Abstract
Abstract. The NO2 annual air quality limit value is systematically exceeded in many European cities. In this context, understanding human exposure, improving policy and planning, and providing forecasts requires the development of accurate air quality models at the urban (street level) scale. We describe CALIOPE-Urban, a system coupling CALIOPE – an operational mesoscale air quality forecast system based on the HERMES (emissions), WRF (meteorology) and CMAQ (chemistry) models – with the urban roadway dispersion model R-LINE. Our developments have focused on Barcelona city (Spain), but the methodology may be replicated for other cities in the future. WRF drives pollutant dispersion and CMAQ provides background concentrations to R-LINE. Key features of our system include the adaptation of R-LINE to street canyons, the use of a new methodology that considers upwind grid cells in CMAQ to avoid double counting traffic emissions, a new method to estimate local surface roughness within street canyons, and a vertical mixing parameterisation that considers urban geometry and atmospheric stability to calculate surface level background concentrations. We show that the latter is critical to correct the night-time overestimations in our system. Both CALIOPE and CALIOPE-Urban are evaluated using two sets of observations. The temporal variability is evaluated against measurements from five traffic sites and one urban background site for April–May 2013. While both systems show a fairly good agreement at the urban background site, CALIOPE-Urban shows a better agreement at traffic sites. The spatial variability is evaluated using 182 passive dosimeters that were distributed across Barcelona during 2 weeks for February–March 2017. In this case, the coupled system also shows a more realistic distribution than the mesoscale system, which systematically underpredicts NO2 close to traffic emission sources. Overall CALIOPE-Urban improves mesoscale model results, demonstrating that the combination of both scales provides a more realistic representation of NO2 spatio-temporal variability in Barcelona.
Highlights
Persistent exposure to high NO2 atmospheric concentrations in cities causes detrimental health effects (e.g. Sunyer et al, 2015; Barone-Adesi et al, 2015)
CALIOPE-Urban shows a greater agreement for hourly, daily means and maximum concentrations but tends to underpredict daily peak concentrations at sites not exposed to very high traffic intensity
This study describes the development of a coupled regionalto street-scale modelling system, CALIOPE-Urban, which provides high-spatial- and high-temporal-resolution NO2 concentrations for Barcelona
Summary
Persistent exposure to high NO2 atmospheric concentrations in cities causes detrimental health effects (e.g. Sunyer et al, 2015; Barone-Adesi et al, 2015). In 2016, 19 out of the 28 European Union (EU) countries reported NO2 exceedances of the annual air quality limit value (40 μg m−3) mostly at urban traffic monitoring stations (EEA, 2018) In this context there is a need for NO2 data at the street level in urban areas that enable individuals and communities to mitigate the problem by, for example, walking in less polluted streets or reducing traffic in school areas. The background model reproduces the variation during both types of days well but overestimates concentrations during night-time (19:00–22:00 UTC), during days with calm conditions
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