Transport and chemistry of isoprene and its oxidation products in deep convective clouds

Abstract

Deep convective clouds can transport trace gases from the planetary boundary layer into the upper troposphere where subsequent chemistry may impact aerosol particle formation and growth. In this modelling study, we investigate processes that affect isoprene and its oxidation products injected into the upper troposphere by an isolated deep convective cloud in the Amazon. We run a photochemical box model with coupled cloud microphysics along hundreds of individual air parcel trajectories sampled from a cloudresolving model simulation of a convective event. The box model simulates gas-phase chemical reactions, gas scavenging by liquid and ice hydrometeors, and turbulent dilution inside a deep convective cloud. The results illustrate the potential importance of gas uptake to anvil ice in regulating the intensity of the isoprene oxidation and associated low volatility organic vapour concentrations in the outflow. Isoprene transport and fate also depends on the abundance of lightning-generated nitrogen oxide radicals (NO x ¼ NO þ NO 2 ). If gas uptake on ice is efficient and lightning activity is low, around 30% of the boundary layer isoprene will survive to the cloud outflow after approximately one hour of transport, while all the low volatile oxidation products will be scavenged by the cloud hydrometeors. If lightning NO x is abundant and gas uptake by ice is inefficient, then all isoprene will be oxidised during transport or in the immediate outflow region, while several low volatility isoprene oxidation products will have elevated concentrations in the cloud outflow. Reducing uncertainties associated with the uptake of vapours on ice hydrometeors, especially HO 2 and oxygenated organics, is essential to improve predictions of isoprene and its oxidation products in deep convective outflows and their potential contribution to new particle formation and growth.

Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation

“…In our base-case simulation, the pH was assumed to have an altitude-dependent profile, reflecting the higher abundance of acids close to the surface and ranging from 4.5 to 5, in accordance with the representative pH values in the EMAC simulation. The base-case water content was as in Bardakov et al 80 and the ice retention coefficient 0.05 in accordance with Ge et al 13 , with no further uptake on ice.…”

“…We conducted a sensitivity study on ammonia transport processes and estimated the fraction remaining of ammonia vapour after convection from the boundary layer to the upper troposphere, using a cloud trajectories framework described in detail in Bardakov et al 79 , 80 . In brief, trajectories from a convective system simulated with the large-eddy simulation (LES) model MIMICA 81 were extracted and a parcel representing the cloud outflow was selected for further analysis (Extended Data Fig.…”

“…8a ). The meteorological profiles and clouds microphysics scheme used here were the same as in Bardakov et al 80 , producing altitude-dependent distributions of water and ice hydrometeors depicted in Extended Data Fig. 8 .…”

Contribution of regional aerosol nucleation to low-level CCN in an Amazonian deep convective environment: results from a regionally nested global model

“…The upward transport of insoluble biogenic precursor gas has been found in the experimental study of Kulmala et al (2006), and the transport process may significantly affect particle nucleation in the upper troposphere. The vertical upward transport efficiency has been further shown to be positively related to biogenic vapour volatility and negatively related to NO x abundance using 100 m resolution large-eddy simulations (LESs), which can resolve the convection and eddies that are important for deep convective transport (Bardakov et al, 2020(Bardakov et al, , 2021(Bardakov et al, , 2022. Upward transport within a relatively short time and domain from the boundary layer to the mid-troposphere was also shown to be efficient in the LES study Bardakov et al (2022), and it indicates the potential important role of deep convection in transporting precursor gases to the free troposphere and upper troposphere.…”

The Role of Convective Up‐ and Downdrafts in the Transport of Trace Gases in the Amazon

“…Potentially, the oxidized VOCs can also trigger NPF on their own, in particular in the UT where temperatures are relatively cold (Frege et al., 2018 ). The Amazon region is of particular interest in this context due to the high abundance of biogenic organic compounds in the BL that could survive transport to the cloud outlfow region (Bardakov et al., 2021 ) and eventually play an important role for NPF (Bianchi et al., 2016 ). Once nucleated and grown by coagulation and condensation, the newly formed particles can be transported back to the BL through large‐scale subsidence, for example, in the outer parts of Hadley cell, and influence low‐level clouds, radiation, and climate (Clarke et al., 1999 ; Williamson et al., 2019 ).…”