Secondary Organic Aerosol Formation Regulates Cloud Condensation Nuclei in the Global Remote Troposphere

Formation of secondary organic aerosols (SOA) through the atmospheric oxidation of organic vapors has potential to enable particle growth to cloud condensation nuclei (CCN)-relevant sizes. In this work, we constrain a global aerosol model by using aircraft measurements to reveal the global importance of SOA formation in CCN production. Our improved model, with explicit size-resolved aerosol microphysics and parametrizations of semivolatile organic oxidation products, presents a state-of-the-art performance in simulating both particle number concentrations and organic aerosol concentrations dominated (80–95%) by SOA in the remote atmosphere, which have been challenges in previous modeling studies. The SOA formation in concert with aerosol nucleation contributes to more than 50% of CCN concentrations in those pristine environments featuring low background aerosol concentrations. We estimate that the SOA-derived CCN alters the magnitude of cloud radiative forcing by ∼0.1 W m −2. Our findings underscore the necessity for aerosol-climate models to represent controls on CCN concentrations by SOA production. Plain Language Summary Atmospheric aerosols with diameters of larger than 60 nm or so can serve as cloud condensation nuclei (CCN) that affect cloud properties and Earth's radiative balance. The determinants of CCN remain one of the largest uncertainties in the assessment of aerosol-radiation-cloud interactions. The secondary organic aerosols (SOA) formed by the oxidation and gas-aerosol partitioning of precursor organic vapors is an important, yet unsettled, source of CCN. The extent to which SOA formation impacts the global distribution of CCN has rarely been reported, in part because previous global aerosol models had a poor ability in simulating both SOA and particle number concentrations. Here, by constraining an aerosol-climate model using in situ aircraft measurements for number and mass concentrations of aerosols, we find that SOAs, formed by the oxidation of primary organic vapors from anthropogenic and natural emissions, dominate the growth of small particles to CCN in the global remote troposphere. The formation of SOAs exerts larger percentage contributions to CCN in preindustrial atmosphere than in the present day, appreciably altering the magnitude of cloud radiative forcing.