Primary organic aerosols are emitted directly into the atmosphere and are thus relatively easy to monitor. Secondary organic aerosols (SOAs), those which form from reactions of precursor gases in the atmosphere, are more elusive. Recent research suggested that clouds are able to uptake water-soluble organics, which are then oxidized and form SOAs after cloud droplet evaporation. To better understand the dynamics of SOA formation through this pathway, Ervens et al. study isoprene, a volatile organic compound and a newly recognized source of atmospheric SOA. Through model studies based on laboratory experiments, the authors find that SOAs form through cloud-processing depend strongly on the initial ratio of isoprene to nitrogen oxides. In this way, combustion emissions (nitrogen oxides) contribute to SOA formation from biogenic hydrocarbons. Further, cloud-derived SOA concentrations increase with increasing cloud-contact time. The authors expect that such information can help improve climate and air quality models.-American Geophysical Union