Cloud processing of isoprene in the presence of NOx

Recent studies show the important role of isoprene in new particle formation (NPF) in the outflow of convective clouds. The oxidation of isoprene combined with low temperatures and low condensation sink yields organic vapours with very low volatility, promoting nucleation (Bardakov et al. 2024, Shen et al. 2024).

This study focuses on the transport of isoprene from low altitudes to the upper troposphere, by convective updrafts. In-cloud isoprene concentration is affected by microphysical mechanisms (uptake by cloud droplets, mixing) and chemical reactions (e.g., OH oxidation). Previous observational and modeling studies have highlighted the survivability of isoprene during transport overnight  due to its high volatility (Bardakov et al. 2024, Murphy et al., 2015). If oxidation can occur inside the convective cloud during the diurnal updraft, further chemical reactions could take place, altering the volatility of the organic vapours present. This would affect the uptake and therefore change the availability of precursors reaching the outflow. NOx, formed during convection due to lightning, could react with isoprene oxidation products to form isoprene nitrates.

We designed a one-month experimental campaign using the smog chamber and the rotating wetted-wall flow reactor (WFR) at PSI. The experimental set up is shown in Figure 1. The isoprene oxidation products are formed in the chamber in the presence of UV lights after the injection of isoprene and HONO (produces OH and NOx) . We varied the oxidation times (OH exposure levels) in the chamber in order to produce first- and second- generation oxidation products (case 1 and 2, respectively). Then, the chamber is used as a reservoir of vapours with the lights off. The vapours are continuously injected into the WFR for 5 hours. In addition, water is injected into the WFR, where due to rotation a microfilm on the inner wall is created. Cases 1 and 2 are carried out either in the absence of light around the WFR (only aqueous uptake), or in the presence of UVB lights, leading to simultaneous aqueous uptake and photochemistry. Another aspect of the experiment is to study the temperature dependence of the aqueous uptake of isoprene oxidation products. The experiment is carried out at 20°C as well as at 5°C in the absence of the light.

Multiple online instruments are deployed in the experiments: two mass spectrometers, Vocus 2R PTR-TOF-MS (proton-transfer-reaction time-of-flight mass spectrometry) and Vocus AIM (Adduct Ionization Mechanism), as well as gas monitors (O3, NO, NO2, NOx) and an SMPS (Scanning mobility particle sizer). 

Figure 1. Schematic of the experimental set-up.

The preliminary analysis shows the dominant presence of MVK/MACR, C4H6O, and isoprene hydroxy nitrates, C5H9NO4 (Figure 2). We will characterize the uptake and chemistry behaviours of the organic vapours under different conditions.

Figure 2. Time series of organic vapours measured with the VOCUS 2R PTR for case 1 at 20°C. Purple shading area indicates chamber measurements.

Grant Agreement number: 101008004.
Grant number: 101073026.

Bardakov, R. et al., (2024) Geophys. Res. Letters, 51
Murphy, B. N. et al., (2015) J. Geophys. Res. Atmos.,120
Shen et al., (2024) Nature, 636