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. Urban air quality issues are closely related to humanhealth and economic development. In order to investigate street-scaleflow and air quality, this study developed the atmospheric photolysiscalculation framework (APFoam 1.0), an open-source computational fluid dynamics (CFD) code based onOpenFOAM, which can be used to examine microscale reactive pollutantformation and dispersion in an urban area. The chemistry module ofAPFoam has been modified by adding five new types of reactions, which canimplement the atmospheric photochemical mechanism (fullO3–NOx–volatile organic compound chemistry) coupled with a CFD model. Additionally,the model, including the photochemical mechanism (CS07A), air flow, and pollutantdispersion, has been validated and shows good agreement with SAPRCmodeling and wind tunnel experimental data, indicating that APFoam hassufficient ability to study urban turbulence and pollutant dispersioncharacteristics. By applying APFoam, O3–NOx–volatile organic compound (VOC) formationprocesses and dispersion of the reactive pollutants were analyzed in anexample of a typical street canyon (aspect ratio H/W=1). The comparison ofchemistry mechanisms shows that O3 and NO2 are underestimated, whileNO is overestimated if the VOC reactions are not considered in thesimulation. Moreover, model sensitivity cases reveal that 82 %–98 % and75 %–90 % of NO and NO2, respectively, are related to the local vehicle emissions,which is verified as the dominant contributor to local reactive pollutantconcentration in contrast to background conditions. In addition, a large amount of NOx emissions, especially NO, is beneficialto the reduction of O3 concentrations since NO consumes O3.Background precursors (NOx/VOCs) from boundary conditions only contribute2 %–16 % and 12 %–24 % of NO and NO2 concentrations andraise O3 concentrations by 5 %–9 %. Weaker ventilation conditionscould lead to the accumulation of NOx and consequently a higherNOx concentration but lower O3 concentration due to thestronger NO titration effect, which would consume O3. Furthermore, inorder to reduce the reactive pollutant concentrations under the odd–evenlicense plate policy (reduce 50 % of the total vehicle emissions), vehicleVOC emissions should be reduced by at least another 30 % to effectivelylower O3, NO, and NO2 concentrations at the same time. Theseresults indicate that the examination of the precursors (NOx and VOCs) fromboth traffic emissions and background boundaries is the key point forunderstanding O3–NOx–VOCs chemistry mechanisms better in street canyonsand providing effective guidelines for the control of local street airpollution.

geoscientific model development

APFoam 1.0: integrated computational fluid dynamics simulation of O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;–NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt;–volatile organic compound chemistry and pollutant dispersion in a typical street canyon