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Abstract Evaluating our understanding of smoke from wild and prescribed fires can benefit from downwind measurements that include inert tracers to test production and transport and reactive species to test chemical mechanisms. We characterized smoke from fires in coniferous forest fuels for >1,000 hr over two summers (2017 and 2018) at our Missoula, Montana, surface station and found a narrow range for key properties. ΔPM2.5/ΔCO was 0.1070 ± 0.0278 (g/g) or about half the age‐independent ratios obtained at free troposphere elevations (0.2348 ± 0.0326). The average absorption Ångström exponent across both years was 1.84 ± 0.18, or about half the values available for very fresh smoke. Brown carbon (BrC) was persistent (~50% of absorption at 401 nm) in both years, despite differences in smoke age. ΔBC/ΔCO doubled from 2017 to 2018, but the average across 2 years was within 33% of recent airborne measurements, suggesting low sampling bias among platforms. Switching from a 1.0 to a 2.5 micron cutoff increased the mass scattering and mass absorption coefficients, suggesting often overlooked supermicron particles impact the optical properties of moderately aged smoke. O3was elevated ~6 ppb on average over a full diurnal period when wildfire smoke was present, and smoke‐associated O3increases were highest (~9 pbb) at night, suggesting substantial upwind production. NOxwas mostly local in origin. NOxspurred high rates of NO3production, including in the presence of wildfire smoke (up to 2.44 ppb hr−1) and at least one nighttime BrC secondary formation event that could have impacted next‐day photochemistry.
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In situ measurements of trace gases, PM, and aerosol optical properties during the 2017 NW US wildfire smoke event
Abstract. In mid-August through mid-September of 2017 a major wildfire smoke and hazeepisode strongly impacted most of the NW US and SW Canada. During this periodour ground-based site in Missoula, Montana, experienced heavy smoke impactsfor ∼ 500 h (up to 471 µg m−3 hourly averagePM2.5). We measured wildfire trace gases, PM2.5 (particulate matter≤2.5 µm in diameter), and black carbon and submicron aerosolscattering and absorption at 870 and 401 nm. This may be the most extensivereal-time data for these wildfire smoke properties to date. Our range oftrace gas ratios for ΔNH3∕ΔCO and ΔC2H4∕ΔCO confirmed that the smoke from mixed, multiple sourcesvaried in age from ∼ 2–3 h to ∼ 1–2 days. Our study-averageΔCH4∕ΔCO ratio (0.166±0.088) indicated a largecontribution to the regional burden from inefficient smoldering combustion.Our ΔBC∕ΔCO ratio (0.0012±0.0005) for our groundsite was moderately lower than observed in aircraft studies (∼ 0.0015)to date, also consistent with a relatively larger contribution fromsmoldering combustion. Our ΔBC∕ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC (black carbon),mostly organic nature of the smoke observed in airborne studies of wildfiresmoke to date. Smoldering combustion is usually associated with enhanced PMemissions, but our ΔPM2.5∕ΔCO ratio (0.126±0.002)was about half the ΔPM1.0∕ΔCO measured in freshwildfire smoke from aircraft (∼ 0.266). Assuming PM2.5 isdominated by PM1, this suggests that aerosol evaporation, at least nearthe surface, can often reduce PM loading and its atmospheric/air-qualityimpacts on the timescale of several days. Much of the smoke was emitted latein the day, suggesting that nighttime processing would be important in theearly evolution of smoke. The diurnal trends show brown carbon (BrC),PM2.5, and CO peaking in the early morning and BC peaking in the earlyevening. Over the course of 1 month, the average single scattering albedo forindividual smoke peaks at 870 nm increased from ∼ 0.9 to ∼ 0.96.Bscat401∕Bscat870 was used as a proxy for the size and“photochemical age” of the smoke particles, with this interpretation beingsupported by the simultaneously observed ratios of reactive trace gases toCO. The size and age proxy implied that the Ångström absorptionexponent decreased significantly after about 10 h of daytime smoke aging,consistent with the only airborne measurement of the BrC lifetime in anisolated plume. However, our results clearly show that non-BC absorption canbe important in “typical” regional haze and moderately aged smoke, with BrCostensibly accounting for about half the absorption at 401 nm on average forour entire data set.
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- Award ID(s):
- 1748266
- PAR ID:
- 10127271
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 19
- Issue:
- 6
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 3905 to 3926
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-19-3905-2019
