Multi-year characterization of aerosol black carbon concentrations over a semiarid tropical site Udaipur

Continuous and multi-year (2008-2012) measurements of black carbon (BC) mass concentrations (M_BC), carried out from the semiarid tropical site Udaipur (24.6° N, 74° E, 580 m a.s.l.) near the western Indian desert, are analyzed for their region-specific features. M_BC varied over a wide range during the period of study, with the hourly mean values occurring as low as 0.09 to as high as 49.1 μg m^-3, with the multi-year average ~ 4.5 ± 2.6 μg m^-3. Annual variations showed the highest concentrations during November and December (winter seasonal mean = 7.4 ± 3.3 μg m^-3) and the lowest in the monsoon months of July and August (monsoon seasonal mean = 2.5 ± 2.2 μg m^-3). M_BC showed significant inverse relationship with the planetary boundary layer height (ρ ~ - 0.50) as well as ventilation coefficient (ρ ~ - 0.72). Alike aforesaid atmospheric dynamic parameters, T, WS, and RH also possessed statistically significant negative correlations with monthly M_BC. Enhancement in annual as well as diurnal amplitude in M_BC during deficient monsoon years relative to excessive monsoon years have given marked indication of BC sink mechanism due to precipitation. Roles of long-range regional air pollutant transport also have been identified. Identical and consistent seasonal variation in percentage contribution of M_BC with PM_2.5 (varying from 2.6 to 9.1%) and absorption Angstrom exponent (α_abs, monthly mean values varying from 0.77 ± 0.04 to 1.16 ± 0.08) gives evidence of substantial amount of enhanced anthropogenic source activities of fossil fuel incomplete combustion in post-monsoon and winter period.