Aerosol size distribution changes in FIREX-AQ biomass burning plumes: the impact of plume concentration on coagulation and OA condensation/evaporation

Abstract

Abstract. The evolution of organic aerosol (OA) and aerosol size distributions within smoke plumes is uncertain due to the variability in rates of coagulation and OA condensation/evaporation between different smoke plumes and at different locations within a single plume. We use aircraft data from the FIREX-AQ campaign to evaluate differences in evolving aerosol size distributions, OA, and oxygen to carbon ratios (O:C) between and within smoke plumes during the first several hours of aging as a function of smoke concentration. The observations show that the median particle diameter increases faster in smoke of a higher initial OA concentration (>1000 µg m−3), with diameter growth of over 100 nm in 8 h – despite generally having a net decrease in OA enhancement ratios – than smoke of a lower initial OA concentration (<100 µg m−3), which had net increases in OA. Observations of OA and O:C suggest that evaporation and/or secondary OA formation was greater in less concentrated smoke prior to the first measurement (5–57 min after emission). We simulate the size changes due to coagulation and dilution and adjust for OA condensation/evaporation based on the observed changes in OA. We found that coagulation explains the majority of the diameter growth, with OA evaporation/condensation having a relatively minor impact. We found that mixing between the core and edges of the plume generally occurred on timescales of hours, slow enough to maintain differences in aging between core and edge but too fast to ignore the role of mixing for most of our cases.

An evaluation of biomass burning aerosol mass, extinction, and size distribution in GEOS using observations from CAMP2Ex

“…A limitation of GOCART, however, is that the particle size distribution cannot vary in time or space for a given aerosol species. This contradicts the variability seen within particle size distribution during the low-level flight segment from CAMP 2 Ex RF9 as well as results from other field campaigns that indicate the particle size distribution of biomass burning aerosol changes with respect to median diameter and modal width as smoke ages (June et al, 2022). If the assumed particle size distribution for brown carbon in GEOS were to be modified such that the mode radius is larger, it would need to be thoroughly evaluated as a change in the particle distribution would also impact fields such as the single-scattering albedo and Ångström exponent.…”

Optical properties of biomass burning aerosol during the 2021 Oregon fire season: comparison between wild and prescribed fires

“…Previous studies have concluded that coagulation has a larger effect on particle growth than OA evaporation/SOA formation, and coagulation rates are faster in denser plumes compared to more dilute plumes. 43,44,127 Using data from a scanning mobility particle sizer (SMPS) deployed at MBO during summer 2015, Laing et al 48 found both MSE and geometric mean particle diameter to be strongly correlated with plume concentration. Our results suggest that about 50% of the observed variability in MSE is explained by particle size (based on the R 2 between SAE and MSE), and that particle growth towards the measurement wavelength of 550 nm is more efficient in denser plumes.…”

“…Denser plumes from larger fires have slower dilution rates which favor coagulation and condensation, while less concentrated smoke from smaller fires is expected to have faster rates of dilution and evaporation, producing more SOA precursors. 42–44…”

Understanding the Evolution of Smoke Mass Extinction Efficiency Using Field Campaign Measurements

“…As shown earlier, proper representation of aerosol size is also important for resolving MEE. Thus, given the correlation of diameter and refractive index, evaluating and developing models to resolve particle diameter (e.g., June et al., 2022) can also support parameterizations of real refractive index evolution.…”

“…Laboratory and field measurements have found a large range in mid‐visible MSE for fresh and aged smoke (1.5–6 m 2 /g for wavelengths on the 532–550 nm range), with values being correlated with changes in aerosol mean diameter as expected from Mie theory (Kleinman et al., 2020; Laing et al., 2016; Levin et al., 2010; J. S. Reid et al., 1998; J. S. Reid, Eck, et al., 2005). Aerosol size increase with age is the result of a combination of coagulation and gas‐to‐particle conversion of semi‐volatile material (Martins et al., 1996; J. S. Reid, Koppmann, et al., 2005), likely primarily coagulation (Hodshire et al., 2021; June et al., 2022).…”