Particulate Matter and Black Carbon in the Brahmaputra Valley of Northeast India: Observations and Model Simulation

Abstract

Brahmaputra Valley of northeast India is relatively pristine, with low local emission levels, but is susceptible to long-range transport from west Asian deserts. Hourly measurements of aerosol particulate matter (PM2.5 and PM10) and black carbon (BC) over a period of 1 year (June 2013–May 2014) over Tezpur station (26.63° N, 92.77° E), located in the Brahmaputra Valley, display annual mean PM2.5 and BC of 42 ± 25 µg m−3 and 6 ± 1.5 µg m−3, respectively, indicating that Tezpur enjoys lower pollution levels than other northeast stations. The lowest concentrations of PM2.5 and PM10 (~ 14.7 and 24 µg m−3, respectively) are observed during the monsoon season (JJAS). The highest values (~ 91 and 114 µg m−3) are observed during the winter (DJF) due to local atmospheric conditions and some additional transport from the west Asian deserts and the Indo-Gangetic Basin (IGB). Local emissions seem to be mainly responsible for moderate values (~ 24 and 42 µg m−3) of PM during the post-monsoon (ON) season with weak winds. Coarse particles (diameter > 2.5 µm) are much higher (approx. 47% of PM10) in the pre-monsoon than in the other seasons due to the long-range transport of dust particles from the IGB and west Asian deserts as well as sea salt particles from the Bay of Bengal. WRF-Chem model simulation underestimated the concentration of PM2.5 and PM10 as compared to daily mean observations. The variation in the “model–observation” time series for relative humidity (RH) and boundary layer height suggests that underestimation of PM10 is due mainly to the underestimation of RH and overestimation of the ventilation coefficient by the model during winter and pre-monsoon seasons.