Abstract. Urban air quality issues are closely related to human
health and economic development. In order to investigate street-scale
flow and air quality, this study developed the atmospheric photolysis
calculation framework (APFoam 1.0), an open-source computational fluid dynamics (CFD) code based on
OpenFOAM, which can be used to examine microscale reactive pollutant
formation and dispersion in an urban area. The chemistry module of
APFoam has been modified by adding five new types of reactions, which can
implement the atmospheric photochemical mechanism (full
O3–NOx–volatile organic compound chemistry) coupled with a CFD model. Additionally,
the model, including the photochemical mechanism (CS07A), air flow, and pollutant
dispersion, has been validated and shows good agreement with SAPRC
modeling and wind tunnel experimental data, indicating that APFoam has
sufficient ability to study urban turbulence and pollutant dispersion
characteristics. By applying APFoam, O3–NOx–volatile organic compound (VOC) formation
processes and dispersion of the reactive pollutants were analyzed in an
example of a typical street canyon (aspect ratio H/W=1). The comparison of
chemistry mechanisms shows that O3 and NO2 are underestimated, while
NO is overestimated if the VOC reactions are not considered in the
simulation. Moreover, model sensitivity cases reveal that 82 %–98 % and
75 %–90 % of NO and NO2, respectively, are related to the local vehicle emissions,
which is verified as the dominant contributor to local reactive pollutant
concentration in contrast to background conditions. In addition, a large amount of NOx emissions, especially NO, is beneficial
to the reduction of O3 concentrations since NO consumes O3.
Background precursors (NOx/VOCs) from boundary conditions only contribute
2 %–16 % and 12 %–24 % of NO and NO2 concentrations and
raise O3 concentrations by 5 %–9 %. Weaker ventilation conditions
could lead to the accumulation of NOx and consequently a higher
NOx concentration but lower O3 concentration due to the
stronger NO titration effect, which would consume O3. Furthermore, in
order to reduce the reactive pollutant concentrations under the odd–even
license plate policy (reduce 50 % of the total vehicle emissions), vehicle
VOC emissions should be reduced by at least another 30 % to effectively
lower O3, NO, and NO2 concentrations at the same time. These
results indicate that the examination of the precursors (NOx and VOCs) from
both traffic emissions and background boundaries is the key point for
understanding O3–NOx–VOCs chemistry mechanisms better in street canyons
and providing effective guidelines for the control of local street air
pollution.