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.
Wildfire smoke may interfere with the use of black carbon as an indicator of traffic exposure
