Influence of urban pollution on the production of organic particulate matter from isoprene epoxydiols in central Amazonia

摘要

pstrongAbstract./strong The atmospheric chemistry of isoprene contributes to the production of a substantial mass fraction of the particulate matter (PM) over tropical forests. Isoprene epoxydiols (IEPOX) produced in the gas phase by the oxidation of isoprene under HOsub2/sub-dominant conditions are subsequently taken up by particles, thereby leading to production of secondary organic PM. The present study investigates possible perturbations to this pathway by urban pollution. The measurement site in central Amazonia was located 4 to 6span class="thinspace"/spanh downwind of Manaus, Brazil. Measurements took place from February through March??2014 of the wet season, as part of the GoAmazon2014/5 experiment. Mass spectra of organic PM collected with an Aerodyne Aerosol Mass Spectrometer were analyzed by positive-matrix factorization. One resolved statistical factor (qIEPOX-SOA factor/q) was associated with PM production by the IEPOX pathway. The IEPOX-SOA factor loadings correlated with independently measured mass concentrations of tracers of IEPOX-derived PM, namely Csub5/sub-alkene triols and 2-methyltetrols (iR/ia??=a??0.a??96 and 0.78, respectively). The factor loading, as well as the ratio if/i of the loading to organic PM mass concentration, decreased under polluted compared to background conditions. For an increase in NOsubiy/i/sub concentration from 0.5 to 2span class="thinspace"/spanppb, the factor loading and if/i decreased by two to three fold. Overall, sulfate concentration explained 37span class="thinspace"/span% of the variability in the factor loading. After segregation of factor loading into subsets based on NOsubiy/i/sub concentration, the sulfate concentration explained up to 75span class="thinspace"/span% of the variability. Considering both factors, the data sets show that the suppressing effects of increased NO concentrations dominated over the enhancing effects of higher sulfate concentrations. The pollution from Manaus elevated NOsubiy/i/sub concentrations more significantly than sulfate concentrations relative to background conditions. In this light, increased emissions of nitrogen oxides, as anticipated for some scenarios of Amazonian economic development, could significantly alter pathways of PM production that presently prevail over the tropical forest, implying changes to air quality and regional climate./p.