Rapid modification of cloud‐nucleating ability of aerosols by biogenic emissions

Although secondary organic aerosol formation is well studied, the extent to which oxidation products of biogenic volatile organic compounds condense onto primary aerosols and modify their cloud‐nucleating properties remains highly uncertain. Here we show that water‐soluble organic acids produced from the reaction between α‐pinene and ozone rapidly accumulate onto preexisting particles forming coatings of organic materials that reach a mass fraction of 80–90% within a time period of 30 to 60 min for the reactant conditions of 7 to 37 ppbv α‐pinene and 20 ppbv ozone. Cloud condensation nuclei (CCN) measurements reveal that the initially hydrophobic aerosols are rapidly converted to efficient CCN at a supersaturation of 0.22%. Our results imply that changes in the activation potential of a significant fraction of the atmospheric aerosol population are controlled by the formation and composition of coatings formed during the aging process, rather than by the original particle size or composition.