An analysis of diurnal cycles in the mass of ambient aerosols derived from biomass burning and agro‐industry

Abstract

[1] Strong diurnal cycles in ambient aerosol mass were observed in a rural region of Southeast Brazil where the trace composition of the lower troposphere is governed mainly by emissions from agro-industry. An optical particle counter was used to record size-segregated aerosol number concentrations between 13 May 2010 and 15 March 2011. The data were collected every 10 min and used to calculate aerosol mass concentrations. Aerosol samples were also collected onto filters during daytime (10:00-16:00 local time) and nighttime (20:00-06:00) periods, for subsequent analysis of soluble ions and water-soluble organic carbon. Biomass burning aerosols predominated during the dry winter, while secondary aerosols were most important in the summer rainy season. In both seasons, diurnal cycles in calculated aerosol mass concentrations were due to the uptake of water by the aerosols and, to a lesser extent, to emissions and secondary aerosol formation. In neither season could the observed mass changes be explained by changes in the depth of the boundary layer. In the summer, nighttime increases in aerosol mass ranged from 2.7-fold to 81-fold, depending on particle size, while in the winter, the range was narrower, from 2.2-fold to 9.5-fold, supporting the possibility that the presence of particles derived from biomass burning reduced the overall ability of the aerosols to absorb water.

Understanding aerosol formation mechanisms in a subtropical atmosphere impacted by biomass burning and agroindustry

“…8). This suggests that freshly emitted biomass burning aerosols were not responsible for the observed diurnal trends, which were therefore probably due to the hygroscopic growth of existing particles into the detectable size range, as reported previously by Caetano-Silva et al (2013).…”

“…The accumulation mode showed a maximum in the summer in 2012 and in the winter (2012). The first of these could be explained by greater hygroscopic particle growth in summer, as reported previously (Caetano-Silva et al, 2013), and the second by greater particle formation due to heterogeneous reactions in the winter. Seasonal trends of the Aitken mode particle number concentrations were less clear, although there was evidence of both summer and winter maximum in number concentrations.…”

“…Nonetheless, generally similar diurnal trends were found for all particles sizes and all seasons. Previous work found that diurnal changes in the depth of the planetary boundary layer of the study region were unable to explain the magnitude of the differences in aerosol number concentrations observed for daytime and nighttime periods (Caetano-Silva et al, 2013). The possibility that the diurnal trends observed in the present work were associated with biomass burning emissions was investigated using data for periods with or without evidence of major fires.…”

“…6 shows the ratios of the number concentrations of aerosols in the three size ranges for the period between August 2011 and November 2012, based on monthly average concentrations, revealing the consistently higher contribution of the nucleation mode particles during the summer period. Previous work has shown that this could be attributed to lower concentrations of aerosols in larger size fractions (up to 10 μm) and higher humidity during the rainy season favoring the nucleation of new particles (Da Rocha et al, 2005;Allen et al, 2004;Caetano-Silva et al, 2013).…”

“…1a and 1b). In addition, turbulent mixing and expansion of the boundary layer caused by strong insolation during the daytime has been estimated to dilute the concentrations of trace components (gases and aerosols) in the lower troposphere of the study region by approximately 60% (Caetano-Silva et al, 2013;Urban et al, 2012).…”