Solubility Considerations for Cloud Condensation Nuclei (CCN) Activity Analysis of Pure and Mixed Black Carbon Species.

Black carbon (BC) is an aerosol that is released into the atmosphere due to the incomplete burning of biomass and can affect the climate directly or indirectly. BC commonly mixes with other primary or secondary aerosols to undergo aging, thereby changing its radiative properties and cloud condensation nuclei (CCN) activity. The composition of aged BC species in the atmosphere is difficult to measure with high confidence, so their associated CCN activity can be uncertain. In this work, the CCN activity analysis of BC was performed using laboratory measurements of proxy aged BC species. Vulcan XC72R carbon black was used as the representative of BC, and three structural isomers of benzenedicarboxylic acid─phthalic acid (PTA), isophthalic acid (IPTA), and terephthalic acid (TPTA)─were mixed with BC to generate three different proxies of aged BC species. Most studies related to CCN activity analysis of BC aerosols use the traditional Köhler theory or an adsorption theory (such as the Frenkel-Halsey-Hill adsorption theory). PTA, IPTA, and TPTA fall in the sparingly water-soluble range and therefore do not fully obey either of the aforementioned theories. Consequently, a novel hybrid activity model (HAM) was used for the CCN activity analysis of the BC mixtures studied in this work. HAM combines the features of adsorption theory via the adsorption isotherm with the features of Köhler theory by incorporating solubility partitioning. The results in this work showed that HAM improves the representation of CCN activity of pure and mixed BC aerosol species with high certainty, evident from generally better goodness of fit, > 0.9. This work implies that the hygroscopicity parameterization based on HAM captures the size-dependent variability in the CCN activity of the pure and aged BC species.