Quantifying the contribution of long-range transport to Particulate Matter (PM) mass loadings at a suburban site in the North-Western Indo Gangetic Plain (IGP)

Many sites in the densely populated Indo Gangetic Plain (IGP) frequently exceed the national ambient air quality standard (NAAQS) of 100 μg m−3 for 24 h average PM10 and 60 μg m−3 for 24 h average PM2.5 mass loadings, exposing residents to hazardous levels of PM throughout the year. We quantify the contribution of long range transport to elevated PM levels and the number of exceedance events through a back trajectory climatology analysis of air masses arriving at the IISER Mohali Atmospheric Chemistry facility (30.667° N, 76.729° E; 310 m a.m.s.l.) for the period August 2011–June 2013. Air masses arriving at the receptor site were classified into 6 clusters, which represent synoptic scale air mass transport patterns and the average PM mass loadings and number of exceedance events associated with each air mass type were quantified for each season. Long range transport from the west leads to significant enhancements in the average coarse mode PM mass loadings during all seasons. The contribution of long range transport from the west and south west (Source region: Arabia, Thar desert, Middle East and Afghanistan) to coarse mode PM varied between 9 and 57% of the total PM10−2.5 mass. Local pollution episodes (wind speed < 1 m s−1) contributed to enhanced coarse mode PM only during winter season. South easterly air masses (Source region: Eastern IGP) were associated with significantly lower coarse mode PM mass loadings during all seasons. For fine mode PM too, transport from the west usually leads to increased mass loadings during all seasons. Local pollution episodes contributed to enhanced PM2.5 mass loadings during winter and summer season. South easterly air masses were associated with significantly lower PM2.5 mass loadings during all seasons. Using simultaneously measured gas phase tracers we demonstrate that most PM2.5 originated from combustion sources. The fraction of days in each season during which the PM mass loadings exceeded the national ambient air quality standard was controlled by long range transport to a much lesser degree. For the local cluster, which represents regional air masses (Source region: NW-IGP), the fraction of days during which the national ambient air quality standard (NAAQS) of 60 μg m−3 for 24 h average PM2.5 was exceeded, varied between 22% of the days associated with this synoptic scale transport during monsoon season and 85% of the days associated with this synoptic scale transport during winter season; the fraction of days during which the national ambient air quality standard (NAAQS) of 100 μg m−3 for the 24 h average PM10 was exceeded, varied between 37% during monsoon season and 84% during winter season. Long range transport was responsible for both, bringing air masses with a significantly lower fraction of exceedance days from the Eastern IGP and air masses with a moderate increase in the fraction of exceedance days from the West (Source region: Arabia, Thar desert, Middle East and Afghanistan). In order to bring PM mass loadings in compliance with the national ambient air quality standard (NAAQS) and reduce the number of exceedance days, mitigation of regional pollution sources in the NW-IGP needs to be given highest priority.