Using NORA3 to investigate the impact of inversion trends on NO2 concentration trends in Oslo and Bergen

Temperature inversions prevents vertical mixing and thereby the distribution and dilution of pollutants, and can cause health hazardous concentrations of NO2 to accumulate. Policies have been implemented to reduce the emissions, and causing the yearly average NO2 concentration to decrease. In this study, we will use NORA3 to investigate if inversion trends have an impact on the observed trends in NO2 concentrations in Oslo and Bergen, or if the trends in NO2 concentrations are only due to reduction in emissions. The statistical relationship between the observed NO2 concentrations and inversions was assessed from 2001 to 2010 in Oslo and 2011 to 2020 in Bergen. The data was filtered to be able to discard daily and weekly variations in the emissions. In Oslo, the inversions, measured between 2 and 25 meter altitude at Valle Hovin, and the NO2 concentrations, measured at Alnabrua, had a correlation of 0.7. In Bergen, the inversions, measured between 2 and 40 meters altitude at Florida/Geofysen, and NO2 concentrations, measured at Danmarks plass, had a correlation of 0.3. Comparison of inversions in NORA3 to the observations showed NORA3 underestimating the yearly average inversion intensity in Oslo, but the variations were similar to the observations in both Oslo and Bergen. The correlation between the modelled and observed inversions were weak for both study areas. Assessing two scenarios each in Oslo and Bergen; one where the modelled and observed inversions were similar in magnitude and one where they differed, showed for the first case that the inversions were sporadic night inversions, while for the second case, the inversions were continuous. The model surface energy balance was consistent with the 2 meter air and skin temperature. Lastly, we investigated 20 year trends in the modelled inversions and observed NO2 concentrations from 2001 to 2020. Both study areas showed a significant negative trend in the NO2 concentrations. There was no significant trend in modelled inversions for Oslo, but there was a significant negative trend for Bergen