Tropospheric photochemistry and the formation of particulate nitrate depend critically on the budget of nitrogen oxides (NOx). The NOx budget in turn is tied to the nocturnal N2O5 hydrolysis reaction, which takes place in the aqueous phase of aerosols. Through this reaction, NOx is removed from the atmosphere and HNO3 is formed, which then partitions between the particle and the gas phase. Recent research has shown that these processes depend crucially on the characteristic development of vertical profiles of gas phase and aerosol phase species in the nocturnal boundary layer. Resolving these profiles adequately is a prerequisite for a good representation of nighttime chemistry and poses an important modeling challenge.
In this work we explore the sensitivity of the model results for N2O5 and aerosol nitrate to different existing planetary boundary layer parameterizations within the WRF-Chem model, as well as the sensitivity to model resolution in the vertical direction. We use a 1-D version of WRF-Chem to systematically investigate a summer and a winter case of meteorological conditions. In the analysis, we compare the resulting temperature profile and the vertical profiles of three chemical species, N2O5, HNO3, and aerosol NO3, from five different planetary boundary layer schemes. Effects of hydrolysis on these chemical species are also investigated, and we quantify how these results depend on the vertical resolution.
For the summer case, using different boundary layer schemes can change the nocturnal boundary layer heights by a factor of 2 and the maximum mixing ratios of N2O5 by 22%. This is in contrast to the winter case, for which the nocturnal planetary boundary depth varies by a factor of 13 when using different boundary layer schemes, and maximum mixing ratios of N2O5 vary by 18%. The impact of the hydrolysis reaction is largest for the QNSE scheme. While changing the vertical resolution has the largest impact on the temperature profile when using the YSU scheme (a nonlocal scheme), the largest impact on the target chemical species is seen for the YSU and the QNSE scheme.