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

Abstract

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.

Keywords: ATom; biomass burning; ozone; troposphere; urban.

Fig. 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.

Fig. 2.

Distributions of tropospheric O₃ mixing ratios (ppbv, y-axis) for each quartile of pollution tracer (x-axis) measured during ATom in four regions of the globe. The colors in the legend indicate the source of the tracers (red for BB, gray for urban). The Northern Hemisphere (NH) extratropics (panels A and B) corresponds to latitudes >20°N, the tropics (panels C and D) are defined within 20°S to 20°N, and the Southern Hemisphere (SH) extratropics (panels E and F) correspond to latitudes >20°S. The NH and SH data are further parsed into a photochemically active period (spring and summer; S&S) and a darker period (fall and winter; F&W). The box and whisker plots show the 10th, 25th, 50th, 75th, and 90th percentiles of O₃ distributions. The mustard-shaded area represents regional background O₃ values defined as the average O₃ mixing ratio in well-mixed and aged air masses. The width of the mustard-shaded area represents the range of background O₃ values obtained when using different pairs of tracers to define the well-mixed and aged air masses. The black dashed lines show regional median O₃ values.

Fig. 3.

Distributions of tropospheric O₃ (y axis) within each air mass classification (x-axis): well-mixed and aged (WMA) air, urban (UR), BB, or mixed pollution (MP) as defined in Sources of O3 to the Remote Troposphere. The Northern Hemisphere (NH) extratropics (panels A and B), the tropics (panels C and D), and the Southern Hemisphere (SH) extratropics (panels E and F) were defined as in Fig. 2. ATom observations classified using HCN and C₂Cl₄ or CH₃CN and CH₂Cl₂ pairs of tracers are shown in yellow and green, respectively. Modeling results are shown by the hashed box and whisker plots with GEOS-Chem1 in violet (2 × 2.5° horizontal resolution), GEOS-Chem2 in pink (4 × 5° horizontal resolution), GFDL-AM3 in light blue, and CAM-chem in gray. The boxes and whiskers show the 10th, 25th, 50th, 75th, and 90th percentiles of O₃ distributions. ΔO₃ corresponds to the difference between average O₃ in polluted air (i.e., UR, BB, or MP) and that in WMA air. ΔO₃ is plotted with square markers for ATom observations and round markers for modeling results. The error bars correspond to the SEs of the average O₃ in polluted air and in WMA air added in quadrature.

Fig. 4.

Median mixing ratios of the four main NO_y species measured during ATom in four air mass types: 1) well mixed and aged, 2) urban, 3) BB, and 4) mixed pollution. NO_y species are indicated with different colors as shown in the legend.

Fig. 5.

Concentration-weighted average O₃ attributed to urban and BB emissions. ATom observations are shown in green (the two markers per symbol are from the two pairs of tracers used to identify air mass influences), and modeling results are shown in brown. The uncertainties are in SI Appendix, Table S3 (observations) and SI Appendix, Table S4 (modeling results).