Abstract
Abstract. The occurrence of nonliquid and liquid physical states of submicron atmospheric particulate matter (PM) downwind of an urban region in central Amazonia was investigated. Measurements were conducted during two intensive operating periods (IOP1 and IOP2) that took place during the wet and dry seasons of the GoAmazon2014/5 campaign. Air masses representing variable influences of background conditions, urban pollution, and regional- and continental-scale biomass burning passed over the research site. As the air masses varied, particle rebound fraction, an indicator of physical state, was measured in real time at ground level using an impactor apparatus. Micrographs collected by transmission electron microscopy confirmed that liquid particles adhered, while nonliquid particles rebounded. Relative humidity (RH) was scanned to collect rebound curves. When the apparatus RH matched ambient RH, 95 % of the particles adhered as a campaign average. Secondary organic material, produced for the most part by the oxidation of volatile organic compounds emitted from the forest, produces liquid PM over this tropical forest. During periods of anthropogenic influence, by comparison, the rebound fraction dropped to as low as 60 % at 95 % RH. Analyses of the mass spectra of the atmospheric PM by positive-matrix factorization (PMF) and of concentrations of carbon monoxide, total particle number, and oxides of nitrogen were used to identify time periods affected by anthropogenic influences, including both urban pollution and biomass burning. The occurrence of nonliquid PM at high RH correlated with these indicators of anthropogenic influence. A linear model having as output the rebound fraction and as input the PMF factor loadings explained up to 70 % of the variance in the observed rebound fractions. Anthropogenic influences can contribute to the presence of nonliquid PM in the atmospheric particle population through the combined effects of molecular species that increase viscosity when internally mixed with background PM and increased concentrations of nonliquid anthropogenic particles in external mixtures of anthropogenic and biogenic PM.
Highlights
Particulate matter (PM) directly affects the Earth’s climate by scattering and absorbing solar radiation and indirectly through effects on clouds (Ramanathan et al, 2001)
The combined set of laboratory and ambient studies shows that the physical state of PM with high Secondary organic material (SOM) content depends on the surrounding relative humidity (RH)
The results reported for GoAmazon2014/5 represent a longer time series (i.e., 550 compared to 30 rebound curves) and reinforce the generality of the earlier results found by Bateman et al (2016) on the prevalence of liquid PM for this forested region under background conditions
Summary
Particulate matter (PM) directly affects the Earth’s climate by scattering and absorbing solar radiation and indirectly through effects on clouds (Ramanathan et al, 2001). For background conditions of the Amazonian tropical forest, a region dominated by isoprene-derived SOM and high RH, PM was mostly liquid (Bateman et al, 2016). The combined set of laboratory and ambient studies shows that the physical state of PM with high SOM content depends on the surrounding relative humidity (RH). This effect arises in part because organic particles are hygroscopic to various extents depending on composition. Compared to prevailing background conditions of SOM dominance over many forests, PM produced by biomass burning leads to a net effect of decreased water uptake when the PM mixes into the background particle population (Dusek et al, 2011). The submicron PM in this region is dominated by biogenic SOM (Chen et al, 2009, 2015)
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