Modeling the Impacts of Organic Aerosol Phase Transitions on Cloud Droplet Number Concentration in the Boreal Forest

Accounting for the condensation of organic vapors along with water vapor (co-condensation) has previously been shown to significantly enhance the number of aerosol particles that activate to form cloud droplets in adiabatic cloud parcel model (CPM) simulations. In these simulations the organic aerosol (OA), which includes organics in both particle and vapor phases, is present and its phase transitioning is efficiently tracked using a volatility basis set (VBS), which lumps the tens of thousands of organic species found in the atmosphere into bins determined by their effective saturation vapor concentration. The VBS has traditionally been estimated from measurements with thermodenuders and dilution chamber experiments, during which the presence of organic vapors (of various volatilities) is mapped through changes in bulk particle-phase OA mass. Due to new advances in online measurement techniques the gas phase can be now thoroughly sampled. By varying the ionization scheme, chemical ionization mass spectrometers (CIMS) can detect a wide variety of vapors in real time and a VBS can be constructed using these measurements. Recent studies suggest that the concentrations of semi- and intermediate volatility organic compounds (S/IVOC) can be potentially higher than previously estimated. The previous studies that have employed CPM to calculate the increase in cloud droplet number due to co-condensation utilized old VBS retrievals, which potentially underestimate the amount of S/IVOC. In this work we show results from CPM simulations initialized with a comprehensive set of co-located aerosol observations from the boreal forest of Finland involving a VBS compiled from various CIMS measurements. We define a volatility range of organics making most impact and describe the environmental conditions that favor co-condensation-driven cloud droplet number enhancements utilizing long-term data sets.