Investigations of aerosol black carbon from a semi-urban site in the Indo-Gangetic Plain region

• A complete seasonal variation of BC from a semi-urban site in the IGP region.
• Large diurnal and seasonal variation with maximum amplitude and levels in winter.
• Surface BC is maximum in winter, unlike emission estimates showing spring maxima.
• Unlike BC, CALIPSO extinctions at higher height and AOD reveal higher values in spring.
• WRF-Chem simulated BC shows important features but underestimate observations.

Long-term (2009–2012) data from ground-based measurements of aerosol black carbon (BC) from a semi-urban site, Pantnagar (29.0°N, 79.5°E, 231 m amsl), in the Indo-Gangetic Plain (IGP) near the Himalayan foothills are analyzed to study the regional characterization. Large variations are seen in BC at both diurnal and seasonal scales, associated with the mesoscale and synoptic meteorological processes, and local/regional anthropogenic activities. BC diurnal variations show two peaks (morning and evening) arising from the combined effects of the atmospheric boundary layer (ABL) dynamics and local emissions. The diurnal amplitudes as well as the rates of diurnal evolution are the highest in winter season, followed by autumn, and the lowest in summer-monsoon. BC exhibits nearly an inverse relation with mixing layer depth in all seasons; being strongest in winter (R2 = 0.89) and weakest (R2 = 0.33) in monsoon (July–August). Unlike BC, co-located aerosol optical depths (AOD) and aerosol absorption are highest in spring over IGP, probably due to the presence of higher abundances of aerosols (including dust) above the ABL (in the free troposphere). AOD (500 nm) showed annual peak (>0.6) in May–June, dominated by coarse mode, while fine mode aerosols dominated in late autumn and early winter. Aerosols profiles from CALIPSO show highest values close to the surface in winter/autumn, similar to the feature seen in surface BC, whereas at altitudes > 2 km, the extinction is maximum in spring/summer. WRF-Chem model is used to simulate BC temporal variations and then compared with observed BC. The model captures most of the important features of the diurnal and seasonal variations but significantly underestimated the observed BC levels, suggesting improvements in diurnal and seasonal varying BC emissions apart from the boundary layer processes.