Impact of urbanization on gas-phase pollutant concentrations: a regional scale, model based analysis of the contributing factors

Abstract

Abstract. Urbanization or rural-urban transformation (RUT) represents one of the most important transformations of land-use. To account for the impact of such process on air-quality, multiple aspects of how this transformation impacts the air has to be accounted for. Here we present a numerical model (regional climate models RegCM and WRF coupled to chemistry transport model CAMx) based study for present day conditions (2015–2016) focusing on a range of central European cities and quantify the individual and combined impact of four potential contributors. Apart from the two most studied impacts, i.e. the urban emissions and the urban canopy meteorological forcing (UCMF, i.e. the impact of modified meteorological conditions) we focus also on two less studied contributors to RUT: the impact of modified dry-deposition due to transformed landuse and the impact of modified biogenic emissions due to urbanization induced vegetation modifications and changes in meteorological conditions affecting these emissions. To quantify each of these RUT components, we performed a series of simulations with CAMx driven with both RegCM and WRF were each effect was added to the simulations one-by-one while we focused on gas-phase key pollutants: nitrogen and sulfur dioxide (NO2 and SO2) and ozone (O3). The validation of the results using surface observations showed an acceptable match between the modelled and observed annual cycles of monthly pollutant concentrations for NO2 and O3 while some discrepancies in the shape of the annual cycle were identified for some of the cities for SO2 pointing to incorrect representation of the annual emission cycle in the emissions model used. We showed on an ensemble 19 European cities that the most important contributors to the impact of RUT are the urban emissions themselves, resulting in increases concentrations for nitrogen dioxide (by 5–7 ppbv on average) and sulfur dioxide (by about 0.5–1 ppbv) and decreases for ozone (by about -2 ppbv) and the urban canopy meteorological forcing resulting in decreases of primary pollutants (by about 2 ppbv for NO2 and 0.2 ppbv for SO2) and increases of those of ozone (by about 2 ppbv). These are the two major drivers of urban air pollution and our results showed that they have to be accounted for simultaneously as the impact of urban emissions without considering UCMF can lead to overestimation of the emission impact. Additionally, we quantified two weaker contributors: the effect of modified landuse on dry-deposition and the effect of modified biogenic emissions. Due to modified dry-deposition summer (winter) NO2 increases (decreases) by 0.05(0.02) ppbv while almost no average effect for SO2 in summer and a 0.04 ppbv decrease in winter is modelled. The impact on ozone is much stronger and reaches a 1.5 ppbv increase on average. Due to modified biogenic emissions, negligible effect on SO2 and winter NO2 is modelled, while for summer NO2, and increase by about 0.01 ppbv is calculated. For ozone, we found a much larger decreases between 0.5–1 ppbv. In summary, when analyzing the overall impact of urbanization on air-pollution for ozone, all four components has to be accounted for while for primary pollutants (i.e. NO2 and SO2), the two minor contributors can be neglected.