The Sensitivity of the CHIMERE Model to Emissions Reduction Scenarios on Air Quality in Northern Italy

Abstract

The sensitivity of the CHIMERE model to emission reduction scenarios on particulate matter PM2.5 and ozone (O3) in Northern Italy is studied. The emissions of NOx, PM2.5 SO2, VOC or NH3 were reduced by 50% for different source sectors for the Lombardy region, together with 5 additional scenarios to estimate the effect of local measures on improving the air quality for the Po valley area. Firstly, we evaluate the model performance by comparing calculated surface aerosol concentrations for the standard case (no emission reductions) with observations for January and June 2005. Calculated monthly mean PM10 concentrations are in general underestimated by a factor of 1.4 for January, while NO3- and NH4+ calculated monthly mean values are in good correspondence with observations. However, SO4= is underestimated by a factor of 2.4 and the sum of elemental carbon, organic material and anthropogenic dust (PPM) is underestimated by a factor of 3.8 when compared to measurements for January 2005. For June, modelled PM10 concentrations slightly overestimate the measurements by a factor of 1.2 and calculated monthly mean SO4=, NO3-, NH4+ concentrations are in good agreement with the observations. PPM is a factor 2 underestimated. Monthly mean calculated ozone concentrations are in general 12% overestimated when compared to the observations for June 2005. Secondly, the model sensitivity of emission reduction scenarios on PM2.5 and O3 calculated concentrations for the Po valley area is evaluated. The most effective 1 scenarios to abate PM2.5 concentration are based on the SNAP2 (non-industrial combustion plants) and SNAP 7 (road traffic) sectors, for which the NOx and PM2.5 emissions are reduced by 50%. These scenarios reduce the monthly calculated PM2.5 concentrations for January for the Po valley area on average by 4-10µg/m3 and 3-8µg/m3 respectively, with maximum reductions of 13.4µg/m3 and 14µg/m3 respectively, compared to the standard case. The number of days that the 2015 PM2.5 limit value of 25µg/m3 in Milan is exceeded by reducing primary PM2.5 and NOx emissions for SNAP 2 and 7 by 50%, does not change in January when compared to the standard case for the Milan area. From the additional scenarios carried out to investigate the impact of local versus regional air pollution, it appears that 60% of the PM2.5 concentration in the greater Milan area is caused by the emissions from the Lombardy region, while 40% of the PM2.5 concentration over the Milan area is due to the emissions surrounding the Lombardy region and from the model boundary conditions. This study also showed that a more effective pollutant reduction (emissions) per tonne of pollutant reduced (concentrations) for the greater Milan area is obtained by reducing the primary PM2.5 emissions for SNAP 7 by 50%. This scenario is almost four times more efficient than reducing the PM2.5 emissions of SNAP 2 by 50%. Reducing the precursor NOx emissions by 50% is the most effective for SNAP 2 on the decrease of PM2.5 concentrations. The most effective scenario on PM2.5 decrease for which the precursor SO2 emissions are reduced is achieved by SNAP 7. Our study showed that during summer time, the largest reductions in O3 concentrations are achieved for SNAP 7 emission reductions (up to 12 ppb over a larger area around Milan), when volatile organic compounds (VOCs) are reduced by 50%