PNNL

Abstract

The occurrence of non-liquid and liquid physical states of submicron atmospheric particulate matter 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 background conditions, urban pollution, and regional and continental scale biomass burning passed over the research site. As the air masses evolved, 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 solid particles rebounded. Relative humidity (RH) was scanned from 5 to 95% to collect rebound curves, meaning particle rebound fraction as a function of RH. The rebound response curve depends on particle hygroscopicity in a homogeneous particle population or alternatively on an increase in the fraction of hydrophobic particles in a heterogeneous particle population. Liquid PM encompassed approximately 95% of the observations when the apparatus RH was the same as that of ambient RH. The liquid PM originated from production of secondary organic material by oxidation of volatile organic compounds emitted from the forest. The remaining 5% of the observations represents the anthropogenic influence, and these observations are the focus of interpretation herein. Time periods affected by biomass burning and urban pollution were identified by auxiliary data sets, including analysis of the particle mass spectra by positive21 matrix factorization (PMF) as well as analysis of concentrations of carbon monoxide concentration, total particle number, and oxides of nitrogen. Enhanced particle rebound correlated with these indicators of anthropogenic influence. Up to 70% in the variance of rebound fraction could be explained by a linear model based on the PMF factors. Microanalysis by scanning transmission X-ray microscopy interfaced with near-edge X-ray fine structure spectroscopy (STXM/NEXAFS) confirmed the chemical differences of the rebounding particles compared to adhering particles. The shifts between liquid and non-liquid PM highlight the several roles of urban pollution and biomass burning, including contributions of molecular species that can increase the viscosity of PM by internal mixing, contributions of hydrophobic particles in external mixtures, and a combination of these effects under real-world conditions.