URMELL – part II: semi-explicit isoprene and aromatics gasSOA modelling†

Oxidation of emitted anthropogenic and biogenic volatile organic compounds (VOCs) and subsequent chemical reactions reduce the volatility of the products formed leading to secondary organic aerosol (SOA) formation. Despite the huge diversity of individual SOA compounds, SOA modelling is often simplified and estimated at the initial oxidation step neglecting chemical and physical process influencing SOA formation e.g. advection, deposition, chemical degradation and aging processes. To overcome this shortcoming, the chemical gas-phase mechanism URMELL was developed. URMELL treats more than 40 distinct oxidised gas-phase SOA (gasSOA) precursors with individual molecular characteristics and physico-chemical partitioning properties enabling a much more explicit gasSOA treatment for products of aromatics and isoprene oxidation. In this study, CTM simulations using COSMO-MUSCAT were performed with URMELL and compared with a simplified gasSOA scheme applying the widely used gas-phase mechanism RACM. The comparison indicates a delayed and thereby locally shifted gasSOA formation when applying URMELL. This effect is caused by the formation of multigenerational and multifunctional products along the transport trajectory whereby accounting for changes in the oxidant regime and leading to a multitude of gasSOA substances with URMELL. For isoprene and aromatics, URMELL simulates higher contributions of products with lower volatilities whereby aromatics generate even non-volatile products which can partition in new particle formation. The non-volatile aromatic products increase the average aromatic surface gasSOA concentration (30% on 20^th of May 2014) and show unexpectedly high concentrations in remote spruce forest areas, away from the emission sources, highlighting the potential of the detailed schemes and its need for application in CTMs.