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

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.

Rapid Evolution of Aerosol Particles and their Optical Properties Downwind of Wildfires in the Western U.S.

“…In the size range of our samples, backscatter ratios and scattering Angstrom exponents decrease with particle size (Selimovic et al, 2019). An anti-correlation between these intensive parameters and MSE is expected and is observed (Figs.…”

“…On a global basis, wildfire aerosols are estimated to have a net direct radiative effect of 0.17 watts m -2 , 1-σ uncertainty = -0.45 -+0.15 W m -2 (Bond et al, 2013) The rapid evolution of aerosol optical properties of fresh smoke contributes to the radiative effect uncertainty (Yokelson et al, 2009;Akagi et al, 2012;May et al, 2015;Vakkari et al, 2018;Selimovic et al, 2019). In this study we are concerned with 35 changes in wildfire optical properties that occur in the near-field, extending from the time it takes a smoke plume to rise to aircraft-sampling altitude to circa three hours downwind.…”

Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol

“…Forrister et al 2015 collected filter samples in the plumes of wildfires with different transport times during the SEAC4RS campaign, and found that the BrC absorbance lifetime at 370 nm was 9-15 hours. Similarly, Selimovic et al 2019 found a significant decrease in AAE 420 after 10 hours of daytime aging during a wildfire event in the Northwestern US. Sumlin et al, 2017 aged smoldering peat BBOA in an OFR, and reported a decrease of ~40-50% in the aerosol mass absorption coefficients at 375 nm and 405 nm after 4.5 equivalent aging days.…”

Emission of trace gases and aerosols from biomass burning – An updated assessment

“…In 20 view of the importance of the number concentrations of aerosol particles, especially cloud condensation nuclei, for climate change, it is unfortunate that there have only been a few additional measurements of their emission factors in the last two decades. Another climate-relevant component, for which we have no emission data at this time, is brown carbon (Andreae and Gelencsér, 2006), which has been shown to account for about half of the aerosol light absorption by biomass smoke at 401 nm (Selimovic et al, 2019) and 25-45 % at 550 nm (Tian et al, 2019). 25…”