Abstract
Abstract. In mid-August through mid-September of 2017 a major wildfire smoke and haze episode strongly impacted most of the NW US and SW Canada. During this period our ground-based site in Missoula, Montana, experienced heavy smoke impacts for ∼ 500 h (up to 471 µg m−3 hourly average PM2.5). We measured wildfire trace gases, PM2.5 (particulate matter ≤2.5 µm in diameter), and black carbon and submicron aerosol scattering and absorption at 870 and 401 nm. 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 h 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 (black carbon), 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 timescale 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 brown carbon (BrC), PM2.5, and CO peaking in the early morning and BC peaking in the early evening. Over the course of 1 month, the average single scattering albedo for individual 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 being supported by the simultaneously observed ratios of reactive trace gases to CO. The size and age proxy implied that the Ångström absorption exponent decreased significantly after about 10 h of daytime smoke aging, consistent with the only airborne measurement of the BrC lifetime in an isolated plume. However, our results clearly show that non-BC absorption can be important in “typical” regional haze and moderately aged smoke, with BrC ostensibly accounting for about half the absorption at 401 nm on average for our entire data set.
Highlights
Biomass burning (BB) emissions are an important source of trace gases and particles that can influence local, regional, and global atmospheric chemistry, air quality, climate forcing, and human health (Crutzen and Andreae, 1990)
There were more than 20 distinct periods of major smoke impacts that are readily identified by large simultaneous enhancements in CO, black carbon (BC), and PM2.5
The highest hourly values were observed on 4 September 2017, the morning after the Rice Ridge Fire doubled in size (PM2.5, 471 μg m−3, CO 2.78 ppm, BC 3.62 μg m−3)
Summary
Biomass burning (BB) emissions are an important source of trace gases and particles that can influence local, regional, and global atmospheric chemistry, air quality, climate forcing, and human health (Crutzen and Andreae, 1990). Over the last few decades, the annual number of wildfires in the US has not changed significantly, but the annual area burned has increased by a factor of about 3 (United States National Interagency Fire Center, 2019), and many of the highest burnedarea years have coincided with many of the warmest years on record (United States Environmental Protection Agency, 2019). Despite these important issues, many of the emissions from BB remain either understudied or completely unstudied. Most of the research on the emissions and evolution of smoke from US fires in the field has targeted prescribed fires (Burling et al, 2011; Akagi et al, 2013; Yokelson et al, 2013a; May et al, 2014; Müller et al, 2016), and while there are studies that probe trace gas and optical property emissions of wildfire smoke sampled in the field (Liu et al, 2017; Lindaas et al, 2017; Landis et al, 2017; Collier et al, 2016; Eck et al, 2013; Sahu et al, 2012; Lack et al, 2012), much of the information is limited in temporal extent or incomplete chemically and fails to assess important issues such as the aging and evolution of smoke over varying and extended amounts of time, nighttime evolution and oxidation, or the contribution of constituents of increasingly recognized importance such as BrC (UV-absorbing OA), to name a few
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