• BC, CO and NOx annual variability is affected by transport from IGP.
• Pollutants from North-East India outflows to southeast China and East-Asia.

The Indo Gangetic Plain (IGP) has been identified from back-trajectory analyses, as one of the most potential region affecting the species transport to the Northeastern region of India (NER). The continental export efficiency (εε) of BC, NOx and SO2 within the boundary layer is estimated in order to examine how efficiently these chemical species are transported towards the NER. For this the measurements carried out at Dibrugarh, a wet tropical location in NER during 2012–2013 have been used as the references in the estimation of the species enhancements above their background. CO is used as a passive tracer of transport due to its longer lifetime in the atmosphere. The emission estimates of BC, NOx, SO2 and CO in the IGP region are adopted from the emission inventories REAS and INTEX-B. The estimated export efficiency is highest in winter (DJF) for BC and NOx, whereas SO2 shows maximum efficiency in monsoon (JJAS). BC due to efficient transportation/removal from the IGP region exhibits highest εε values compared to the other species. NOx and SO2 on the other hand get transformed to other chemical species shortly after emission into the atmosphere and hence are less efficiently transported towards the study region. The export of BC, CO, NOx and SO2 are expected to supplement the chemical atmosphere in NER, which is further studied through the annual variability in their distribution in Dibrugarh. Pearson correlation analyses of BC, NOx and SO2 with CO is carried out to examine the similarity or dissimilarity among the sources.