PNNL

Abstract

Fundamental to quantifying the influence of human activities on climate and air quality is the understanding of how anthropogenic emissions affect the concentrations and composition of airborne particulate matter (PM). The central Amazon basin has experienced rapid changes in the past decades due to ongoing urbanization, especially around the city of Manaus, Brazil. Herein, changes in the concentration and composition of submicron PM downwind of the Manaus metropolitan region are reported as part of the GoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a suite of other gas- and particle-phase instruments were deployed at the “T3” research site, 70 km downwind of Manaus, during the wet season. At this site, the organic component represented on average 79 ± 7 % of the non-refractory PM1 mass concentration, which was consistent across sites in central Amazonia. The organic PM1 was, however, considerably more oxidized at T3 than at upwind sites. Positive matrix factorization (PMF) was applied to the time series of organic mass spectra at the T3 site, yielding three factors representing secondary processes (73 ± 15 % in factor loading) and three factors representing primary anthropogenic emissions (27 ± 15 %). Fuzzy c-means clustering (FCM) was applied to the afternoon time series of concentrations of NOy, ozone, total particle number, black carbon, and sulfate. Four clusters were identified and characterized by distinct airmass origins and particle compositions. Two clusters, labeled Bkgd-1 and Bkgd-2, were associated with background conditions, the first of which appeared to represent near-field atmospheric PM production and oxidation of a day or less and the second of which appeared to represent material transported and oxidized for two or more days. Two other clusters, labeled Pol-1 and Pol-2, represented the Manaus influence, one originating from the northern region of Manaus and the other from the southern region of the city. A composite of the PMF and FCM analyses provided insights on the anthropogenic effects on PM concentration and composition. The mass concentration of submicron PM increased by 25 % to 200 % under polluted compared to background conditions, including contributions from both primary and secondary PM. A comparison of PMF factor loadings among clusters demonstrates a shift in the pathways of PM production under polluted conditions. Increased emissions of nitrogen oxides may play the dual role of shifting pathways of PM production from HO2-dominant to NO-dominant as well as increasing the concentration of oxidants in the atmosphere, thereby accelerating the oxidation of reactive organic precursor gases and the aging of pre-existing atmospheric PM. The combined set of results demonstrates the susceptibility of atmospheric chemistry over tropical rainforests and the associated implications for climate dynamics to anthropogenic perturbations.