Impact of humidity on aerosol growth from methanesulfonic acid

Methanesulfonic acid (MSA; CH₃SO₃H), produced by oxidation of dimethylsulfide (DMS; CH₃SCH₃), is a key precursor for aerosols in the marine boundary layer and free troposphere. Laboratory experiments show that MSA contributes to both particle nucleation and subsequent growth, often together with sulfuric acid (H₂SO₄) and base vapours such as ammonia (NH₃). However, the influence of relative humidity (RH) on MSA condensation to particles remains uncertain. Here, in experiments conducted under low NH₃ conditions (<4 parts per trillion by volume, pptv) at the CERN Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we find that RH critically regulates the participation of MSA in the initial growth of newly formed particles. Between +10 °C and -10 °C, MSA drives rapid particle growth at high RH (>50%), whereas its contribution at low RH (<16%) is negligible. When comparing with aerosol process models such as the Model for Acid-Base Chemistry in Nanoparticle Growth (MABNAG), we find that the model fails to match our laboratory results. In particular, our measurements show that MSA drives rapid particle growth at substantially warmer temperatures and lower relative humidities than predicted by the Extended Aerosol Inorganic Model (E-AIM). This highlights the importance of further experiments to fully quantify the effect of RH on MSA particle growth and evaporation, and incorporating these measurements in aerosol models. This will be essential for accurately representing the important role of MSA in marine aerosols in global models, particularly in cold, low-NH₃ environments like polar regions and the free troposphere.