CCN closure study: Effects of aerosol chemical composition and mixing state

This study presents a detailed cloud condensation nuclei (CCN) closure study that investigates the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols. Measurements of the chemical composition, aerosol size distribution, total number concentration, and CCN concentration at supersaturation (SS = 0.2-1.0%) were performed during the winter season in Kanpur, India. Among the two cases considered here, better closure results are obtained for case 1 (low total aerosol loading, 49.54 26.42 g m(-3), and high O:C ratio, 0.61 0.07) compared to case 2 (high total aerosol loading, 101.05 18.73 g m(-3), and low O:C ratio, 0.42 0.06), with a maximum reduction of 3-81% in CCN overprediction for all depleted SS values (0.18-0.60%). Including the assumption that less volatile oxidized organic aerosols represent the soluble organic fraction reduced the overprediction to at most 40% and 129% in the internal and external mixing scenarios, respectively. At higher depleted SS values (0.34-0.60%), size-resolved chemical composition with an internal mixing state performed well in CCN closure among all organic solubility scenarios. However, at a lower depleted SS value (0.18%), closure is found to be more sensitive to both the chemical composition and mixing state of aerosols. At higher SS values, information on the solubility of organics and size-resolved chemical composition is required for accurate CCN predictions, whereas at lower SS values, information on the mixing state in addition to the solubility of organics and size-resolved chemical composition is required. Overall, (total) values are observed to be independent of the O:C ratio [(total)= (0.36 +/- 0.01) x O:C - (0.03 +/- 0.01)] in the range of 0.2<0.81, which indicates that the variation in the chemical composition of aerosols is not well represented by the changes in the O:C ratio alone.