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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Nov 2018

Research article | 21 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

In-situ measurements of trace gases, PM, and aerosol optical properties during the 2017 NW US wildfire smoke event

Vanessa Selimovic1, Robert J. Yokelson1, Gavin R. McMeeking2, and Sarah Coefield3 Vanessa Selimovic et al.
  • 1Department of Chemistry, University of Montana, Missoula, 59812, USA
  • 2Handix Scientific LLC, 5485 Conestoga Court, Suite 104B, Boulder, CO, 80301, USA
  • 3Missoula City-County Health Department, Missoula, MT, 59801, USA

Abstract. In mid-August through mid-September of 2017 a major wildfire smoke/haze episode strongly impacted most of the NW US and SW Canada. During this period our ground-based site in Missoula, MT experienced heavy smoke impacts for ~500 hours (up to 471µgm−3 hourly average PM2.5). We measured wildfire trace gases, PM2.5, and black carbon and sub-micron aerosol scattering and absorption at 870 and 401nm. This may be the most extensive real-time data for these wildfire smoke properties to date. Our range of trace gas ratios for ΔNH3/ΔCO and ΔC2H4/ΔCO confirmed that the smoke from mixed, multiple sources varied in age from ~2–3 hours to ~1–2 days. Our study-average ΔCH4/ΔCO ratio (0.166±0.088) indicated a large contribution to the regional burden from inefficient “smoldering” combustion. Our ΔBC/ΔCO ratio (0.0012±0.0005) for our ground site was moderately lower than observed in aircraft studies (~0.0015) to date, also consistent with a relatively larger contribution from smoldering combustion. Our ΔBC/ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC, mostly organic nature of the smoke observed in airborne studies of wildfire smoke to date. Smoldering combustion is usually associated with enhanced PM emissions, but our ΔPM2.5/ΔCO ratio (0.126±0.002) was about half the ΔPM1.0/ΔCO measured in fresh wildfire smoke from aircraft (~0.266). Assuming PM2.5 is dominated by PM1, this suggests that aerosol evaporation, at least near the surface, can often reduce PM loading and its atmospheric/air-quality impacts on the time scale of several days. Much of the smoke was emitted late in the day suggesting that nighttime processing would be important in the early evolution of smoke. The diurnal trends show BrC, PM2.5, and CO peaking in early morning and BC peaking in early evening. Over the course of one month, the average single scattering albedo for individual smoke peaks at 870nm 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 being supported by the simultaneously-observed ratios of reactive trace gases to CO. The size/age proxy implied that the Ångström absorption exponent decreased significantly after about ten hours of daytime smoke aging, consistent with the only airborne measurement of the brown carbon (BrC) lifetime in an isolated plume. However, our results clearly show that non-BC absorption can be important in “typical” regional haze/moderately-aged plumes with BrC ostensibly accounting for about half the absorption at 401nm on average for our entire data set.

Vanessa Selimovic et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Vanessa Selimovic et al.
Vanessa Selimovic et al.
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Short summary
A massive wildfire smoke episode impacted the western US and Canada in summer 2017. We measured CO, other trace gases, PM, BC, and aerosol optical properties at a heavily-impacted, ground-based site embedded in this event. Brown carbon diminished as smoke aged, but was a persistent component of the regional smoke, accounting for about half of aerosol absorption at 401 nm on average. The PM/CO ratios suggested that aerosol evaporation was dominant at the surface at smoke ages of up to ~1-2 days.
A massive wildfire smoke episode impacted the western US and Canada in summer 2017. We measured...