Urban influence on the concentration and composition of submicron particulate matter in central Amazonia

Abstract

<p><strong>Abstract.</strong> Fundamental to quantifying the influence of human activities on climate and air quality is an understanding of how anthropogenic emissions affect the concentrations and composition of airborne particulate matter (PM). The central Amazon basin, especially around the city of Manaus, Brazil, has experienced rapid changes in the past decades due to ongoing urbanization. Herein, changes in the concentration and composition of submicron PM due to pollution 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 <q>T3</q> research site, 70 km downwind of Manaus, during the wet season. At this site, organic components represented on average 79 ± 7 % of the non-refractory PM1 mass concentration, which was in the same range as several upwind sites. The organic PM<sub>1</sub> was, however, considerably more oxidized at T3 compared to upwind measurements. Positive-matrix factorization (PMF) was applied to the time series of organic mass spectra collected at the T3 site, yielding three factors representing secondary processes (73 ± 15 % of total organic mass concentration) and three factors representing primary anthropogenic emissions (27 ± 15 %). Fuzzy c-means clustering (FCM) was applied to the afternoon time series of concentrations of NO<sub>y</sub>, ozone, total particle number, black carbon, and sulfate. Four clusters were identified and characterized by distinct airmass origins and particle compositions. Two clusters, Bkgd-1 and Bkgd-2, were associated with background conditions. Bkgd-1 appeared to represent near-field atmospheric PM production and oxidation of a day or less. Bkgd-2 appeared to represent material transported and oxidized for two or more days, often with out-of-basin contributions. Two other clusters, Pol-1 and Pol-2, represented the Manaus influence, one apparently associated with the northern region of Manaus and the other with the southern region of the city. A composite of the PMF and FCM analyses provided insights into the anthropogenic effects on PM concentration and composition. The increase in mass concentration of submicron PM ranged from 25 % to 200 % under polluted compared to background conditions, including contributions from both primary and secondary PM. Furthermore, a comparison of PMF factor loadings for different clusters suggested a shift in the pathways of PM production under polluted conditions. Nitrogen oxides may have played a critical role in these shifts. Increased concentrations of nitrogen oxides can shift pathways of PM production from HO<sub>2</sub>-dominant to NO-dominant as well as increase the concentrations of oxidants in the atmosphere. Consequently, the oxidation of biogenic and anthropogenic precursor gases as well as the oxidative processing of pre-existing atmospheric PM can be accelerated. The combined set of results demonstrates the susceptibility of atmospheric chemistry, air quality, and associated climate forcing to anthropogenic perturbations over tropical forests.</p>

CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: seasonal and diel variations and impact of anthropogenic emissions

“…xylenes, but not high NO x isoprene products (e.g., methylglyceric acid; de Sá et al, 2017b). This factor likely represents SOA formed from aromatics emitted from urban areas, possibly combined with a mixture of freshly oxidized biogenic compounds within the urban-influenced air (de Sá et al, 2017b).…”

“…The six factors for IOP2 included IEPOX-SOA, MO-OOA, LO-OOA, an aged biomass burning organic aerosol factor (aged BBOA), a fresh biomass burning organic aerosol factor (fresh BBOA), and HOA. Further details of PMF analysis and the characteristics of the factors can be found in de Sá et al (2017b). The O : C ratio and calculated density for each factor are presented in Table 1.…”

“…For the dry season, the measurements were classified into three groups each for day and night periods based on the bulk aerosol organic mass fraction, and the mass size distribution of each species was averaged from measurements in each of the six groups. The size-resolved mass concentrations of sulfate, nitrate, and organics at the time resolution of the CCN measurement were then derived by scaling the total mass concentration using the average mass size distributions for the corresponding group (based on the bulk organic mass fraction) of either wet or dry season (de Sá et al, 2017b). The shape of the NH + 4 mass size distribution was assumed to be the same as that of sulfate, as ammonium cations were primarily associated with sulfate.…”

“…PMF was applied to the AMS mass spectra (Lanz et al, 2008;, and six organic factors were identified for each of the two IOPs (de Sá et al, 2017b). For IOP2 the PMF analysis included data from 24 August to 15 October 2014, excluding a major regional biomass burning event from 16 to 23 August, which was treated separately in the PMF analysis.…”

Aircraft-based observations of isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) in the tropical upper troposphere over the Amazon region

“…Feb/Mar 2008; GoAmazon2014/5: Feb/Mar 2014) and represent ground measurements (Chen et al, 2009;de Sá et al, 2018). showing that the organic aerosol is oxidized.…”