Response to reviewers of acp-2022-265

Abstract

The goal of this study is to investigate the role of organic aerosols emitted with sea spray or formed from marine gas phase emissions of volatile organic compounds (VOCs) in influencing the stability of stratiform marine clouds. We aim at pointing out the processes and drivers that could be relevant in larger scale and should thus be considered in global climate models. We employ large eddy simulator that includes a detailed description of aerosols, cloud droplets and rain drops together with different model parameterizations for emission of sea salt, primary organic aerosol and VOCs from sea surface, and oxidation of the emitted VOCs and partitioning of the resulting semi-volatile organic species between vapor and aerosol phases. As a case study, we apply the model to simulate the conditions of the DYCOMS-II observational campaign characterized by low level stratocumulus clouds transitioning from closed cells to drizzling open cell structure. We find that the inclusion of online sea spray emissions can both extend and shorten the lifetime of the cloud layer based on the parameterization employed. Fine sea spray provides extra cloud condensation nuclei (CCN) and delays the onset of drizzle as the collision-coalescence process is slowed down due to smaller cloud droplet mean size. From the same emissions, the coarse mode has an opposite effect due to giant CCN (GCCN) speeding up the drizzle formation through the enhanced collision-coalescence processes. The balance between two process depends on the model parameterization employed. Compared to differences between different sea spray parameterizations, the sensitivity of the clouds to the variations in organic fraction of sea spray and hygroscopicity of the emitted fine aerosols is relatively limited. However, our results show that it is important to account for the size dependence of the sea spray organic fraction as attributing organic emissions to coarse mode noticeably reduces the GCCN effect. In addition, including the secondary aerosol formation from VOCs can potentially have a noticeable impact, but only when emitting the highest observed fluxes of monoterpenes. This impact is also highly sensitive on the size distribution of the background aerosol population. SOA production from isoprene is visible only if aqueous phase SOA production pathways are included in the model, and even then, the effect is lower than from monoterpenes.