Seasonal Variability of Atmospheric Aerosols in Karachi, Pakistan

A variety of in situ and satellite-derived data of aerosols like atmospheric black carbon concentrations wereused to probe the seasonal differences of aerosol concentration in Karachi, Pakistan for one year. Daily [black carbon]varied from about 4000 to 50,000 ng/m3 with the mean maximum of 14700 ng/m3 in February, primarily duringmornings and evenings. The [black carbon] concentrations were at a maximum during winter months of November toFebruary i.e. around 12000 ng/m3 and were at minimum value during summer from June to September (3000 ng/m3).Short term and long-term variabilities were mostly affected by meteorological parameters. Apart from industrial andindiscriminate solid waste burning, most important source of BC emissions in Karachi was vehicular traffic, since overa million vehicles were registered in the city. Aerosol Optical Depth (AOD) from multi-band AOD, AERONET, andMODIS satellites showed a similar trend of its concentrations similar to BC. Aeronet 500 nm AODs were at amaximum for July (0.95 monsoons) and minimum (around 0.4) in November-February. Seasonal variation of AOD(Aeronet) was matching at other wavelengths, while the deviation in the spectral dependency of AOD was uncertain. Itimplied that a columnar spectral optical depth represented different aerosol type association having advection fromvarious directions and sources. Relevant stakeholders should play their role to reduce BC emissions to mitigate illhealth impacts in this metropolitan city.