Assessment of PM2.5 and PM10 over Guwahati in Brahmaputra River Valley: Temporal evolution, source apportionment and meteorological dependence

Temporal evolution, source apportionment and transport pathways of particulate matter (PM2.5 and PM10) are analysed over Guwahati, located in the Brahmaputra River Valley (BRV), as a function of meteorological dynamics. During the study period (July 2013-June 2014), the mean PM2.5 and PM10 mass concentrations were found to be 52 ± 37 and 91 ± 60 μg m−3, respectively, both exhibiting higher concentrations during December-March and very low during summer. The annual mean ratio of PM2.5/PM10 was 0.57 ± 0.11, varying from 0.24 to 0.86, suggesting dominance of anthropogenic vs natural emissions during winter and spring, respectively. Diurnal variation reveals higher PM concentrations during morning (∼9:00 local time (LT)) and evening (∼23:00 LT) and lowest around ∼14:00 to 17:00 LT due to influence of dilution processes and higher mixing-layer height over the region. Bivariate plots and Conditional Bivariate Probability Function (CBPF) analysis showed that the highest PM2.5 and PM10 concentrations are mostly associated with weak northwestern winds (<1.5 ms−1) in all seasons except spring, when the highest PM10 are for southwestern winds above 4-6 ms−1, indicating dust transport from SW Asia. Analysis reveals that the local emissions, transported aerosols, along with seasonally-changed air masses, meteorology and boundary-layer dynamics control the concentrations, evolution and fractions of PM over BRV. The turbid air masses transported over Guwahati mostly from western and southwestern directions contribute to higher PM concentrations, either carrying anthropogenic pollution from Indo-Gangetic Plains or locally and LRT dust from BRV and western India, respectively.