Characterization of carbonaceous aerosols in urban air

Concentrations of black carbon, [BC], were determined with an Aethalometer every 5 min at Lahore, a city of about 10 million in Pakistan, from 22 November 2005 to 31 January 2006. [BC] were very high, ranging from about 5 to 110μgm-3, with a mean of 21.7μgm-3. A distinct diurnal variation was observed: concentrations were lowest from about 10 a.m. to 4 p.m. local time (LT), and highest around 5–9 p.m. No clear relationship was observed between surface wind directions and [BC], although some of the highest concentrations were observed when the airflow was from southwest to northwest. The daily variations in concentrations were strongly affected by the diurnal variations in the mixing height; BC concentrations were low during the day when the mixing heights were high, ∼1000m, and very high at night when the mixing heights were low <250m. Periods of light to dense fog occurred from 22 December through 4 January. [BC] were generally lower than average when fog occurred during the night and early morning, but they were not necessarily lower during daytime fogs. We also collected aerosols on quartz filters every 3, 6, or 12 h and determined the concentrations of elemental, [EC], and organic carbon, [OC], using the thermal–optical method. The [BC] were highly correlated with EC (r2=0.71)(r2=0.71), but on average 25% higher than [EC]. The [EC] and [OC] concentrations were moderately correlated (r2=0.65)(r2=0.65). The [OC]/[EC] ratios varied from 2.8 to 12, with a mean of 5.6. Although a large component of the carbonaceous aerosols in Lahore originated from fossil fuel combustion, a significant fraction was derived from biomass burning.