Evidence of Entrainment of Particles from the Free Troposphere into the Marine Boundary Layer over the Tropical Oceans

It is well known that newly formed particles are entrained into the marine boundary layer (MBL) from the free troposphere (FT) as a result of frontal passages in the mid-latitudes and tropics. The MBL aerosol is also replenished by a persistent subsidence from the FT in large regions of the tropics and subtropics. During the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC), entrainment was observed at 18 m asl in the absence of frontal activity. ATOMIC took place during the dry season in the northwest tropical Atlantic from January 6 to February 13, 2020. Between January 22nd and 29^th, three episodic events of coinciding enhanced ozone and particle number concentration were observed with ozone indicating a free tropospheric source. The stability parameter and buoyancy frequency calculated from radiosonde soundings indicate air masses descending to the surface during this period. Similar instances of non-frontal related episodic entrainment were also observed during DYNAMO which took place in the Indian Ocean September to December, 2011. Evidence and explanations will be presented.