Yee Jun Tham

Ozone (O3) formation can regulate the atmospheric chemistry and climate, but O3 formation over the coastal marine atmosphere remains poorly understood, particularly in the tropical regions. To address this gap, our study combines shipborne measurements, coastal observations, and WRF-Chem simulations along the East Coast of Peninsular Malaysia (ECPM) to examine the spatial distribution and controlling processes of tropospheric O3. Shipborne observations showed a clear north-to-south offshore ozone gradient, with a maximum hourly concentration of 97.7 ppbv in the northern sector. Offshore O3 was generally higher than at nearby coastal land stations, indicating active ozone production and/or accumulation over the coastal marine atmosphere. Model results suggest that this enhancement was mainly driven by eastward transport of anthropogenic and biogenic precursors under the south-westerly monsoon. During transport, photochemical ageing promoted O3 formation, while lower nitrogen oxides over the marine boundary layer reduced O3 loss through nitric oxide titration. Strong solar radiation, warm temperatures, shallow boundary layers, and weak winds further favoured ozone production and accumulation. Sensitivity simulations showed that removing biogenic volatile organic compound emissions reduced surface ozone by 6–15 ppbv over the ECPM and adjacent South China Sea, highlighting the important role of tropical vegetation emissions in O3 formation. Our study shows that under the influence of continental precursors outflow, the atmosphere above tropical coastal waters can act as active ozone accumulation zones, with important implications for future coastal air quality management and regional ozone chemistry studies in Southeast Asia.

Dimethylsulfide (DMS; CH₃SCH₃) from marine phytoplankton is a major source of atmospheric sulfur ¹. Its oxidation products include sulfuric acid (SA; H₂SO₄) and methanesulfonic acid (MSA; CH₃SO₃H), which has a higher yield than SA below 10 °C ². Whereas SA is known to drive the formation of new particles ³, which may subsequently grow and act as cloud condensation nuclei (CCN), the role of MSA remains unclear ⁴. Here, in experiments performed under atmospheric conditions at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we show that MSA nucleates together with ammonia (NH₃) below −10 °C, at rates comparable to SA-NH₃. Moreover, MSA and SA nucleate synergistically below −10 °C, forming multi-acid molecular clusters with NH₃. Even at ultra-low NH₃ levels, MSA drives particle growth at or near the kinetic limit below 9 °C and above 40 % relative humidity (RH). Since MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that nucleation rates may be accelerated up to tenfold and growth rates up to twofold compared with SA-NH₃ alone. Our global model simulations indicate that MSA can enhance CCN concentrations, especially in polar regions. We propose that MSA might be an important driver of biogenic particles in cool, pristine marine regions of both the present-day and pre-industrial atmospheres, and yet is unaccounted for in global climate models ⁵.

Chloric acid (HOClO₂) is a newly detected compound in the atmosphere and is involved in ozone depletion. The reaction of HOClO₂ with OH radical is likely one of its main atmospheric loss processes. Aqueous surfaces have been reported as critical media for reactions of some chlorine acids, such as HOCl/HCl. However, their effect on the reaction of HOClO₂ with OH remains unexplored. Here, we use Born-Oppenheimer molecular dynamics simulations to investigate the reaction of HOClO₂ with OH on an aqueous surface. The interfacial behaviors of the reactant (HOClO₂) and product (ClO₃) involved in this reaction were investigated. Gas phase HOClO₂ can be absorbed on the aqueous surface by forming H-bonds between the acidic H atoms and water. As HOClO₂ penetrates the sublayer, it rapidly dissociates into OClO₂^- and H₃O⁺; however, before this dissociation, a competitive reaction with OH is likely due to the rapid reactivity of the radical. The reaction of HOClO₂ with OH on the aqueous surface follows hydrogen abstraction mechanisms; these include the direct and water monomer/dimer-mediated abstraction pathways. Direct abstraction has no energy barrier and is most kinetically favored, compared to ∼0.5 kcal mol^-1 and ∼3.0 kcal mol^-1 for the water monomer- and dimer-mediated mechanisms, respectively. The resulting ClO₃ can reside on the surface with its Cl atom exposed to the air, increasing the potential for further reactions with other atmospheric species. Overall, the specific reaction of HOClO₂ with OH on the water surface, could lead to new chlorine recycling by converting HOClO₂ into reactive species.

Chlorine atoms (·Cl) exhibit high reactivity and exert a substantial influence on the atmospheric oxidation capacity and chlorine cycle in the troposphere. The photolysis of molecular chlorine (Cl₂) is a crucial source of ·Cl. However, the daytime peaks of Cl₂ concentration cannot be entirely accounted for by the currently known pathways. Here we show that the photolytic oxidation of ammonium chloride (NH₄Cl), a ubiquitous component of atmospheric aerosol, serves as an important daytime source of Cl₂. Laboratory experiments demonstrate that oxygen, water vapour, and acidity are indispensable for Cl₂ generation and release, and that Cl₂ production is enhanced in the presence of black carbon aerosol. Field observation combined with model simulation demonstrates that the mechanism explains 12-55% of daytime Cl₂ concentration. These results reveal a photoactivation pathway for chlorine production that depends only on chlorine salts and solar radiation, with significant implications in regions with abundant chloride salts.