Calculations of the conversion factor from organic carbon to organic matter for aerosol mass balance

Aerosol mass balance studies based on filter samples require a conversion factor to derive organic matter (OM) concentrations from organic carbon (OC) measurements from thermo-optical methods. This factor provides indirect insights on the molecular structure of OM needed in chemical transport models. Site- and season-specific ratios of OC to OM (f_OM:OC) were calculated using data from five rural background sites in France between 2012 and 2021 by relating the unidentified chemical fraction in PM_2.5 samples to thermo-optical OC concentrations. Further, multiple linear formulations were used to evaluate the impact of possible artefacts on the determination of f_OM:OC. The resulting f_OM:OC was then compared to other estimates derived from online aerosol mass spectrometry data, showing good agreement. The spatial and temporal variability in f_OM:OC is discussed considering factors such as seasonality, meteorological conditions and the atmospheric oxidative potential. Linear-mixed effect models were formulated to quantitatively determine the drivers which influence the f_OM:OC at the French rural background sites. Both ozone and relative humidity were variables with statistically significant effects on f_OM:OC, indicating that differences in the contributions from both photooxidation and water content, explain the variability in f_OM:OC observed at the French rural background sites. Site-specific f_OM:OC yielded more accurate PM_2.5 mass closure and are therefore recommended in mass-balance exercises. Accurate f_OM:OC are critical to maintain consistency in OM time series, especially in cases where filter-based time series may be replaced by state-of-the-art online instrumentation.