Simulation of atmospheric organic aerosol with the 2D volatility basis during the SPRUCE-22 field campaign

Organic aerosol (OA) constitutes a major fraction of the sub-micrometer atmospheric particulate matter and is either emitted directly into the atmosphere as primary organic aerosol (POA) or formed by the partitioning onto pre-existent particles of low vapor pressure products of the oxidation of volatile, intermediate volatility, and semivolatile organic compounds (VOCs, IVOCs, and SVOCs respectively) as secondary organic aerosol (SOA). The oxidation of these compounds results in thousands of mostly unspecified oxygenated products making our understanding of SOA formation mechanisms incomplete. The volatility basis set (VBS) is a framework that has been designed to simplify these oxidation systems and to allow SOA simulation in chemical transport models (CTMs). The VBS describes the evolution of OA using a set of surrogate species with effective saturation concentrations that vary by 1 order of magnitude (referred to as 1D-VBS). This framework was extended by a second dimension (2D-VBS) to include the oxidation state (2D-VBS), which is important to quantify the degree of oxidation. Three main reasons led to this extension. First, the disadvantage of the 1D-VBS is that compounds with similar saturation concentrations can have different properties and reactivities. Second, the available measuring techniques have increasing capabilities, and they can provide detailed information about the composition of ambient and smog chamber OA (e.g., oxidation state). Third, CTMs often have difficulties in reproducing field observations.

In this work, different parametrization schemes on the 2D-VBS framework were evaluated using measurements collected in the SPRUCE-22 field campaign in a remote forest area of the eastern Mediterranean site (Pertouli, Greece) in the summer of 2022. Field measurements suggested that most of the OA in the site was highly processed secondary anthropogenic and biogenic OA and also aged biomass burning OA.  The results of the default version of the model indicated both underprediction of the total OA level and its oxygen-to-carbon ratio (O:C). A series of hypotheses are tested involving the chemical aging of atmospheric OA to close the gap between the measurements and model predictions.