Chemical composition of rainwater collected at a southwest site of Mexico City, Mexico

Measurements of the trace metals Cd, Cr, Mn, Ni, Pb, V and Al in soluble and insoluble rain fractions and SO4− 2, NO3−, Cl−, HCO3−, Ca2+, Mg2+, Na+, K+, NH4+ and H+ in soluble fractions were performed in rainwater collected at a southwest site of Mexico City during the rainy seasons of 2001 and 2002. Aluminum presented the highest volume-weighted mean concentration (VWMC) in both insoluble and soluble fractions. In the insoluble fractions, the VWM of the other trace metals decreased in the order Mn, Pb, Ni, V, Cr and Cd, and in the soluble fractions in the order Mn, V, Ni, Pb, Cd and Cr. Ammonium presented the higher VWMC, followed by SO42−, NO3−, HCO3−, Ca2+, Cl−, H+, Na+, Mg2+ and K+. Air mass back trajectories were associated to the concentrations of trace metals and of SO42−, Ca2+, Mg2+, NH4+ and H+ observed during each rainy day. Trace metal concentrations were not clearly related to wind direction. Enrichment factors related to the relative abundance of elements in crustal material were calculated using Mg as reference. The high enrichment factors (EFc) suggested that, in general, trace metals and major ions had an anthropogenic origin. Aluminum, K+, and Ca2+ were the only elements that had a significant crustal source. Factor analysis (Principal Component Analysis) with Varimax normalized rotation grouping the elements analyzed into three factors. Factor 1 indicated a crustal contribution for Ca2+, K+, Mg2+ and anthropogenic sources for SO42−, NH4+ and V. Factor 2 indicated a high loading for Al, Ni and Mn, that indicate possible contribution of anthropogenic sources but with a significant crustal contribution for Al. Factor 3 indicated an anthropogenic origin for H+ and NO3−. Pearson's correlations show that Al correlated with all the metals, including Ca2+ and Mg2+. The solubility of trace metals did not depend on rainwater pH. As it was expected, Al presented the highest wet deposition flux.