Considerable increases in accumulation mode aerosol concentrations and low aerosol hygroscopicity limit impacts of recent particle formation to Northwest Atlantic post-frontal cloud deck

Mid-latitude postfrontal clouds often undergo overcast-to-broken cloud regime transitions, which are typically initiated by substantial rain and are characterized by relatively sharp decreases in regional albedo. The interplay between microphysical processes and large-scale cloud response creates a challenge to earth system models (ESMs), resulting in radiative biases connected to postfrontal clouds. Increased concentrations of aerosol available as cloud condensation nuclei (CCN) have been demonstrated to exert a control to the rain onset and thereby the timing of these transitions. New particle formation (NPF) may deliver such CCN after size growth from nucleation towards Aitken mode. Here we aim to understand how aerosol populations shaped by NPF impact a downwind cloud field.

The recently concluded, multi-year NASA campaign, ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment), dedicated its off-summer resources to postfrontal clouds. Two aircraft repeatedly measured atmospheric constituents in and around marine boundary layer (MBL) clouds through remote sensing and in-situ probes. During half of these flights, the MBL upwind of clouds showed elevated total aerosol concentrations and a characteristic diurnal aerosol size growth from 3 to 30 nm diameter over the course of several hours. Using the flight day of strongest growth, we perform Lagrangian large-eddy simulations to investigate implications to the downwind cloud deck. First, we use a trimodal aerosol configuration that reflects conditions prior to NPF. We infer a likely aerosol hygroscopicity by comparing two extreme setups (i.e., kappa set to 0.05 and 0.70) against downwind observational targets collected from aircraft as well as several satellite platforms and find a low hygroscopicity to provide a good match. Assuming all else being equal, we then change initial aerosol properties to late-NPF conditions. We find a concomitant increase in accumulation mode concentration (here from 250 to 1100 cm-3) that often coincides with seen NPF events to dominate the response to the downwind cloud deck and to prevent activation of Aitken mode aerosol by establishing an updraft-limited regime. Lastly, we explore whether NASA GISS ModelE3.1’s aerosol scheme, MATRIX (Multiconfiguration Aerosol TRacker of mIXing state), produces plausible aerosol properties during postfrontal conditions – a first step to an upcoming wider assessment of interactive cloud and aerosol properties that is hoped to help resolve long-standing ESM radiative biases.