MSA and sulfuric acid as important components of particle composition in the tropical upper troposphere of the Indo-Pacific

Aircraft campaigns have shown high concentrations of ultrafine particles related to tropical convective outflow (Andreae et al. 2018; Williamson et al., 2019; Curtius et al., 2024). In the marine environment, dimethylsulfide (DMS) is a likely precursor for aerosols. It is a major sulfur source to the atmosphere and can be oxidised to sulfuric acid (SA) and methanesulfonic acid (MSA), which play important roles in the formation and growth of aerosol particles in the boundary layer (Kirkby et al., 2011; Hodshire et al., 2019; Shen et al., 2022). However, direct observations of the particle compositions at high altitudes and their connection to convection are sparse.

The CAFE-Pacific (Chemistry of the Atmosphere Field Experiment - Pacific) campaign provided valuable insights into the chemical composition of the tropical troposphere over Australia and the Indo-Pacific region around North-Eastern Australia. Seventeen research flights from Cairns were conducted with the HALO (High Altitude and LOng range) aircraft, ranging from the boundary layer up to 14 km altitude. With our nitrate CI-APi-TOF specially adapted for aircraft operation, we measured MSA and SA, among other species. Due to adiabatic heating in our inlet and subsequent evaporation of particles our instrument responds also to the particle phase composition in addition to the gas phase concentration. This evaporation effect is largest at high altitudes, where a large fraction of the total signal can be attributed to particle phase mass. It enables us to derive the composition of particles smaller than the aerosol size cut-off diameter of the AMS, which was also part of the HALO payload.

We find high concentrations of MSA throughout the entire measurement region. While SA is more variable, MSA is usually the dominant acid and mainly responsible for the particle mass, often exceeding SA by more than a factor of 10. Using our measurements in combination with HYSPLIT back trajectories and satellite data, we were able to trace back most of our data points to deep convective events in the past five days and thereby identify transport and oxidation of DMS as a source for ultrafine particles in the upper troposphere. The highest values of both acids are detected 15–20 hours after contact with a convective system, aligning well with the DMS lifetime.

Due to the large spatial extent and high frequency of convection around the marine ITCZ, this process represents most likely a substantial production mechanism of high-altitude aerosols which is not yet properly represented in most current models.

Andreae, M. O. et al. (2018), Atmospheric Chemistry and Physics 18, 921–961.

Curtius, J. et al. (2024), Nature, 636, 124–130.

Hodshire, A.L. et al. (2019), Atmospheric Chemistry and Physics 19, 3137-3160.

Kirkby, J. et al. (2011), Nature 218, 429-433.

Shen et al. (2022), Environ. Sci. Technol. 56, 13931–13944.

Williamson, C. J. et al. (2019), Nature 574, 399-403.