Elemental carbon (EC) and organic carbon (OC) contributes large fractions of aerosol mass in PM2.5. EC strongly absorbs solar radiation, changes the optical properties and life cycle of the clouds. Aerosol size distribution can provide information about radiation balance in the atmosphere, chemical and physical processes, such as, the primary carbonaceous substance and secondary organic aerosols (SOA), which results from the condensation of organic compounds formed by chemical reactions involving primary precursors.

EC and OC concentrations varied from 10 to 30 µg/m3 and from 80 to 300 µg/m3 respectively in Lahore, Pakistan. OC/EC ratios varied from ~ 6 to 12 with an average about 8.6. EC and OC were highly correlated, which suggested that most of the EC and OC in aerosols were from the same source. From 74% to 98% EC were found in the fine particles with diameters <1 µm. 33% - 61% OC were found in the accumulation and coarse phases of particles with diameters in the range of 1-10 µm. The OC/EC ratios in the stage of particle size less than 0.5 µm presented a general feature with values about ~ 4. The rapidly increasing OC/EC ratios with the particle sizes (in the stages 0.5-0.99, 1-1.49, 1.5-2.99, 3-7.19, 7.2-10 µm) suggested a significant secondary organic aerosol (SOA) production in this region. SOA was estimated up to 16 µg/m3 in Karachi and 75 µg/m3 in Lahore, which the higher SOA most possibly were enhanced by the fog events in Lahore.