Anthropogenic influences on the physical state of submicron particulate matter over a tropical forest

Abstract

<p><strong>Abstract.</strong> The occurrence of non-liquid 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, respectively, 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, which is an indicator of the mix of physical states in a sampled particle population, was measured in real time at ground level using an impactor apparatus. Micrographs collected by transmission electron microscopy confirmed that liquid particles adhered while non-liquid particles rebounded. Relative humidity (RH) was scanned to collect rebound curves. When the apparatus RH matched ambient RH, 95 % of the particles were liquid as a campaign average, although this percentage dropped to as low as 60 % during periods of anthropogenic influence. Secondary organic material, produced for the most part by the oxidation of volatile organic compounds emitted from the forest, was the largest source of liquid PM. 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 non-liquid PM 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 appear to favor non-liquid PM by providing molecular species that increase viscosity when internally mixed with background PM, by contributing non-liquid particles in external mixtures of PM, and a by combination of these effects under real-world conditions.</p>

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

“…For example, pollution may mask some of the influences of forest loss on Amazonian rainfall (Cox et al 2008). Interactions can also be complex, for example, the isoprene derived particles found over the Amazon typically adhere to each other allowing aggregation and increased size but smoke reduces this aggregation (Bateman et al 2017). As another example, while forest loss generally reduces BVOC emissions, some land uses are major sources.…”

Mechanism of SOA formation determines magnitude of radiative effects

“…The IM_OC scheme was designed as a sensitivity test to consider observations that showed that OC from anthropogenic sources can be nonliquid even at high relative humidity (RH) and exist in an EM state in the Amazon ( 33 ). Thus, these particles would not participate in thermodynamic processes that allow semivolatile gases to partition to the particle phase.…”

“…In addition, the sensitivity study for the distribution of SOA associated with OC (based on the finding from ref. 33 ) is simplified. As new knowledge is developed, our simplified treatment will need to be expanded, based on knowledge of the physical and chemical characteristics of aerosols and their mixing with SOA.…”

“…A recent measurement has indicated that primary OC from anthropogenic sources are nonliquid even at high RH and exist in an EM state in the Amazon ( 33 ). We tested the effect of this assumption in the IM_OC scheme which was developed by modifying the IM scheme to include the assumption of no SOA partitioning to primary OC via the thermodynamic process.…”

Mobile and stationary sources of air pollutants in the Amazon rainforest: a numerical study with WRF-Chem model

“…Thus, the region is a valuable laboratory for studying the impact of anthropogenic emissions of air pollutants. Although, there are a number of studies involving the measurements of atmospheric pollutants in the Amazon region (e.g., Kuhn et al, 2010;Trebs et al, 2012;Baars et al, 2012;Artaxo et al, 2013), most of the contribution was published very recently with the results of the GoAmazon project (Martin et al, 2016a;Martin et al, 2016b;Alves et al, 2016;Pöhlker et al, 2016;Sá et al, 2016;Kourtchev et al, 2016;Hu et 55 al., 2016;Bateman et al, 2016;Cecchini et al, 2016;Jardine et al, 2016). In terms of numerical studies, only a few studies focusing the role of the anthropogenic emission sources in urban area are available (Freitas et al, 2006;Andreae et al, 2012;Beck et al, 2013;Bela et al, 2014).…”