Abstract
Abstract. Biomass burning emits vapors and aerosols into the atmosphere that can rapidly evolve as smoke plumes travel downwind and dilute, affecting climate- and health-relevant properties of the smoke. To date, theory has been unable to explain observed variability in smoke evolution. Here, we use observational data from the Biomass Burning Observation Project (BBOP) field campaign and show that initial smoke organic aerosol mass concentrations can help predict changes in smoke aerosol aging markers, number concentration, and number mean diameter between 40–262 nm. Because initial field measurements of plumes are generally >10 min downwind, smaller plumes will have already undergone substantial dilution relative to larger plumes and have lower concentrations of smoke species at these observations closest to the fire. The extent to which dilution has occurred prior to the first observation is not a directly measurable quantity. We show that initial observed plume concentrations can serve as a rough indicator of the extent of dilution prior to the first measurement, which impacts photochemistry, aerosol evaporation, and coagulation. Cores of plumes have higher concentrations than edges. By segregating the observed plumes into cores and edges, we find evidence that particle aging, evaporation, and coagulation occurred before the first measurement. We further find that on the plume edges, the organic aerosol is more oxygenated, while a marker for primary biomass burning aerosol emissions has decreased in relative abundance compared to the plume cores. Finally, we attempt to decouple the roles of the initial concentrations and physical age since emission by performing multivariate linear regression of various aerosol properties (composition, size) on these two factors.
Highlights
Elevated f60 values are indicative of “levoglucosan-like” species (Aiken et al, 2009; Cubison et al, 2011; Lee et al, 2010) that are known tracers of smoke primary organic aerosol (POA) (Cubison et al, 2011); f44, the OA fractional component observed by the SP-AMS as the high-resolution ion fragment CO2+ as well as some acid groups, is a proxy for Secondary organic aerosol (SOA) arising from oxidative aging (Alfarra et al, 2004; Cappa and Jimenez, 2010; Jimenez et al, 2009; Volkamer et al, 2006)
S14–S18 provides this information for the other pseudo-Lagrangian transect flight sets studied. (Here, black carbon (BC) represents the refractory BC from the SP2; Sect. 2.) We have divided each transect into four regions: between the 5th–15th, 15th–50th, 50th–90th, and 90th–100th percentile of CO within each transect. (As discussed above, we exclude the lowest 5 % in order to provide a buffer between the plume edge and background air.) Note that in Fig. 1
These flights have allowed us to examine near-field (
Summary
Smoke from biomass burning is a major source of atmospheric primary aerosol and vapors (Akagi et al, 2011; Gilman et al, 2015; Hatch et al, 2015, 2017; Jen et al, 2019; Koss et al, 2018; Reid et al, 2005; Yokelson et al, 2009), influencing air quality, local radiation budgets, cloud properties, and climate (Carrico et al, 2008; O’Dell et al, 2019; Petters et al, 2009; Ramnarine et al, 2019; Shrivastava et al, 2017) as well as the health of impacted communities (Ford et al, 2018; Gan et al, 2017; Reid et al, 2016). Fires span an immense range in size, from small agricultural burns, which may be only a few square meters in total area and last a few hours, to massive wildfires, which may burn tens of thousands of kilometers over the course of weeks (Andela et al, 2019). This range in size leads to variability in initial plume size and extent of dilution by the time of the first measurement. Capturing variability in plume aerosol concentrations and dilution between fires and within fires can aid in understanding how species change within the first few hours of emission for a range of plume sizes
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