Modelling molecular composition of SOA from toluene photo-oxidation at urban and street scales†

Near-explicit chemical mechanisms representing toluene SOA formation are reduced using the GENOA algorithm and used in 3D simulations of air quality over Greater Paris and in the streets of a district near Paris. The SOA concentrations formed by the toluene photo-oxidation are found to mostly originate from molecular rearrangement with ring opening of a bicyclic peroxy radical (BPR) with an O–O bridge (45%), followed by OH-addition on the aromatic ring (22%), Highly Oxygenated organic Molecules (HOM) formation without ring opening (13%), condensation of methylnitrocatechol (8%), irreversible formation of SOA from methylglyoxal (6%), and ring-opening pathway (3%). The concentrations simulated using the most comprehensive reduced chemical scheme (rdc. Mech. 3) are also compared to those simulated with a SOA scheme based on chamber measurements, and one reduced from the Master Chemical Mechanism. Using rdc. Mech 3 leads to between 50% and 75% more toluene SOA concentrations than the other schemes, mostly because of molecular rearrangement. The SOA compounds from rdc. Mech. 3 are more oxidized and less volatile, with molecules of different functional groups. Concentrations of methylbenzoquinones, which may be of particular health interest, represent about 0.5% of the toluene SOA concentrations. Those are slightly higher in streets than in the urban background (by 2%).