New particle formation in deep convective cloud environment over the Amazon

Abstract:

It has been estimated that over 50% of the present-day global low-level cloud condensation nuclei (CCN) are formed from new particle formation (NPF), and that this process has a substantial effect on the radiative properties of shallow clouds (Gordon et al. 2017). Observational studies have confirmed the importance of NPF from upper troposphere (UT) to supply boundary layer CCN and they suggest deep convection may efficiently transport gas precursors and particles (Clarke et al., 1998; Clarke et al., 1999; Andreae et al., 2018; Williamson et al., 2019). However, we have a very limited understanding of how deep convection affects NPF. Deep clouds could interact strongly with NPF because they extend into the high free troposphere where most new particles are formed, and they are responsible for most of the vertical transport of the nucleating vapours. Andreae et al. (2018) hypothesised from ACRIDICON-CHUVA campaign that organic gas molecules are transported by deep convection to the UT where they are oxidised and produce new particles, which are then be entrained into the boundary layer and grow to CCN-relevant sizes.

Here we study the interaction of deep convection and NPF using the United Kingdom Chemistry and Aerosols (UKCA) model coupled with the Cloud-AeroSol Interacting Microphyics (CASIM) embedded in the regional configuration of UK Met Office Hadley Centre Global Environment Model (HadGEM3). We simulate 3 days over a 1000 km region of the Amazon at 4 km resolution in Sept 2014.

Our simulations highlight three findings. Firstly, deep convection transports insoluble gases such as monoterpenes vertically. Monoterpene oxidation takes around 1-2 hours, which points to the importance of simulating biogenic nucleation over the Amazon in a cloud-resolving model. Secondly, merely monoterpene is not enough to sustain Amazon aerosol population, therefore, more organics with lower volatilities are needed. Finally, we examine the Andreae et al (2018) hypothesis of aerosol supply to the boundary layer by quantifying clear and cloudy up- and downdraft transport. Less than 2000 cm^-3 newly formed particles are found below 2 km and almost all of them are in clear regions. Aitken mode aerosols are seen more frequently in both cloudy and clear downdraft in the boundary layer than nucleation mode aerosols.