Chemical Composition of the Droplet Residuals of Coastal Low-level Clouds during EPCAPE

The chemical composition of the aerosols that serve as cloud condensation nuclei (CCN) for cloud droplets both determines the activation of those aerosols into cloud droplets and is changed by the chemical reactions that occur within cloud droplets. To better understand these effects, we analyze year-long aerosol mass spectrometer (AMS) measurements at the Mt. Soledad site of the recent Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, CA. We identify 55 in-cloud cases at Mt. Soledad, totaling to 346 hrs of cloud droplet residual composition measurements taken by the AMS that sampled on a ground-based counterflow virtual impactor (GCVI), which sampled residuals from cloud droplets >9 μm in diameter. For each in-cloud case, we also select pre- and post-cloud periods of similar duration that have AMS measurements of the composition of the out-of-cloud non-refractory submicron aerosol particles. The largest contributions to the cloud droplet residual non-refractory mass were from nitrate and organics in 53 of the 55 in-cloud cases. This is in contrast to sulfate and organics, which had the largest contributions to the out-of-cloud non-refractory submicron particle mass. The mean nitrate mass concentration in the cloud droplet residuals is slightly lower than in the pre-cloud particles. This difference in nitrate is relatively small when compared to the difference in sulfate and organics, which have significantly lower mass concentrations in the cloud droplet residuals than in the pre-cloud particles. Preliminary results from a k-means clustering analysis of the AMS event-trigger measurements reveal single particles containing large amounts of nitrate were a higher fraction of the cloud droplet residual number compared to their fraction of the particle number during out-of-cloud periods. Single particles composed of primarily sulfate or organic fragments contributed less to the cloud droplet residual number as opposed to the out-of-cloud particle number. These results provide a long-term chemical characterization of cloud droplet residuals and suggest a role for nitrate in CCN found in this coastal region.