资源环境科技发展态势分析平台

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 similar to 500 h (up to 471 mu g m(-3) hourly average PM2.5). We measured wildfire trace gases, PM2.5 (particulate matter <= 2.5 mu m 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 Delta NH3/Delta CO and Delta C2H4/Delta CO confirmed that the smoke from mixed, multiple sources varied in age from similar to 2-3 h to similar to 1-2 days. Our study-average Delta CH4/Delta CO ratio (0.166 +/- 0.088) indicated a large contribution to the regional burden from inefficient smoldering combustion. Our Delta BC/Delta CO ratio (0.0012 +/- 0.0005) for our ground site was moderately lower than observed in aircraft studies (similar to 0.0015) to date, also consistent with a relatively larger contribution from smoldering combustion. Our Delta BC/Delta PM2.5 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 Delta PM2.5/Delta CO ratio (0.126 +/- 0.002) was about half the Delta PM1.0/Delta CO measured in fresh wildfire smoke from aircraft (similar to 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 similar to 0.9 to similar to 0.96. B(scat)401/B(scat)870 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 Angstrom 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.