The Far-Reaching Consequences of Wildfire Smoke Plumes

Smoke from wildfires burning in the western United States carries harmful pollutants across the country.

Characterization of aerosol particles during a high pollution episode over Mexico City

More than 7 thousand wildfires were recorded over Mexico in 2019, affecting almost 640 thousand hectares. Most of these fires occurred during the spring season generating dense smoke plumes, impacting urban areas in the central part of the Mexican plateau. From May 10 to 17, 2019, biomass burning (BB) plumes affected Mexico City (MC) and diffused across the basin, producing PM2.5 levels ~ 2 times higher than the nation's air quality standards. Average PM2.5 concentrations increased sharply from 29.4 ± 7.2 µg m−3 to 65.1 ± 13.6 µg m−3 when the dense smoke plumes were detected. The higher particle concentration impacted the aerosol optical depth (AOD) as values ~ 3 times greater than the annual mean (0.32 ± 0.12) were measured, which resulted in a 17% loss of global horizontal irradiation (GHI). Under these severe pollution conditions, the visibility (Va) was reduced by ~ 80%. The high incidence of strong absorbent particles, such as soot and tarballs was revealed through electron microscopy and X-ray fluorescence (XRF) analysis. These techniques show chemical similarities between MC aerosols and those from the high-altitude (~ 4010 m. a. g. l.) Altzomoni Atmospheric Observatory, evidencing a strong influence of the BB emissions, suggesting a regional transport of these pollutants.

Methods for estimate the dynamic and thermal characteristics of smoke plumes

Space observations of the propagation of smoke flares from the chimneys of industrial enterprises provide information on the physical characteristics of the emitted gas-air mixtures. Models for estimating the parameters of the rise of impurities under the influence of dynamic and thermal factors are proposed. The basic relations in the estimation models are the solutions of the equations of hydrothermodynamics of the atmosphere. The case of neutral atmospheric stratification is considered in detail. Using satellite information and meteorological observation data, a numerical study of the stage of ascent of smoke jets from the chimneys of the Gusinoozerskaya State District Power Plant was carried out.

The Gas and Aerosol Phase Composition of Smoke Plumes From The 2019-2020 Black Summer Bushfires and Potential Implications For Human Health

The 2019-2020 Australian bushfire season was historically large, long, and intense. Smoke from fires burning in southeast Australia blanketed population centres for weeks to months. This study reports the chemical composition in the gas and aerosol phase of aged plumes measured near Wollongong, NSW in early 2020. Enhancement ratios to CO are presented for thirteen species. Plume composition is largely similar to that measured in fresh smoke during previous studies. It is hoped enhancement ratios reported here will assist in plume modelling of landscape scale fires and allow concentration estimates of infrequently measured atmospheric pollutants at monitoring stations. The relative toxicological contribution of species present in the plumes was determined for dilute plume exposure at the measurement site and for concentrated plumes at a heavily impacted population centre case study location. Similar results were determined for both sites. Respirable particles, formaldehyde and acrolein were found to contribute significantly to the toxicological loading, with respirable particles contributing approximately half of the loading. This is a reminder to consider not only the toxicological contributions of particles when studying health impacts of bushfire smoke exposure.

A novel method of identifying and analysing oil smoke plumes based on synergic satellite data

Black carbon aerosols are the second largest contributor to global warming while also being linked to respiratory and cardiovascular disease. These particles are generally found in smoke plumes originating from biomass burning and fossil fuel combustion. They are also heavily concentrated in smoke plumes originating from oil fires exhibiting the largest ratio of black carbon to organic carbon. In this study, we identified and analyzed oil smoke plumes derived from 30 major industrial events within a 12-year timeframe. To our knowledge, this is the first study of its kind that utilized a synergetic approach based on satellite remote sensing techniques. One objective of this study is to highlight the importance of satellite remote sensing techniques in identifying these types of events. As opposed to ground stations, satellite data offers access to remote areas all over the globe which would otherwise be very difficult to reach. Satellite data offers access to these events which, as seen in this study, are mainly located in war prone or hazardous areas. This study focuses on the use of MODIS (Moderate Resolution Imaging Spectroradiometer) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) products regarding these types of aerosol while also highlighting their intrinsic limitations. By using data from both MODIS instruments onboard Terra and Aqua satellites we addressed the temporal evolution of the smoke plume while assessing Lidar specific properties and plume elevation using CALIPSO data. We present several aerosol properties in the form of plume specific averaged values. The MODIS ocean algorithms were successful in retrieving aerosol properties which, on average, ranged from −0.06 to 0.16 for plume specific AOD, −0.18 to 1.25 for Ångström exponent and 0.29 to 1.73 µm for the effective radius. CALIPSO measurements showed values of plume AOD ranging from 0 to 0.14 (532 nm) and 0 to 0.13 (1064 nm) except for one event where AOD values showed 1.52 (532 nm) and 1.42 (1064 nm). AE values ranged from 0.11 to 0.33 which were in agreeance with MODIS values. A large discrepancy can be found in one event where CALIPSO measured AOD values 5 times higher than MODIS. This event also produced the largest lidar ratio at 109 sr (532 nm) and 86 (1064 nm). Other lidar ratio values ranged from 37 to 55 sr however these unconstrained solutions were obtained for the entire layer of which the plumes were a part of and thus did not reflect specific plume conditions. Particulate backscatter values ranged from 0.002 to 0.0017 km−1 sr−1 while extinction coefficient values ranged from 0.10 to 1.65 km−1. On average backscatter and extinction coefficient values were 2 to 9 times higher than local background values. Particulate depolarization ratios ranged from 0.11 to 0.15 in 4 out of 6 cases while the remaining two ranged from 0.27 to 0.32 where dust was highly dominant. The values represented in this study are in good agreement with similar studies that used ground based and flight measurements. We believe that MODIS values are a conservative estimation of plume AOD since MODIS algorithms rely on general aerosol models and various atmospheric conditions within the look-up tables which do not reflect the highly absorbing nature of these smoke plumes. CALIPSO measurements are heavily dependent on lidar ratios which are not directly measured if plumes within the planetary boundary layer. We also believe that AOD values based on CALIPSO measurements are conservative in nature since heavy absorbing smoke would yield larger lidar ratios and AOD values. Based on this study we conclude that the MODIS land algorithms are not yet suited for retrieving aerosol properties for these types of smoke plumes due to the strong absorbing properties of these aerosols. We believe that these types of studies are a strong indicator for the need of improved aerosol models and retrieval algorithms.

Fire-weather interaction fed the 2020 western USA gigafire

Wildfires threaten human lives, destroy infrastructure, disrupt economic activity, and damage ecosystem services. A record-breaking gigafire event ravaged the western United States (USA) in mid-September 2020, burning 1.2 million acres (4,900 km2) in Oregon and California, and resulting in severe smoke pollution with daily fine particulate matter (PM2.5) concentrations over 300 µg/m3 for multiple days in many cities. Although previous studies have shown that regional warming escalates wildfire in the western USA, such an unprecedented fire cannot be explained by climate variability alone. Here we show that the synoptic-scale feedback between the wildfires and weather played an unexpectedly important role in accelerating the spread of this fire and also trapped pollutants in the shallow boundary layer over valley cities. Specifically, we find that aerosol-radiation interaction of the smoke plumes over the Cascade Mountains enhanced the downslope winds and weakened the moisture transport, thereby forming a positive feedback loop that amplified the fires and contributed to ~54% of estimated air-pollution related deaths. Our study underscores the complexity of the Earth system and the importance of understanding fundamental mechanisms to effectively mitigate disaster risks in a changing climate.

Evaluation and intercomparison of wildfire smoke forecasts from multiple modeling systems for the 2019 Williams Flats fire

Wildfire smoke is one of the most significant concerns of human and environmental health, associated with its substantial impacts on air quality, weather, and climate. However, biomass burning emissions and smoke remain among the largest sources of uncertainties in air quality forecasts. In this study, we evaluate the smoke emissions and plume forecasts from 12 state-of-the-art air quality forecasting systems during the Williams Flats fire in Washington State, US, August 2019, which was intensively observed during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign. Model forecasts with lead times within 1 d are intercompared under the same framework based on observations from multiple platforms to reveal their performance regarding fire emissions, aerosol optical depth (AOD), surface PM2.5, plume injection, and surface PM2.5 to AOD ratio. The comparison of smoke organic carbon (OC) emissions suggests a large range of daily totals among the models, with a factor of 20 to 50. Limited representations of the diurnal patterns and day-to-day variations of emissions highlight the need to incorporate new methodologies to predict the temporal evolution and reduce uncertainty of smoke emission estimates. The evaluation of smoke AOD (sAOD) forecasts suggests overall underpredictions in both the magnitude and smoke plume area for nearly all models, although the high-resolution models have a better representation of the fine-scale structures of smoke plumes. The models driven by fire radiative power (FRP)-based fire emissions or assimilating satellite AOD data generally outperform the others. Additionally, limitations of the persistence assumption used when predicting smoke emissions are revealed by substantial underpredictions of sAOD on 8 August 2019, mainly over the transported smoke plumes, owing to the underestimated emissions on 7 August. In contrast, the surface smoke PM2.5 (sPM2.5) forecasts show both positive and negative overall biases for these models, with most members presenting more considerable diurnal variations of sPM2.5. Overpredictions of sPM2.5 are found for the models driven by FRP-based emissions during nighttime, suggesting the necessity to improve vertical emission allocation within and above the planetary boundary layer (PBL). Smoke injection heights are further evaluated using the NASA Langley Research Center's Differential Absorption High Spectral Resolution Lidar (DIAL-HSRL) data collected during the flight observations. As the fire became stronger over 3–8 August, the plume height became deeper, with a day-to-day range of about 2–9 km a.g.l. However, narrower ranges are found for all models, with a tendency of overpredicting the plume heights for the shallower injection transects and underpredicting for the days showing deeper injections. The misrepresented plume injection heights lead to inaccurate vertical plume allocations along the transects corresponding to transported smoke that is 1 d old. Discrepancies in model performance for surface PM2.5 and AOD are further suggested by the evaluation of their ratio, which cannot be compensated for by solely adjusting the smoke emissions but are more attributable to model representations of plume injections, besides other possible factors including the evolution of PBL depths and aerosol optical property assumptions. By consolidating multiple forecast systems, these results provide strategic insight on pathways to improve smoke forecasts.

Ground-based Investigation of HO_x and Ozone Chemistry in Biomass Burning Plumes in Rural Idaho

Ozone (O3), a potent greenhouse gas that is detrimental to human health, is typically found in elevated concentrations within biomass burning (BB) smoke plumes. The radical species OH, HO2, and RO2 (known collectively as ROx) have central roles in the formation of secondary pollutants including O3 but are poorly characterized for BB plumes. We present measurements of total peroxy radical concentrations ([XO2] ≡ [HO2] + [RO2]) and additional trace-gas and particulate matter measurements from McCall, Idaho during August 2018. There were five distinct periods in which BB smoke impacted this site. During BB events, O3 concentrations were enhanced as evidenced by ozone enhancement ratios (ΔO3/ ΔCO) that ranged up to 0.25 ppbv ppbv−1. [XO2] was similarly elevated during some BB events. Overall, quantified instantaneous ozone production rates (P(O3)) were only slightly impacted by the presence of smoke as NOx enhancements were minimal. Measured XO2 concentrations were compared to zero-dimensional box modeling results to evaluate the effectiveness of the Master Chemical Mechanism (MCM) and GEOS-Chem mechanisms during periods of BB influence and overall agreed within 31 %. One period of BB influence had distinct measured enhancements of 15 pptv XO2 that were not reflected in the model output, likely due to the presence of an unmeasured HOx source, quite likely nitrous acid (HONO). To our knowledge, this is the first BB study featuring peroxy radical measurements.