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. 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  µ g m −3 hourly averagePM 2.5 ). We measured wildfire trace gases, PM 2.5 (particulate matter ≤2.5 µ m 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-average ΔCH4∕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 PM 2.5 isdominated by PM 1 , 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),PM 2.5 , 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 Angstrom 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.

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