Abstract
Abstract. Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NOx = NO + NO2). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO2) observations. Improvements to simulated NOx production from lightning have increased NO2 predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (T < 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO2 and radicals, is currently over-estimated by 22% in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.
Highlights
Ozone in the upper troposphere is an efficient greenhouse gas (0.25–0.65 Wm−2; Solomon et al, 2007) with a long chemical lifetime (100–365 days; Kley, 1997; Wang et al, 1998).In the troposphere, ozone (O3) is produced by net photolysis of nitrogen dioxide (NO2) shown in Reactions (R1–R3)
The production of nitric acid (R12), though less influential with respect to A, effectively removes a radical and a nitrogen dioxide from the cycles because nitric acid is chemically stable in the upper troposphere
This study has constrained the uncertainty of reaction rates using observational data from the Intercontinental Chemical Transport Experiment-North America (INTEX-A) campaign
Summary
Ozone in the upper troposphere is an efficient greenhouse gas (0.25–0.65 Wm−2; Solomon et al, 2007) with a long chemical lifetime (100–365 days; Kley, 1997; Wang et al, 1998). The reactions that cycle NOx and produce ozone each have uncertainty as reported in the literature. Each paper in the literature estimates a rate from observations of reactants or products in an experimental system. The uncertainty of each reaction rate has a non-linear impact on model estimates of NOx and ozone. Models that predict NOx and ozone use uncertain emissions, transport, and chemical reactions. For a model to accurately predict NOx or ozone, the reactive cycling of NOx must be in balance with the physical transport and emissions of radical precursors and NOx. Model evaluations of NOx, using data from the Intercontinental Chemical Transport Experiment-North America (INTEX-A) campaign, have shown a low-bias for simulated NO2 using GEOS-Chem (Hudman et al, 2007). This temperature sensitivity needs to be confirmed in laboratory experiments
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