Trace Element Chemistry in Urban Atmospheric Aerosols

Unlike in the United States, the concentration of trace elements in urban air is still high enough in South Asian cities to study the impact of trace elements on climate and human health. Hence, continuous sampling of PM2.5 (particulate matter of <2.5 μm aerodynamic diameter)was carried out using low volume sampler in winter (2005-2006) in Lahore, the second largest city of Pakistan, which is highly impacted by urban and agricultural emissions and has remained unexplored in terms of atmospheric chemistry. Aerosols collected on this campaign are likely to carry the signatures of emissions from Afghanistan, North and Central Pakistan, North India in addition to the local pollution sources. During sampling from December 2005 to January 2006, it was possible to collect several samples during brief fog episodes. Samples were analyzed for 25 elements (Be, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Tl and Pb) using inductively coupled plasma mass spectrometry. High pollutant concentrations were observed throughout the study, for instance, Cr concentrations up to 1.4 μgm-3, As, 135 ngm-3, Cd, 93 ngm-3, Sn, 988 ngm-3 and Sb, 157 ngm-3. Pb and Zn concentrations respectively up to 12 and 48 μgm-3 were observed. Calculation of enrichment factor and crustal correction illustrate the attribution of Cr, Co, Ni, Zn, As, Se, Mo, Ag Cd, Sn, Sb, Tl and Pb to non-crustal sources. Air parcel back trajectories, interelemental relationships and meteorological observations have been used to explain the sources and the impacts of fog chemistry and mixing heights on atmospheric processing of trace elements in PM2.5. Atmospheric stagnation appeared to be one of factors causing episodic high concentrations. Crustal correction and interelemental relationships apparently suggest the emissions from coal and oil combustion, industrial processes, building construction sites and biomass burning as the prime role players in the atmospheric pollution in that region. Source apportionment will be done using positive matrix factorization. Findings will improve the understanding of the critical roles and interactions of chemical composition, size and mixing state of atmospheric particles that manipulate the chemistry of an urban atmosphere. This work was partially funded by the US National Science Foundation.