ATom Observations of New Particle Formation in the Tropical Upper Troposphere: The Role of Convection and Nucleation Mechanisms

Measurements of size distributions during the Atmospheric Tomography Mission (ATom) reveal high number concentrations (>>1000 cm^-3) of nucleation mode particles at high altitudes in the tropics and subtropics under low condensation sink conditions and are associated with upwelling in convective clouds. The broad spatial extent of these newly formed particles shows that the upper free troposphere (FT) of the tropics and subtropics is a globally significant source.

In this study, we investigate the link between convection and new particle formation (NPF) by exploring the processes that govern NPF and growth in the tropical and subtropical FT of the Pacific and Atlantic Oceans. We use measurements of the size distributions made with a suite of fast-response instruments on board of a NASA DC-8 aircraft during ATom mission. ATom maps the remote atmosphere over the Pacific and Atlantic basins (~80 °N and ~65 °S) in continuous ascents and descents (0.2 and ~13 km), providing the latitudinal and vertical information on the greenhouse gases, reactive and tracer species and aerosol properties and their seasonal variability.

We couple measurements of size distributions between 0.003 and 4.8 µm and potential aerosol precursor vapors measured on ATom (August 2016 and February 2017) with calculated air mass back trajectories and the TwO-Moment Aerosol Sectional (TOMAS) box model. The back trajectories identify air masses potentially influenced by recent convection. We then use TOMAS to model particle nucleation, condensation and coagulation along that trajectory to investigate the link between convection and NPF. Through TOMAS, we explore the influence of different nucleation mechanisms (such as binary, ternary or the one with organics) and gas-phase aerosol precursors (such as sulfur dioxide) on observed particle size distributions.

We discuss similarities and differences in NPF over the Pacific and Atlantic Oceans and their relationship to convection, examine particle composition and volatility from in situ measurements, and examine which nucleation schemes are most consistent with the observations.