Black carbon aerosols over a high altitude station, Mahabaleshwar: Radiative forcing and source apportionment

Abstract Observations of Black Carbon (BC) aerosol concentrations were carried at Mahabaleshwar, a high altitude station in southwest India during March 2017–February 2019 using an Aethalometer (AE42) from Magee Scientific. BC concentration at Mahabaleshwar, varied from 0.016 μg/m3 to 16.03 μg/m3, with an average of 1.56 ± 0.88 μg/m3, indicating significant burden of these absorbing aerosols even at the free tropospheric altitudes (1348 m AMSL) as the station height remains above the Mixing Layer Height (MLH) throughout the year (MLH varies from 60 to 1293.6 m). BC is found to exhibit strong seasonal variation with the maximum concentration during winter and minimum during monsoon. The concentrations of BC, BC from fossil fuel (BCff), and BC from biomass burning (BCbb) exhibit diurnal variation with maximum values in the morning and evening hours and lower values around the afternoon due to the changes in the mixing layer height. The BCff and BCbb show the maximum value in winter (BCff: 1.47 μg/m3, BCbb: 0.69 μg/m3) which may be due to more biomass burning activity in the surrounding area along with the lower atmospheric temperatures and wind speeds which trap the pollution near the surface. However, in summer and post-monsoon the BCff remains around 1.0 μg/m3 and BCbb remains around 0.4 μg/m3. The lowest values of BCff (0.47 μg/m3) and BCbb (0.15 μg/m3) are exhibited in monsoon. Large negative radiative forcing was observed at the surface (SUF) and at top of atmosphere (TOA) whereas that at the atmosphere (ATM) was positive during all the seasons for both composite aerosol and without BC scenarios thus inferring to the significant cooling at SUF and warming at ATM leading to high atmospheric heating rates. Overall, the contribution of BC to the total atmospheric aerosol radiative forcing (ARF) was found to be 43% for the whole observational period. Temporal variations of MLH, BCff, and BCbb along with meteorological parameters and bivariate analysis of winds with respect to BC have been also discussed.