<p>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 471 µg m<sup>−3</sup> hourly average PM<sub>2.5</sub>). We measured wildfire trace gases, PM<sub>2.5</sub>, and black carbon and sub-micron 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 ΔNH<sub>3</sub>/ΔCO and ΔC<sub>2</sub>H<sub>4</sub>/ΔCO confirmed that the smoke from mixed, multiple sources varied in age from ~ 2–3 hours to ~ 1–2 days. Our study-average ΔCH<sub>4</sub>/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/ΔPM<sub>2.5</sub> 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 ΔPM<sub>2.5</sub>/ΔCO ratio (0.126 ± 0.002) was about half the ΔPM<sub>1.0</sub>/ΔCO measured in fresh wildfire smoke from aircraft (~ 0.266). Assuming PM<sub>2.5</sub> is dominated by PM<sub>1</sub>, 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, PM<sub>2.5</sub>, 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 870 nm increased from ~ 0.9 to ~ 0.96. B<sub>scat401</sub>/B<sub>scat870</sub> 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 401 nm on average for our entire data set.</p>