Simulation of the effects of low-volatility organic compounds on aerosol number concentrations in Europe

PMCAMx-UF, a three-dimensional chemical transport model focusing on the simulation of the ultrafine particle size distribution and composition has been extended with the addition of reactions of chemical aging of semivolatile anthropogenic organic vapors, the emissions and chemical aging of intermediate-volatility organic compounds (IVOCs), and the production of extremely low-volatility organic compounds (ELVOCs) by monoterpenes. The model is applied in Europe to quantify the effect of these processes on particle number concentrations. The model predictions are evaluated against both ground measurements collected during the PEGASOS 2012 summer campaign across many stations in Europe and airborne observations by a zeppelin measuring above Po Valley, Italy. PMCAMx-UF reproduces the ground level daily average concentrations of particles with a diameter larger than 100 nm (N-100) with normalized mean error (NME) of 45 % and normalized mean bias (NMB) close to 10 %. For the same simulation, PMCAMx-UF tends to overestimate the concentration of particles with a diameter larger than 10 nm (N-10) with a daily NMB of 23 % and a daily NME of 63 %. The model was able to reproduce more than 75 % of the N-10 and N-100 airborne observations (zeppelin) within a factor of 2. According to the PMCAMx-UF predictions, the ELVOC production by monoterpenes leads to surprisingly small changes of the average number concentrations over Europe. The total number concentration decreased due to the ELVOC formation by 0.2 %, N-10 decreased by 1.1 %, N-50 (particles with a diameter larger than 50 nm) increased by 3 %, and N-100 increased by 4% due to this new secondary organic aerosol (SOA) source. This small change is due to the nonlinearity of the system, with increases predicted in some areas and decreases in others, but also the cancelation of the effects of the various processes like accelerated growth and accelerated coagulation. Locally, the effects can be significant. For example, an increase in N-100 by 20 %-50% is predicted over Scandinavia and significant increases (10 %-20 %) are predicted over some parts of central Europe. The ELVOCs contributed on average around 0.5 mu gm(-3) and accounted for 10 %-15% of the PM2.5 OA. The addition of IVOC emissions and their aging reactions led to a surprising reduction of the total number of particles (N-tot) and N-10 by 10 %-15% and 5 %-10 %, respectively, and to an increase in the concentration of N-100 by 5 %-10 %. These were due to the accelerated coagulation and reduced nucleation rates.