WRF-Chem model predictions of the regional impacts of N₂O₠heterogeneous processes on night-time chemistry over north-western Europe

Two flights from the campaign were used as test cases -- one with low relative humidity (RH) (60-70%), the other with high RH (80-90%). N₂₠heterogeneous chemistry was found to not be important in the low-RH test case; but in the high-RH test case it had a strong effect and significantly improved the agreement between modelled and measured NO₃ and N₂O₠. When the model failed to capture atmospheric RH correctly, the modelled NO3 and N₂O₠mixing ratios for these flights differed significantly from the measurements.

This demonstrates that, for regional modelling which involves heterogeneous processes, it is essential to capture the ambient temperature and water vapour profiles. The night-time NO₃ oxidation of VOCs across the whole region was found to be 100-300 times slower than the daytime OH oxidation of these compounds. The difference in contribution was less for alkenes (× 80) and comparable for dimethylsulfide (DMS).

However the suppression of NO₃ mixing ratios across the domain by N₂O₠heterogeneous chemistry has only a very slight, negative, influence on this oxidative capacity. The influence on regional particulate nitrate mass loadings is stronger. Night-time N₂O₠heterogeneous chemistry maintains the production of particulate nitrate within polluted regions: when this process is taken into consideration, the daytime peak (for the 95th percentile) of PM10 nitrate mass loadings remains around 5.6 µg kg⁻¹air, but the night-time minimum increases from 3.5 to 4.6 µg kg⁻¹air.

The sustaining of higher particulate mass loadings through the night by this process improves model skill at matching measured aerosol nitrate diurnal cycles and will negatively impact on regional air quality, requiring this process to be included in regional models.