Black carbon aerosols over source vs. background region: Atmospheric boundary layer influence, potential source regions, and model comparison

Abstract

Black carbon (BC) aerosols measured simultaneously over a source (Ahmedabad, urban) and background (Gurushikhar, high-altitude) region are analysed. The influence of atmospheric boundary layer (ABL), and potential source regions for BC are examined, and compared with model, for the first time. BC mass in Ahmedabad is significantly higher (2 to 5 times) than Gurushikhar. BC mass concentrations in Ahmedabad peak during morning and evening hours when ABL is shallow and anthropogenic emissions are high. In contrast over Gurushikhar BC emissions are higher in afternoon due to a fully evolved ABL and upward transport of pollutants from the valley/foothills during winter when Gurushikhar is within ABL. During other seasons ABL is higher and diurnal variation is suppressed. BCmax over Gurushikhar is close to BCmin over Ahmedabad, which occurs during afternoon hours when ABL is fully evolved and anthropogenic sources are negligible over Ahmedabad while emissions get transported from foothills surrounding Gurushikhar. Potential source contribution function (PSCF) analysis reveals that probable source regions for BC in these two locations are distinctly different, especially, during postmonsoon and winter, long range transport from northwestern India and the Indo-Gangetic Plain (IGP) are the probable source regions (agricultural waste and biomass burning) to the BC over the high-altitude location. Regional transport of BC emitted from urban areas of Ahmedabad to high-altitude Gurushikhar is evident in premonsoon from PSCF analysis. BC mass concentrations simulated by 2 models - MERRA-2 (fine resolution) and ECHAM6 (coarse resolution) are significantly underestimated over the source region Ahmedabad though the models capture the seasonal cycle of winter high and monsoon low. The models simulate BC mass over the background region Gurushikhar well. The magnitude of BC mass concentrations simulated by both models over Ahmedabad and Gurushikar are quite similar. This observation-model comparative analysis reveals that anthropogenic aerosol emissions, especially BC and its long range transport are inadequately represented in models, in particular over the source regions strongly suggesting that improvements in emission inventories of aerosol sources as well as inclusion of missing aerosol source categories are needed. Such quantitative evaluation of BC aerosols over a source and a background region due to the influence of ABL, and potential source regions are hitherto unavailable over tropics which can be used to improve the representation of aerosols in models and thereby estimates of radiative and climate effects due to aerosols.