Wildfire atmospheric chemistry: climate and air quality impacts

Abstract

Wildfires emit significant amounts of material into the atmosphere. To fully understand the impact of these emissions an accurate understanding of wildfire smoke chemistry is needed. This perspective highlights our chemical understanding and research gaps regarding the impacts of wildfire smoke on air quality and climate. Wildfires emit significant amounts of material into the atmosphere. To fully understand the impact of these emissions an accurate understanding of wildfire smoke chemistry is needed. This perspective highlights our chemical understanding and research gaps regarding the impacts of wildfire smoke on air quality and climate. Higher temperatures and increased fuel aridity are driving more frequent and intense wildfire events, such as in the western USA [1.Abatzoglou J.T. Williams A.P. Impact of anthropogenic climate change on wildfire across western US forests.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 11770-11775Crossref PubMed Scopus (1043) Google Scholar] where they have led to increases in levels of total atmospheric particulate matter (PM) [2.Jaffe D.A. et al.Wildfire and prescribed burning impacts on air quality in the United States.J. Air Waste Manag. Assoc. 2020; 70: 583-615Crossref PubMed Scopus (51) Google Scholar]. Heat waves and extreme wildfire seasons are partly driven by climate change [3.Kirchmeier-Young M.C. et al.Attributing extreme fire risk in Western Canada to human emissions.Clim. Chang. 2017; 144: 365-379Crossref PubMed Scopus (59) Google Scholar], with increased wildfire activity also caused by human activity and forest management [2.Jaffe D.A. et al.Wildfire and prescribed burning impacts on air quality in the United States.J. Air Waste Manag. Assoc. 2020; 70: 583-615Crossref PubMed Scopus (51) Google Scholar]. Our attention is drawn to extreme events, such as occurred in Lytton, British Columbia, which set the record in summer 2021 for the highest temperature (49.6 oC) ever recorded in Canada and burned to the ground a few days later. The focus of this article is on the chemistry of atmospheric wildfire emissions. While wildfire science has traditionally encompassed the fields of forestry and ecology, the atmospheric chemistry community is now addressing the role of fires in climate warming and air quality. The challenges are to decipher the atmospheric chemical transformations associated with photochemical activity and cloud processing, and to then connect the chemical composition, hygroscopic and optical properties of the evolving wildfire plumes to their climate and air quality impacts. A schematic summary of the atmospheric processes discussed throughout this text and their impacts are presented in Figure 1. Both lightning and humans initiate fires, which then pass through two distinct phases. The high temperatures of the burning phase lead to higher emissions of carbon dioxide (CO2), soot (sometimes called black carbon, BC), and reactive nitrogen oxides (NOx) relative to a lower temperature smoldering phase, which releases relatively more carbon monoxide (CO) and organic compounds. The thousands of different organic compounds emitted from either phase range from greenhouse gases such as methane (CH4) to molecules with much lower volatility, such as multi-ring polyaromatic hydrocarbons (PAHs), that are largely particulate-bound. While oxygen can be fully consumed in the fire, air is plentifully entrained into the atmospheric plume. As a result, the oxidation processes initiated in the fire continue in the atmosphere, albeit at a considerably slower rate. Sunlight-driven photochemistry leads to gas-phase radical production [for example, from photolysis of nitrous acid (HONO)], which in turn drives the oxidation of gas-phase organic molecules and the formation of secondary products, such as ozone (O3). As in urban settings, this chemistry is highly nonlinear, dependent on the chemical conditions present in the plume, the time of day, and the distance downwind of the fire [4.Jaffe D.A. Wigder N.L. Ozone production from wildfires: A critical review.Atmos. Environ. 2012; 51: 1-10Crossref Scopus (291) Google Scholar]. In particular, the ozone production rate drops as the plume ages, transitioning from a volatile organic compound (VOC)-sensitive regime to one that is NOx-sensitive [5.Xu L. et al.Ozone chemistry in western U.S. wildfire plumes.Sci. Adv. 2021; 7eabl3648Crossref Scopus (6) Google Scholar]. This opens up the potential for additional O3 formation when the plume encounters urban NOx. Wildfire PM chemical composition also evolves in the plumes. Air entrainment and dilution lead to the evaporation of semivolatile particulate organics to the gas phase. Offsetting this loss of particulate mass, simultaneous oxidation of gas-phase organic emissions forms more oxidized molecules, increasing the pool of secondary semivolatile species that partition to the aerosol particles [6.Palm B.B. et al.Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes.Proc. Natl. Acad. Sci. U. S. A. 2020; 117: 29469-29477Crossref PubMed Scopus (41) Google Scholar]. The net result is that the organic PM is sustained as a wildfire plume for ages in the atmosphere, even though significant chemical change and dilution is occurring. Meteorological conditions, such as relative humidity and temperature, play an important role in mediating these transformations, with cold, dry environments increasing the viscosity of organic PM and slowing down the chemistry it undergoes. Wildfire CO2 and BC PM have well recognized climate-warming effects, being two of the strongest radiative forcing agents in the atmosphere. The atmospheric lifetime of CO2 is sufficiently long that emissions at one location are felt globally. The behavior of BC is more complex because these particles deposit to the Earth’s surface by precipitation, which can darken the surfaces of mountain glaciers and Arctic ice sheets. Not only may this accelerate melting but also potentially impact the hydrological cycle and freshwater supply in some regions. Understanding these effects requires knowledge of the hygroscopic properties of BC-containing particles and their ability to be scavenged by clouds. The evolving chemical composition and associated optical properties of wildfire organic PM are also important. These particles cool the atmosphere by scattering light. However, they also contain chromophores, such as functionalized aromatics and PAHs, that absorb visible and ultraviolet sunlight, giving rise to brown carbon (BrC) PM. Climate models estimate the warming role of BrC to be 25% that of BC, with most BrC having wildfire origins [7.Zhang A. et al.Modeling the global radiative effect of brown carbon: a potentially larger heating source in the tropical free troposphere than black carbon.Atmos. Chem. Phys. 2020; 20: 1901-1920Crossref Scopus (39) Google Scholar]. Initial evidence from the field is that BrC particles 'whiten' with a timescale of a day or two in the atmosphere, but the chemical processes driving this reduction in absorption are not fully known [8.Hems R.F. et al.Aging of atmospheric brown carbon aerosol.ACS Earth Space Chem. 2021; 5: 722-748Crossref Scopus (31) Google Scholar]. Laboratory studies have indicated a variety of mechanisms that can change the optical properties of BrC particles, driven by photoreactions with sunlight, heterogeneous oxidation with OH, NO3 or O3, and cloud processing. As well, the secondary organic aerosol material that forms upon atmospheric oxidation can absorb light. It is important to determine the composition and optical properties of wildfire smoke close to the fires in the near field, given the huge impact of fires on regional climate. However, BC absorption and residual BrC absorption after whitening drive the climate impacts on the global atmosphere far from the wildfire. Pollutants such as PM and O3 in wildfire smoke have strong negative health impacts. Epidemiological studies have linked exposure to wildfire smoke to negative respiratory outcomes (asthma, chronic obstructive pulmonary disease) and potentially negative cardiovascular effects [9.Reid C.E. et al.Critical review of health impacts of wildfire smoke exposure.Environ. Health Perspect. 2016; 124: 1334-1343Crossref PubMed Scopus (423) Google Scholar]. Initial indications are that wildfire smoke particles are more toxic by mass than other forms of carbonaceous aerosol [10.Aguilera R. et al.Wildfire smoke impacts respiratory health more than fine particles from other sources: observational evidence from Southern California.Nat. Commun. 2021; 12: 1493Crossref PubMed Scopus (65) Google Scholar]. The reasons for this are not well known but may lie in part with the PAH content and high aromaticity of wildfire PM. As well, functionalized aromatics in the smoke, such as quinones, are efficient at promoting redox processes, which can drive oxidative stress in the body after inhalation. Current standard air-quality monitoring techniques cannot uniquely identify wildfire smoke, making assessment of human exposure levels not straightforward in regions that routinely experience both urban and wildfire pollution [11.Schneider S.R. et al.Quality data approach for defining wildfire influence: impacts on PM2.5, NO2, CO, and O3 in Western Canadian cities.Environ. Sci. Technol. 2021; 55: 13709-13717Crossref PubMed Scopus (2) Google Scholar]. Indeed, many cities in western North America are regularly exposed to summer wildfire smoke at the same time they experience enhanced photochemical urban smog. As well, there are indications that the toxicity of chemically processed wildfire PM may be higher than that of freshly emitted wildfire PM [12.Wong J.P.S. et al.Effects of atmospheric processing on the oxidative potential of biomass burning organic aerosols.Environ. Sci. Technol. 2019; 53: 6747-6756Crossref PubMed Scopus (36) Google Scholar]. With their increasing prevalence, wildfire air pollutants – especially PM – provide a challenge to current air-quality regulations [13.Knorr W. et al.Wildfire air pollution hazard during the 21st century.Atmos. Chem. Phys. 2017; 17: 9223-9236Crossref Scopus (39) Google Scholar]. While marked progress has been made in industrially developed countries in lowering primary emissions from industry and traffic, wildfire pollution is harder to control. In the western USA, reductions in anthropogenic PM have been offset by increases in wildfire PM (Figure 2; [14.McClure C.D. Jaffe D.A. US particulate matter air quality improves except in wildfire-prone areas.Proc. Natl. Acad. Sci. U. S. A. 2018; 115: 7901-7906Crossref PubMed Scopus (154) Google Scholar]). In the USA and Canada, air pollution during wildfire 'extreme events' is disregarded when reporting yearly average air quality levels because the fires are not readily amenable to environmental regulation. Recently, the World Health Organization changed their air-quality guidelines for PM to values below most national levels, reflecting the severe health effects of PM and creating a challenge for regions experiencing frequent wildfire events to achieve those standards. It is important to note that wildfire air-quality issues have been shown to disproportionately affect vulnerable populations [15.Davies I.P. et al.The unequal vulnerability of communities of color to wildfire.PLoS One. 2018; 13e0205825Crossref Scopus (59) Google Scholar]. As we continue to lower anthropogenic emissions, with additional improvements on the horizon as renewable energy and electric cars take hold, there are prospects for relatively remote regions having routinely worse air quality during wildfire seasons than some highly populated regions. The atmospheric chemistry community aspires to connect the chemical complexity and evolution of wildfire smoke to its associated toxicological, optical, and hygroscopic properties to generate increasingly accurate climate and air-quality assessments. Appropriately representing the detailed chemical composition and transformations of smoke presents a significant challenge to laboratory studies that relate smoke composition, evolution, and properties because composition depends on fuel type, water content, fire state, and dilution. With a full chemical characterization of wildfire smoke, a particularly ripe field for future studies is deciphering the mechanistic connections of chemical composition to adverse health outcomes. To compliment laboratory experiments, there is also the need for additional on-line, in situ measurements that capture chemical evolution within plumes, both in the near- and far-field, and are sensitive to short-lived species such as radicals and weakly bound, multifunctional species. More accurate representations of wildfire smoke in air quality and climate assessments empower the public and policy makers to take meaningful action to minimize risks to climate and health associated with wildfire smoke. No interests are declared.