Abstract
Abstract. Chemical transport models frequently evaluate their simulation of surface ozone with observations of the maximum daily 8 h average (MDA8) concentration, which is the standard air quality policy metric. This requires successful simulation of the surface ozone diurnal cycle including nighttime depletion, but models often have difficulty simulating this diurnal cycle for a number of reasons, including (1) vertical grid structure in the surface layer, (2) timing of changes in mixed layer dynamics and ozone deposition velocity across the day–night transition, (3) poor representation of nighttime stratification, and (4) uncertainties in ozone nighttime deposition. We analyze the problem with the GEOS-Chem model, taking as a representative case study the Southeast US during the NASA SEAC4RS aircraft campaign in August–September 2013. The model is unbiased relative to the daytime mixed layer aircraft observations but has a mean +8 ppb bias at its lowest level (65 m) relative to MDA8 surface ozone observations. The bias can be corrected to +5 ppb by implicit sampling of the model at the 10 m altitude of the surface observations. The model does not capture frequent observed occurrences of <20 ppb MDA8 surface ozone on rainy days, possibly because of enhanced ozone deposition to wet surfaces that is unaccounted for. Restricting the surface ozone evaluation to dry days still shows inconsistencies with MDA8 ozone because of model errors in the ozone diurnal cycle. Restricting the evaluation to afternoon ozone completely removes the bias. We conclude that better representation of diurnal variations in mixed layer dynamics and ozone deposition velocities is needed in models to properly describe the diurnal cycle of ozone.
Highlights
Ground-level ozone is harmful to human health and vegetation
In the US, the current ozone National Ambient Air Quality Standard (NAAQS) set by the Environmental Protection Agency (EPA) is 70 ppb, as the fourth-highest maximum daily 8 h average (MDA8) concentration per year averaged over 3 years (EPA, 2015)
We evaluate the use of the MDA8 ozone metric in the GEOS-Chem chemical transport models (CTMs), a global model frequently used in studies of regional ozone air quality and evaluated for this purpose with MDA8 ozone (Racherla and Adams, 2008; Lam et al, 2011; Zhang et al, 2011, 2014; Zoogman et al, 2011; Emery et al, 2012)
Summary
Ground-level ozone is harmful to human health and vegetation. It is produced when volatile organic compounds (VOCs) and carbon monoxide (CO) are photochemically oxidized in the presence of nitrogen oxide radicals (NOx ≡ NO + NO2). CTMs tend to overestimate surface ozone, in the Southeast United States (Fiore et al, 2009; Makar et al, 2017) Some of this overestimate is likely due to bias in the NOx emission inventories (Anderson et al, 2014; Travis et al, 2016), but the choice of comparison metric could play a role. Jacob : Systematic bias in evaluating chemical transport models tions relevant to air quality standards (Fiore et al, 2009; Mueller and Mallard, 2011; Emery et al, 2012; Lin et al, 2012; Rieder et al, 2015) The use of this metric implicitly requires the successful simulation of the diurnal cycle in surface ozone, but models are often too high at night, apparently because they cannot resolve the local stratification and associated depletion from surface deposition. The ultimate solution of this problem will require improved representation of boundary layer physics, but we propose in the meantime some simple corrective measures
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