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Many sites in the densely populated Indo-Gangetic Plain (IGP) frequentlyexceed the national ambient air quality standard (NAAQS) of100 μg m−3 for 24 h average PM10 and60 μg m−3 for 24 h average PM2.5mass loadings, exposing residents to hazardous levels of particulate matter (PM) throughoutthe year. We quantify the contribution of long-range transport to elevated PMlevels and the number of exceedance events through a back-trajectoryclimatology analysis of air masses arriving at the IISER MohaliAtmospheric Chemistry facility (30.667° N,76.729° E; 310 m a.m.s.l.) for the periodAugust 2011–June 2013. Air masses arriving at the receptor site wereclassified into six clusters, which represent synoptic-scale air-masstransport patterns. Long-range transport from the west leads to significant enhancementsin the average fine- and coarse-mode PM mass loadings during all seasons. Thecontribution of long-range transport from the west and south-west(source regions: Arabia, Thar Desert, Middle East and Afghanistan) tocoarse-mode PM varied between 9 and 57 % of the totalPM10–2.5 mass. Local pollution episodes (wind speed< 1 m s−1) contributed to enhanced PM2.5 massloadings during both the winter and summer seasons and to enhancedcoarse-mode PM onlyduring the winter season. South-easterly air masses (source region:eastern IGP) were associated with significantly lower fine- and coarse-mode PMmass loadings during all seasons. The fraction of days in each season during which the PM mass loadingsexceeded the national ambient air quality standard was controlled bylong-range transport to a much lesser degree. For the local cluster, which represents regional air masses (sourceregion: NW-IGP), the fraction of days during which the nationalambient air quality standard (NAAQS) of 60 μg m−3 for24 h average PM2.5 was exceeded varied between36 % of the days associated with this synoptic-scaletransport during the monsoon, and 95 % during post-monsoon and winter seasons;the fraction of days during which the NAAQS of 100 μg m−3 for the 24 haverage PM10 was exceeded, varied between 48 %during the monsoon and 98 % during the post-monsoon season. Long-range transport was responsible for both, bringing air masseswith a significantly lower fraction of exceedance days from theeastern IGP and air masses with a moderate increase in the fraction ofexceedance days from the west (source regions: Arabia, Thar Desert,Middle East and Afghanistan). In order to bring PM mass loadings into compliance with the NAAQS and to reduce the number ofexceedance days, mitigation of regional combustion sources in theNW-IGP needs to be given highest priority

atmospheric chemistry and physics

Quantifying the contribution of long-range transport to particulate matter (PM) mass loadings at a suburban site in the north-western Indo-Gangetic Plain (NW-IGP)