Assessing sensitivity regimes of secondary inorganic aerosol formation in Europe with the CALIOPE-EU modeling system

Sulfur dioxide and nitrogen oxides form two of largest contributors to PM2.5 in Europe; ammonium sulfate ((NH4)2SO4) and ammonium nitrate (NH4NO3). In-situ observations of many chemical components are rather sparse, and thus neither can accurately characterize the distribution of pollutants nor predict the effectiveness of emission control. Understanding (and controlling) the formation regimes for these components is important for the achievement of the reduction objectives established in the European legislation for PM2.5 (20% of PM2.5 triennial for the mean of urban background levels between 2018 and 2020). For this purpose, the present work uses the CALIOPE high-resolution air quality modeling system (12 km × 12 km, 1 h) to investigate the formation of SIA (SO42−, NO3− and NH4+, which involve an important part of PM) and their gaseous precursors (SO2, HNO3 and NH3) over Europe during the year 2004. The CALIOPE system performs well at estimating SIAs when compared to the measurements from EMEP monitoring network, but errors are larger for gaseous precursors. NH3 is underestimated in the warmest months, HNO3 tends to be overestimated in the summer months, and SO2 appears to be systematically overestimated. The temporal treatment of ammonia emission is a probable source of uncertainty in the model representation of SIA. Furthermore, we discuss the annual pattern for each inorganic aerosol and gas precursor species over Europe estimated with the EMEP data and CALIOPE outputs, comparing the performance with other European studies. Spatial distribution of key indicators is used to characterize chemical regimes and understand the sensitivity of SIA components to their emission precursors. Results indicate that SO42− is not usually fully neutralized to ammonium sulfate in ambient measurements and is usually fully neutralized in model estimates. CALIOPE and EMEP observations agree that the continental regions in Europe tend to be HNO3-limited for nitrate formation. Regulatory strategies in such regions should focus on reductions in NOx (NO + NO2) rather than NH3 to control ammonium nitrate. This work assesses how well the CALIOPE system reproduces the spatial and temporal variability of SIAs and their gaseous precursors over Europe and complements the measurement findings