Abstract
Abstract. Regional and global chemical transport models underpredict NOx (NO + NO2) in the upper troposphere where it is a precursor to the greenhouse gas ozone. The NOx bias has been shown in model evaluations using aircraft data (Singh et al., 2007) and total column NO2 (molecules cm−2) from satellite observations (Napelenok et al., 2008). The causes of NOx underpredictions have yet to be fully understood due to the interconnected nature of simulated emission, transport, and chemistry processes. Recent observation-based studies, in the upper troposphere, identify chemical rate coefficients as a potential source of error (Olson et al., 2006; Ren et al., 2008). Since typical chemistry evaluation techniques are not available for upper tropospheric conditions, this study develops an evaluation platform from in situ observations, stochastic convection, and deterministic chemistry. We derive a stochastic convection model and optimize it using two simulated datasets of time since convection, one based on meteorology, and the other on chemistry. The chemistry surrogate for time since convection is calculated using seven different chemical mechanisms, all of which predict shorter time since convection than our meteorological analysis. We evaluate chemical simulations by inter-comparison and by pairing results with observations based on NOx:HNO3, a photochemical aging indicator. Inter-comparison reveals individual chemical mechanism biases and recommended updates. Evaluation against observations shows that all chemical mechanisms overpredict NOx removal relative to long-lived methanol and carbon monoxide. All chemical mechanisms underpredict observed NOx by at least 30%, and further evaluation is necessary to refine simulation sensitivities to initial conditions and chemical rate uncertainties.
Highlights
Total oxidized nitrogen [NOy = NO + NO2 + NO3 + N2O5+ HNO2 + HNO3 + HO2NO2 + CH3(CH2)nC(O)OONO2+ RNO3] includes many compounds with a wide variety of physical properties and environmental roles
The partitioning between NOy compounds influences the efficiency of NOy wet scavenging, the availability of HNO3 for acid rain, and the amount of NOx for production of the greenhouse gas ozone
Is the rate of chemical aging consistent between chemical mechanisms and observations? Second, are biases consistent for all chemical mechanisms, and fundamental to the state of the science, or can mechanism differences identify misrepresentations? Third, to what extent can chemical mechanisms’ photochemical aging cause underprediction of NO2? we evaluate factors that contribute to partitioning biases for total oxidized nitrogen in an attempt to improve the individual chemical mechanisms
Summary
Nitrogen oxides (NOx = NO + NO2) are water insoluble, chemically reactive in the atmosphere, and serve as precursors to ozone. Peroxy nitrates (PNs = HO2NO2 + CH3(CH2)nC(O)OONO2) are insoluble, their chemical reactivity is temperature dependent, and they act primarily as a reservoir for NOx. Nitric acid, on the other hand, is highly water soluble, chemically stable, and is a primary component of acid rain. The partitioning of the NOy between component compounds is controlled by a mix of physical (i.e., emissions and transport) and chemical (i.e., aqueous, particle, and gas-phase) processes and is critical to accurate simulation of environmental stress. The partitioning between NOy compounds influences the efficiency of NOy wet scavenging, the availability of HNO3 for acid rain, and the amount of NOx for production of the greenhouse gas ozone. 13.16 pptv 77.76 ppbv 95.52 pptv 203.3 pptv 280.1 pptv. 174.5 pptv 83.80 pptv 1475. pptv 71.25 pptv 374.9 pptv 172.8 pptv Initial 9149 m 300.6 hPa
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