Country and species-dependent parameters for the Heating Degree Day method to distribute NO_x and PM emissions from residential heating in the EU-27: application to air quality modelling and multi-year emission projections

Abstract. The combustion of fossil and biofuels in the residential sector can cause high background levels of air pollutants in winter, but also pollution peaks during cold periods. Its emissions are dominated by space heating and show strong daily variations linked to changes in outside temperatures. The Heating Degree Days (HDDs) approach allows to represent daily variations in space heating emissions. The method depends on a temperature threshold ("Tb") below which building heating is activated, and a fraction ("f") considering the relative contribution of space heating to total residential combustion emissions. These parameters are fixed in the literature. However, they are likely to vary according to the country and pollutant. Using statistics on household energy consumption, we provide country- and species-dependent Tb and f parameters to derive daily temporal factors distributing PM and NO_x emissions from the residential sector in the EU-27. Tested in the CHIMERE model, the simulations show better performance scores (temporal correlation and threshold exceedance detection) in winter, especially for PM, when compared to the simulation with a monthly temporal factor, or based on HDDs but using fixed parameters from the literature. Finally, the HDDs with fitted parameters are used as a method to project official annual residential combustion emissions in subsequent years, as these are typically reported with a 2-year time lag. Results show that this method performs better regarding the persistence method and remains within emission uncertainties for both PM and NO_x emissions, indicating the importance of considering HDDs for air quality forecasting.