When is cloud condensation nuclei activity sensitive to particle characteristics at emission?

The cloud condensation nuclei (CCN) activity of an aerosol population depends on individual particles' size and composition. However, these properties are modified shortly after emission by condensation of semivolatile substances and coagulation with other particles. It is, therefore, unclear to what extent particle characteristics at emission affect CCN activity after aging by condensation and coagulation. We present a process-level analysis of particles emitted from a particular source, diesel engines, in which we isolate the separate effects of condensation and coagulation on the CCN activation of primary aerosol. We simulated aerosol dynamics in a polluted area with the Particle Monte Carlo model coupled to the Model for Simulating Aerosol Interactions and Chemistry and evaluated three factors influencing particle CCN activity shortly after emission: (1) particle characteristics at the time of emission, (2) aging conditions near the emission source, and (3) the water vapor supersaturation at which CCN activity is evaluated. CCN concentrations were sensitive to particle properties at emission only under specific environmental conditions. Diesel emissions did not strongly influence local CCN concentrations at low cloud supersaturation thresholds (s<0.2%), regardless of particle characteristics at emission or aging conditions. At higher supersaturation thresholds (s>0.2%) and under conditions of rapid secondary aerosol formation, changes in CCN concentrations showed a greater sensitivity to particles' emission size than composition. Key Points Modeled aerosol aging and evaluated factors influencing particle CCN activity Increased number of CCN only after condensation aging and only at s>0.2% Change in CCN concentration more sensitive to emission size than composition