Methodologies to assess the influence of chemistry on urban nitrogendioxide concentrations using an atmospheric dispersion model underlimited monitoring conditions

Abstract

Assessing the performance of an atmospheric dispersion model to estimate nitrogen dioxide (NO2) remains challenging inregions with limited monitoring, where observations are scarce and key variables like ozone (O3) are often unavailable. Inthis study, we explore different methodologies to assess the influence of chemistry on modelled high NO2 concentrationsusing DAUMOD-GRS, an urban atmospheric dispersion model with a simplified chemical scheme, applied to the Metro-politan Area of Buenos Aires (MABA) over four years of hourly simulations. We evaluate three complementary approaches:(i) clustering of input/output conditions associated with NO2 exceedances; (ii) analysis of the NO2/NOx concentration ratio;and (iii) sensitivity simulations to key parameters in the chemical scheme, such as reaction time and background O3. Clus-tering proved useful for detecting metadata issues and identifying consistent conditions associated with exceedances. Ourresults highlight that, although the NO2/NOx ratio is widely used as a chemical diagnostic, its value depends strongly onthe accuracy of NOx estimates, which can limit its applicability for evaluating chemical processes in practice. A bivariateanalysis of modelled-to-observed concentration fractions of NO2 vs NOx proved useful for quickly visualizing the distribu-tion of errors in NO2, NOx, and their ratio, along with associated input data conditions. Sensitivity tests revealed that thefraction of NO2 in NOx emissions had greater impact on hourly exceedances, while background O3 had a larger influenceon annual means. These methods offer complementary insights and are particularly valuable in data-scarce contexts. Futurework may benefit from incorporating O3 observations and comparisons with chemical transport models to further supportchemical evaluations and to explore additional reactions beyond those included in simplified schemes.