Size distribution measurements of wildfire smoke-influences aerosol at Yosemite National Park

Dec 2004.Includes bibliographical references.Sponsored by National Park Service H2380040002 to 04-56

Aerosol Mass and Optical Properties, Smoke Influence on O\u3csub\u3e3\u3c/sub\u3e, and High NO\u3csub\u3e3\u3c/sub\u3e Production Rates in a Western U.S. City Impacted by Wildfires

Evaluating our understanding of smoke from wild and prescribed fires can benefit from downwind measurements that include inert tracers to test production and transport and reactive species to test chemical mechanisms. We characterized smoke from fires in coniferous forest fuels for \u3e1,000 hr over two summers (2017 and 2018) at our Missoula, Montana, surface station and found a narrow range for key properties. ΔPM2.5/ΔCO was 0.1070 ± 0.0278 (g/g) or about half the age-independent ratios obtained at free troposphere elevations (0.2348 ± 0.0326). The average absorption Ångström exponent across both years was 1.84 ± 0.18, or about half the values available for very fresh smoke. Brown carbon (BrC) was persistent (~50% of absorption at 401 nm) in both years, despite differences in smoke age. ΔBC/ΔCO doubled from 2017 to 2018, but the average across 2 years was within 33% of recent airborne measurements, suggesting low sampling bias among platforms. Switching from a 1.0 to a 2.5 micron cutoff increased the mass scattering and mass absorption coefficients, suggesting often overlooked supermicron particles impact the optical properties of moderately aged smoke. O3 was elevated ~6 ppb on average over a full diurnal period when wildfire smoke was present, and smoke-associated O3 increases were highest (~9 pbb) at night, suggesting substantial upwind production. NOx was mostly local in origin. NOx spurred high rates of NO3 production, including in the presence of wildfire smoke (up to 2.44 ppb hr−1) and at least one nighttime BrC secondary formation event that could have impacted next-day photochemistry

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

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 ∼ 500h (up to 471μ-3 hourly average PM2.5). We measured wildfire trace gases, PM2.5 (particulate matter ≤2.5μm in diameter), and black carbon and submicron aerosol scattering and absorption at 870 and 401nm. 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 ∼ 2-3h 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 870nm increased from ∼ 0.9 to ∼ 0.96. B/scat401 B/scat870 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 10h 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 401nm on average for our entire data set

Rapidly evolving ultrafine and fine mode biomass smoke physical properties: Comparing laboratory and field results

Combining field and laboratory results, we present biomass smoke physical properties. We report sub-0.56 µm diameter (Dp) particle sizing (fast mobility particle sizer, FMPS) plus light absorption and scattering at 870nm (photoacoustic extinctiometer). For Dp\u3c200 \u3enm, the FMPS characterized sizing within ±20% compared to standards. As compared to the traditional scanning mobility particle sizer, the FMPS responded most accurately to single-mode polydispersions with mean Dp\u3c200 \u3enm, which characterized the smoke sampled here. Smoke was measured from laboratory fresh emissions (seconds to hours old), the High Park Fire (hours to\u3c1 \u3eday), and from regional biomass burning (several days). During a High Park Fire episode, light extinction rapidly reached a maximum of σep = 569 ± 21Mm-1 (10 min) with aerosol single scattering albedo peaking at ω= 0.955 ± 0.004. Concurrently, number concentration and size peaked with maximum Dp = 126nm and a unimodal distribution with σg = 1.5. Long-range transported smoke was substantially diluted (Ntot factor of 7 lower) and shifted larger (maximum Dp = 143) and wider (σg = 2.2). We compared ambient data to laboratory burns with representative western U.S. forest fuels (coniferous species Ponderosa pine and Alaska black spruce). Smoldering pine produced an aerosol dominated by larger, more strongly light scattering particles (Dp\u3e100 nm), while flaming combustion produced very high number concentrations of smaller (Dp ~ 50 nm) absorbing particles. Due to smoldering and particle growth processes, Dp approached 100nm within 3 h after emission. Increased particle cross-sectional area and Mie scattering efficiency shifted the relative importance of light absorption (flaming maximum) and light scattering (smoldering maximum), increasing ω over time. Measurements showed a consistent picture of smoke properties from emission to aging

Strong impact of wildfires on the abundance and aging of black carbon in the lowermost stratosphere

Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (∼140 ng·m$^{-3}$, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m$^{-3}$) with more than 100-fold enhanced peak values (up to ∼720 ng·m$^{-3}$). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼1 nm·h$^{-1}$ due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate

High Relative Humidity as a Trigger for Widespread Release of Ice Nuclei

<div><p>Copyright 2014 American Association for Aerosol Research</p></div

Open-path, closed-path, and reconstructed aerosol extinction at a rural site

<p>The Handix Scientific open-path cavity ringdown spectrometer (OPCRDS) was deployed during summer 2016 in Great Smoky Mountains National Park (GRSM). Extinction coefficients from the relatively new OPCRDS and from a more well-established extinction instrument agreed to within 7%. Aerosol hygroscopic growth (<i>f</i>(RH)) was calculated from the ratio of ambient extinction measured by the OPCRDS to dry extinction measured by a closed-path extinction monitor (Aerodyne’s cavity-attenuated phase shift particulate matter extinction monitor [CAPS PMex]). Derived hygroscopicity (relative humidity [RH] < 95%) from this campaign agreed with data from 1995 at the same site and time of year, which is noteworthy given the decreasing trend for organics and sulfate in the eastern United States. However, maximum <i>f</i>(RH) values in 1995 were less than half as large as those recorded in 2016—possibly due to nephelometer truncation losses in 1995. Two hygroscopicity parameterizations were investigated using high-time-resolution OPCRDS+CAPS PMex data, and the <i>κ</i>_ext model was more accurate than the gamma model. Data from the two ambient optical instruments, the OPCRDS and the open-path nephelometer, generally agreed; however, significant discrepancies between ambient scattering and extinction were observed, apparently driven by a combination of hygroscopic growth effects, which tend to increase nephelometer truncation losses and decrease sensitivity to the wavelength difference between the two instruments as a function of particle size. There was not a statistically significant difference in the mean reconstructed extinction values obtained from the original and the revised IMPROVE (Interagency Monitoring of Protected Visual Environments) equations. On average, IMPROVE reconstructed extinction was ~25% lower than extinction measured by the OPCRDS, which suggests that the IMPROVE equations and 24-hr aerosol data are moderately successful in estimating current haze levels at GRSM. However, this conclusion is limited by the coarse temporal resolution and the low dynamic range of the IMPROVE reconstructed extinction.</p> <p><i>Implications</i>: Although light extinction, which is directly related to visibility, is not directly measured in U.S. National Parks, existing IMPROVE protocols can be used to accurately infer visibility for average humidity conditions, but during the large fraction of the year when humidity is above or below average, accuracy is reduced substantially. Furthermore, nephelometers, which are used to assess the accuracy of IMPROVE visibility estimates, may themselves be biased low when humidity is very high. Despite reductions in organic and sulfate particles since the 1990s, hygroscopicity, particles’ affinity for water, appears unchanged, although this conclusion is weakened by the previously mentioned nephelometer limitations.</p