Composition, sources, and chemical processes of submicrometric aerosol over Southern Amazonian city highly impacted by biomass burning emissions

The southeastern Amazon is part of the so-called deforestation arc region that begins in the South of Pará and goes towards Acre state. The region is experiencing intense pressure for land use change as it shapes the agricultural borders that over the years advance towards the forest. Along with deforestation, there is an increase in fires occurrence that has severe implications on regional air quality, climate, and human health. In this study, an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) coupled to a variety of instruments were deployed providing an overview of the chemical and physical characteristics of the aerosols in Rio Branco (AC) during the dry season of 2018. Median aerosol number concentration was 4,200 cm^-3, size distribution was centered at 85 nm, whilst Black Carbon equivalent (BCe) concentration ranged around 2.2 μ g m^-3 with maximum of 21.2 μ g m^-3. Relative mass composition of PM₁ is largely dominated by organics (76%), followed by BCe (16%); whereas inorganics (SO₄^-2, NO₃^-, NH₄⁺ and Cl^-) accounted only for the remaining fraction (8%). Using positive matrix factorization (PMF) analysis , sources of organic aerosol (OA) were identified such as hydrocarbon-like OA (HOA), biomass burning OA (BBOA), and oxygenated OA (OOA). Two profiles of OOA were resolved, one related to regional background, and another associated with biomass burning emissions. After the peak of the biomass burning (BB) season in September, the aerosol load gradually decreased, consistent with a decrease in fire outbreaks towards a transition to the rainy season. The fraction of f60, a tracer for BB emissions, presented average values around 0.4% while f60> 0.3% is indicated as a threshold for fresh BB. It is interesting to note that even with the decrease in wildfires events f60 was above 0.3% suggesting that to a lower extent, the region continued to be impacted by BB emissions, probably transported from more distant sources in Amazonia. Regarding optical properties, single scattering albedo (637nm) had median of 0.96 for the period of major BB influence but declined slightly to 0.93 during the second part indicating that aerosols in the dry season efficiently scatter light due to the large dominance of OA. Our results shed light on the aging process of BB aerosols and how chemical and physical properties correlate.