Reductions in nitrogen oxides over the Netherlands between 2005 and 2018 observed from space and on the ground: Decreasing emissions and increasing O3 indicate changing NOx chemistry

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Satellite measurements of tropospheric NO2 columns are valuable for monitoring long-term changes in air quality. However, direct linkage of satellite-derived NO2 trends with changes in underlying NOx emissions and NOx surface concentrations is complicated by the contribution of background NO2 to the column, by changes in the chemical regime wherein emissions take place, and by data sampling differences. Here we study the 2005–2018 changes in nitrogen oxides concentrations over the polluted Netherlands. We use the QA4ECV OMI NO2 retrievals, RIVM surface measurements of NO, NO2, and O3, wet deposition fluxes of nitrate, and NOx emissions reported in two European inventories (EMEP and TNO-MACC-III). We interpret the observed changes in concentrations with simulations by CLASS, a box model accounting for boundary layer dynamics and chemistry. Nationally averaged, OMI column NO2 and RIVM surface NO2 concentrations are reduced by 30% and 32% respectively between 2005 and 2018. This is in line with Dutch national NOx emissions from the TNO-MACC-III and EMEP inventories showing decreases of 32%–39%, respectively, between 2005 and 2018. There is no indication that the decrease in NO2 concentrations slows down after 2010. The observed reductions in nitrogen oxides differ between winter and summer and turn out to be modulated by ozone chemistry. The RIVM surface measurements show a stronger reduction in NOx than in NO2, accompanied by an increase in O3 of 4–6 ppbv, which is due to diminished NO-titration following the lower NOx emissions, especially in winter. CLASS simulations confirm that daytime O3 increases have shifted the NO–NO2 equilibrium more towards NO2, explaining the weaker reductions in NO2 (−30%) than in NOx (−40%) concentrations over time. The O3 increases occur both during day and during night, and have likely shortened the NOx lifetimes both in summer (via faster OH + NO2+M during daytime) and in winter (via faster nighttime N2O5 formation and subsequent hydrolysis). Our findings suggest a reduction in anthropogenic NOx emissions of approximately 30% in the southern part of the Netherlands, and propose that interpreting satellite NO2 trends as a proxy for trends in NOx emissions is well-possible over a high-NOx region, but requires careful analysis in terms of changes in the chemical regimes and NOx lifetime.