From the Cover: Large contribution of biomass burning emissions to ozone throughout the global remote troposphere

Ozone is the third most important anthropogenic greenhouse gas after carbon dioxide and methane but has a larger uncertainty in its radiative forcing, in part because of uncertainty in the source characteristics of ozone precursors, nitrogen oxides, and volatile organic carbon that directly affect ozone formation chemistry. Tropospheric ozone also negatively affects human and ecosystem health. Biomass burning (BB) and urban emissions are significant but uncertain sources of ozone precursors. Here, we report global-scale, in situ airborne measurements of ozone and precursor source tracers from the NASA Atmospheric Tomography mission. Measurements from the remote troposphere showed that tropospheric ozone is regularly enhanced above background in polluted air masses in all regions of the globe. Ozone enhancements in air with high BB and urban emission tracers (2.1 to 23.8 ppbv [parts per billion by volume]) were generally similar to those in BB-influenced air (2.2 to 21.0 ppbv) but larger than those in urban-influenced air (−7.7 to 6.9 ppbv). Ozone attributed to BB was 2 to 10 times higher than that from urban sources in the Southern Hemisphere and the tropical Atlantic and roughly equal to that from urban sources in the Northern Hemisphere and the tropical Pacific. Three independent global chemical transport models systematically underpredict the observed influence of BB on tropospheric ozone. Potential reasons include uncertainties in modeled BB injection heights and emission inventories, export efficiency of BB emissions to the free troposphere, and chemical mechanisms of ozone production in smoke. Accurately accounting for intermittent but large and widespread BB emissions is required to understand the global tropospheric ozone burden.

Tropospheric ozone (O₃) has been the focus of decades of scientific research due to its central role in atmospheric chemistry (1), its adverse impact on human and ecosystem health (2, 3), and its role as a climate forcer (4, 5). Despite this focus, there remains considerable uncertainty in tropospheric O₃ production pathways, precursor sources, and long-term trends. Sources of tropospheric O₃ include downward transport from the stratosphere and photochemical production from a complex set of coupled reactions between nitrogen oxides (NO_x) and volatile organic compounds (VOCs), each of which is in turn emitted from both anthropogenic and natural sources (1, 6). The contribution of fossil fuel combustion to tropospheric O₃ has recently declined in the United States and in Europe, proportionally increasing the contribution from natural sources (7–10). However, the spatial distribution of anthropogenic O₃ precursor emissions have shifted to lower latitudes (11, 12), where they are still increasing (13). Additionally, globally averaged tropospheric O₃ has increased over the past five decades (14, 15). Understanding the sources of tropospheric O₃ is thus essential to explain this trend and to inform the development of effective mitigation strategies from regional to hemispheric scales.Biomass burning (BB) is an important source of O₃ precursors (16–19). A recent study based on observed O₃ to carbon monoxide (CO) enhancements in smoke plumes attributed 3.5% of the global tropospheric chemical O₃ production to BB emissions (19). Other studies have accounted for the numerous production and destruction pathways of O₃ in the troposphere using global chemical transport models (CTMs) to estimate the global budget of O₃ (20, 21). However, few studies separately quantify the contributions of fossil fuel combustion and BB emissions to global tropospheric O₃ (22). Global inventories attribute five times more NO_x (23, 24) but roughly equal VOC emissions (17, 25) to fossil fuel combustion (hereafter referred to as urban sources) compared with BB. However, precursor emissions do not necessarily determine tropospheric O₃ production close to the sources because of the nonlinearity of O₃ formation chemistry (26, 27). Additionally, global CTMs do not always agree on the tropospheric O₃ burden, suggesting possible deficiencies with emission inventories of O₃ precursors and/or an incomplete representation of O₃ chemistry (21, 28–30), although a recent model intercomparison study showed that the model ensemble reproduced well the salient spatial, seasonal, and decadal variability and trends of tropospheric O₃ (31).Large-scale in situ observational constraints commensurate with the grid resolution of current global CTMs are rare. Instead, modeling studies often rely on ozonesonde-derived climatologies and satellite-based remote sensing observations to constrain tropospheric O₃ distributions and precursor sources (20, 32, 33). The recent NASA Atmospheric Tomography (ATom) mission provides global-scale and seasonally resolved in situ measurements of O₃ and CO and a comprehensive suite of trace gases and aerosol parameters, including tracers of BB and urban emissions (34). ATom sampled the remote troposphere from the Arctic to the Antarctic over the Pacific and Atlantic Oceans using repeated vertical profiles from ∼0.2 to ∼13 km in altitude during four seasonal deployments between 2016 and 2018 (Fig. 1). Recently, the ubiquitous presence of dilute BB smoke in the remote troposphere and its significant contribution to aerosol mass loading was established using ATom observations (35). Here, we use ATom measurements to quantify the individual contributions of urban and BB emissions to O₃ in the remote global troposphere using tracers specific to each source. We compare this analysis with simulations from three global CTMs that alternatively set BB and urban emissions to zero to evaluate their impact on modeled tropospheric O₃.Open in a separate windowFig. 1.Map of ATom flight tracks from the four seasonal global circuits colored by tropospheric O₃ mixing ratios. Note that the color scale terminates at 70 ppbv of O₃, and higher values are shown in red. Measurements with a strong stratospheric influence were parsed out as indicated in Materials and Methods.