Relationships Between Supermicrometer Sea Salt Aerosol and Marine Boundary Layer Conditions: Insights From Repeated Identical Flight Patterns

The MONterey Aerosol Research Campaign (MONARC) in May–June 2019 featured 14 repeated identical flights off the California coast over the open ocean at the same time each flight day. The objective of this study is to use MONARC data along with machine learning analysis to evaluate relationships between both supermicrometer sea salt aerosol number (N_>1) and volume (V_>1) concentrations and wind speed, wind direction, sea surface temperature (SST), ambient temperature (T_amb), turbulent kinetic energy (TKE), relative humidity (RH), marine boundary layer (MBL) depth, and drizzle rate. Selected findings from this study include the following: (i) Near surface (<60 m) N_>1 and V_>1 concentration ranges were 0.1–4.6 cm⁻³ and 0.3–28.2 μm³ cm⁻³, respectively; (ii) four meteorological regimes were identified during MONARC with each resulting in different N_>1 and V_>1 concentrations and also varying horizontal and vertical profiles; (iii) the relative predictive strength of the MBL properties varies depending on predicting N_>1 or V_>1, with MBL depth being more highly ranked for predicting N_>1 and with TKE being higher for predicting V_>1; (iv) MBL depths >400 m (<200 m) often correspond to lower (higher) N_>1 and V_>1 concentrations; (v) enhanced drizzle rates coincide with reduced N_>1 and V_>1 concentrations; (vi) N_>1 and V_>1 concentrations exhibit an overall negative relationship with SST and RH and an overall positive relationship with T_amb; and (vii) wind speed and direction were relatively weak predictors of N_>1 and V_>1.