Anthropogenic hydrocarbons are present in lower concentrations and their oxidation products contribute insignificantly. These organic oxidation products are hydrophilic but not hygroscopic, and hence do not absorb water at RH < 100%. Upon sunrise around 7 am, the gaseous oxidation products begin to build up, peaking in the late afternoon and then decreasing in concentration (Fig. ) as a result of an interplay between their generation from precursor hydrocarbons (Supplementary Fig. and), partitioning to existing aerosol, and dilution due to the variation in boundary layer height (Supplementary Fig. ). The vapor saturation, 𝑆 tot , which is the sum of individual saturations of condensable species, lags behind the concentrations due to elevated daytime temperatures (Supplementary Fig. ). When no absorbing material is initially present on soot particles, the condition 𝑆 tot > 1 is used as a threshold for the formation of organic condensate. In the biogenic scenario, this condition is met during a relatively narrow period between 4:30 pm and 10:30 pm.