Airborne particulate in Varanasi over middle Indo-Gangetic Plain: variation in particulate types and meteorological influences

Abstract

The variation in particulate mass and particulate types (PM_2.5 and PM₁₀) with respect to local/regional meteorology was analyzed from January to December 2014 (n = 104) for an urban location over the middle Indo-Gangetic Plain (IGP). Both coarser (mean ± SD; PM₁₀ 161.3 ± 110.4 μg m^-3, n = 104) and finer particulates (PM_2.5 81.78 ± 66.4 μg m^-3) revealed enormous mass loading with distinct seasonal effects (range: PM₁₀ 12-535 μg m^-3; PM_2.5 8-362 μg m^-3). Further, 56% (for PM_2.5) to 81% (for PM₁₀) of monitoring events revealed non-attainment national air quality standard especially during winter months. Particulate types (in terms of PM_2.5/PM₁₀ 0.49 ± 0.19) also exhibited temporal variations with high PM_2.5 loading particularly during winter (0.62) compared to summer months (0.38). Local meteorology has clear distinguishing trends in terms of dry summer (March to June), wet winter (December to February), and monsoon (July to September). Among all the meteorological variables (average temperature, rainfall, relative humidity (RH), wind speed (WS)), temperature was found to be inversely related with particulate loading (r_PM10 -0.79; r_PM2.5 -0.87) while RH only resulted a significant association with PM_2.5 during summer (r_PM10 0.07; r_PM2.5 0.55) and with PM₁₀ during winter (r_PM10 0.53; r_PM2.5 0.24). Temperature, atmospheric boundary layer (ABL), and RH were cumulatively recognized as the dominant factors regulating particulate concentration as days with high particulate loading (PM_2.5 >150 μg m^-3; PM₁₀ >260 μg m^-3) appeared to have lower ABL (mean 660 m), minimum temperature (<22.6 °C), and high RH (∼79%). The diurnal variations of particulate ratio were mostly insignificant except minor increases during night having a high wintertime ratio (0.58 ± 0.07) over monsoon (0.34 ± 0.05) and summer (0.30 ± 0.07). Across the region, atmospheric visibility appeared to be inversely associated with particulate (r_PM2.5 -0.84; r_PM10 -0.79) for all humid conditions, while at RH ≥80%, RH appeared as the most dominant factor in regulating visibility compared to particulate loading. The Lagrangian particle dispersion model was further used to identify possible regions contributing particulate loading through regional/transboundary movement.

Keywords: Boundary layer; Humidity; Indo-Gangetic Plain; Particulate; Visibility.