Fire Influences on Atmospheric Composition, Air Quality and Climate
Current Pollution Reports
Fires impact atmospheric composition through their emissions, which range from long-lived gases to short-lived gases and aerosols. Effects are typically larger in the tropics and boreal regions, but can also be substantial in highly populated areas in the northern mid-latitudes. In all regions, fire can impact air quality and health. Similarly, its effect on large-scale atmospheric processes, including regional and global atmospheric chemistry and climate forcing, can be substantial, but this
... mains largely unexplored. The impacts are primarily realised in the boundary layer and lower free troposphere, but can also be noticeable in upper troposphere/lower stratosphere (UT/LS) region, for the most intense fires. In this review, we summarise the recent literature on findings related to fire impact on atmospheric composition, air quality, and climate. We explore both observational and modelling approaches, and present information on key regions and on the globe as a whole. We also discuss the current and future directions in this area of research, focusing on the major advances in emissions estimates, the emerging efforts to include fire as a component in Earth system modelling, and the use of modelling to assess health impacts of fire emissions. Introduction Fire is a central component of the Earth system [1, 2] . There are several key linkages between fire, the atmosphere, and the biosphere, with implications for climate change, ecosystems, human health, and the economy. Forest fire activity is strongly affected by climate change, since temperature, humidity, and precipitation can affect the flammability of vegetation [3, 4] , and since vegetation itself undergoes changes driven by climate change. Furthermore, fire activity depends on the ignition sources available in a region, which can be natural (lightning) or anthropogenic (accidental or deliberate)  , with the natural sources potentially influenced by climate change as well. Fire activity has changed drastically in the past, and is expected to change in the future. Specifically, it has been suggested that from the preindustrial to the present-day, fire activity increased until 1950 and subsequently decreased due to anthropogenic management  , while in the future it will potentially increase due to the climate warming expected [5, 6, 7] . At the same time, fire activity shows large interannual variations both on global and on regional scales  . The atmosphere is affected strongly by these changes in fire activity  through large amounts of emissions that can affect pollution levels, atmospheric chemistry, and climate. There are both gaseous and aerosol emissions from fires and associated biomass burning. Carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), and non-methane volatile organic compounds (NMVOCs) are the most important emitted gases, and black carbon (BC) and organic carbon (OC) are the most important aerosol species [10, 11, 12] . CO2 emissions can have important implications for CO2 concentrations and understanding the carbon cycle, but do not directly affect atmospheric chemistry or contribute to air quality degradation. Other carbonaceous emissions (BC, OC, CO, NMVOCs, and methane (CH4), for which biomass burning is only a minor source) and NOx can be important for both air pollution/chemistry and climate change. NOx, NMVOCs, CO, and methane are important precursors of tropospheric ozone, which is a secondary pollutant and is not directly emitted by fires [9,10]. Such strong effects have been demonstrated in measurement and modelling studies, on scales ranging from local to global, but significant uncertainty still remains. In this paper, we provide a review of the current understanding of such effects, focusing on the recent literature. We examine the impacts of fire emissions on the abundances of gases and aerosols, and continue with a section on related radiative forcing and climate effects. Subsequently, we provide a critical discussion on future directions that will lead to better understanding of fire effects on atmospheric composition, air quality, and climate, with an emphasis on improved emissions estimates, emerging integrated modelling, and understanding of implications for human health. Effects on atmospheric composition Emissions estimates In an atmospheric composition context, the starting point for biomass-burning studies at large scale is to estimate emissions for different species. Satellite observations are the only practical means of estimating biomass-burning emissions at a global scale in a consistent way. There are currently two approaches to doing so. The first is to estimate total fuel consumption as the product of the area burned and the fuel consumed per unit area. This approach is used for the series of Global Fire Emissions Database (GFED) products , Fire Inventory from NCAR (FINN)  and, for particulate emissions, the Fire Locating and Modeling of Burning Emissions inventory (FLAMBE) . The other approach is to infer fuel consumption from the Fire Radiative Power (FRP) determined from thermal infrared measurements. The Global Fire Assimilation System (GFAS)  and Quick Fire Emissions Database (QFED)  both use this approach. FRP-based estimates can be made in near-real time because they use thermal infrared signatures directly, rather then an areaburned estimate based on the difference in burn scar signature before and after a fire. This makes FRP-based approaches better suited to real time air quality forecasting systems such as the EU Monitoring Atmospheric Composition and Climate (MACC) forecasting system and the NASA GEOS-5 Aerosol Modeling and Data Assimilation System. FRP is converted to dry matter fuel consumption via biome-specific conversion factors. Emissions estimates for key species, namely CO2, CO, methane, NOx, OC and BC from GFEDv3, FINNv1 and GFAS, and for total particulate matter (TPM) from the above databases and from FLAMBE are summarized in Table 1 to provide a sense, globally, of the spread across estimates. Gases In the latest (Fifth) Assessment Report of the Intergovernmental Panel for Climate Change (IPCC AR5), it is stated that no estimate for the role of fire in driving long-term (e.g. Holocene) CO2 and climate changes can be given because fire is a newly studied component of carbon cycle studies . A growing body of work, however, indicates an important role. Liu et al.  suggest the fires are one of the important atmospheric CO2 sources and contribute substantially to the global greenhouse effect. On interannual timescales, recent analysis  suggested that fires can be a significant driver of atmospheric CO2 variability, along with the influences of temperature and drought on net ecosystem exchange. Non-CO2 effects of fires are also very important. To obtain emissions of different constituents for use in atmospheric modelling, emission factors are used. These represent the amount of the chemical species released per mass of biomass burned, and are typically obtained from measurement campaigns in biomass burning regions [11, 21, 22] . The largest gaseous non-CO2 fire emissions are those of CO, which are a result of incomplete combustion, especially from smouldering peat fires  . Amongst the short-lived constituents emitted from fires, CO has the largest potential to impact atmospheric composition on large geographical scales (hemispheric, or even global), due to its comparatively long lifetime of a few months. Mao et al.  found that the global CO burden increases linearly with step-wise increases in gaseous biomass burning emissions. Using a nudged global composition-climate model in conjunction with observations, the interannual variability of the global CO burden has been shown to be almost entirely driven by biomass burning emissions by Voulgarakis et al. [24, 9] , overshadowing the influence of other factors such as direct anthropogenic emissions and meteorology. The "hot-spots" of CO interannual variability  were found to be the Maritime Continent and South America, with strong variability also over Siberia and Canada. The interannual variations in firegenerated CO are closely linked to El Niño Southern Oscillation (ENSO) climate variability, which affects drought conditions primarily in Equatorial Asia and South America and subsequently fire activity  with both regional and global impacts. Along the same lines, observational analysis  showed that fires drive the interannual variability of tropical upper tropospheric CO, with the Maritime Continent being the primary contributor. The effect of ENSO on tropospheric CO via fires is also realised in regions far from the tropics. A chemistry-transport model (CTM) study  found that fire emissions drive 66-93% of Arctic CO interannual variability, with ENSO being a major driver. They suggest that high latitude winter/spring precipitation is influenced by ENSO, which drives fluctuations in fire activity, and, therefore, fire emissions and atmospheric CO. This effect, they claim, could become more severe in the future, as boreal forest regions become hotter and drier. Without examining ENSO influences, Strode et al.  discuss how biomass burning strongly impacts the interannual variability of CO in northern mid-latitudes, which affects the ability to detect anthropogenically-driven trends in CO. Fire also has significant impacts on regional CO pollution levels. For Southeast Asia, Aouizerats et al.  found that during the fire season of 2006 (El Niño year), CO concentrations in the station of Bukit Kototabang (Sumatra) were elevated above 250 ppb in several occasions, with the most pronounced enhancements (>1000 ppb) found in October. They demonstrate that these large enhancements can be simulated successfully with a regional meteorology/chemistry model using the GFED3 emissions dataset, while the smaller increases due to smaller fires in earlier months cannot be fully captured. Reddington et al.  used back-trajectories and global modelling to examine the contributions from fires to surface CO over Singapore between 2004-2007, and found that the highest amount of fire-produced CO transported to Singapore occurs in September and October, with the strongest contribution coming from fires in southern Sumatra and a secondary contribution from fires in Indonesian Borneo. They also found substantial seasonal and interannual variability in those transport patterns. Using global modelling, Marlier et al.  examined six tropical stations in Equatorial Asia, Africa and South America, and found that including fire emissions from GFED3 improved the ability of the model to capture observed CO in most stations. Other important gaseous species emitted from fires mainly include NOx and NMVOCs [10, 11, 22] , with their atmospheric effects being less thoroughly examined in the literature, but with evidence that they can be important (for example it was suggested that fires have a widespread impact on atmospheric abundances of benzene, an important NMVOC pollutant  ). These fire-driven NOx and NMVOC increases, along with the earlier mentioned CO increases, can lead to significant tropospheric ozone production, since all the above-mentioned species are ozone precursors. Jaffe and Wigder  thoroughly examined ozone production from fires in the literature, and particularly focused on observed enhancement ratios of ozone to CO (dO3/dCO) as indicative of ozone production. They found dO3/dCO to range from -0.1 to 0.9, due to factors such as fire emissions, efficiency of combustion, chemical and photochemical reactions, aerosol effects on chemistry and radiation, and local and downwind meteorological patterns. However, other recent studies suggest that positive dO3/dCO is not always an indicator of an ozone producing region [32, 33] , especially in cases where there is substantial CO loss in a plume due to intense production of OH from ozone. Parrington et al. [34, 35] discussed in detail ozone production in fire plumes in North America and the northern Atlantic. Their analysis suggests that plumes younger than 4 days feature low ozone production efficiencies that increase for older plumes, associating this feature with the relatively quick reduction of aerosol loading with plume age. Aerosols attenuate shortwave radiation and thus slow down the photolysis of NO2, which is essential for ozone production in the troposphere, while also they provide surfaces for heterogeneous uptake and loss of ozone and NOy (collective name for oxidized forms of nitrogen in the atmosphere, which facilitate ozone production)  . Global chemistry-climate model analysis  found that global tropospheric ozone production and burden linearly increase with fire emissions, as a result of the high NMVOC/NOx ratio in such emissions. They showed that co-emission of trace gases and aerosols from present-day biomass burning increases the global tropospheric ozone burden by 5.1%. Biomass burning aerosols actually drive decreases in global ozone burden in their model, primarily due to heterogeneous processes, and not due to photolysis effects, which are not important globally, but are potentially important regionally. In terms of interannual variability, Voulgarakis et al.  suggested that fire emissions had little influence on global variations of tropospheric ozone, due to global ozone being a well-buffered quantity in atmospheric models. The effect of fire emissions on regional ozone pollution levels can be significant.