In-cloud Sulfur

Research statement

Aqueous-phase atmospheric chemistry contributes to the formation of particulate matter (PM) and gas-phase chemistry. Sulfur dioxide (SO2) oxidation leads to sulfate production in cloud water, the second most important constituent of PM. Understanding these oxidation pathways provide the tools to estimate the sulfate concentrations on a global and regional scale and evaluate the enhancing and quenching mechanism in its formation. Globally, hydrogen peroxide (H2O2) has been considered the dominant oxidant of dissolved SO2, while multifunctional organic hydroperoxides have not been investigated. Organosulfur compounds which compete with sulfate formation have generally been neglected. The role of reactive organic carbon to particulate matter formation requires in-depth investigation in order to understand the processes occurring in fog and cloud water. Research efforts in this field face challenges with measurement techniques for distinguishing organic and inorganic sulfur, especially S(IV) and S(VI) species under dim conditions.

Schematic representation of particulate matter processes. The figure presents some main processes that are of interest to the Keutsch Group.

Figure created by E. Dovrou


(1) Measurements techniques for distinguishing S(IV) and S(VI) species

Mass spectrum of HMS at pH=5.5 and ion chromatographs of HMS and sulfate at pH=12.

Figure obtained by Dovrou et al., 2019, AMT

The ability of measurements techniques to identify and quantify sulfur-species have been investigated by examining an ion chromatography (IC) method and aerosol mass spectrometry (AMS). Specifically, we presented an improved IC method that has the ability to separate efficiently sulfate and hydroxymethanesulfonate (HMS), a sulfur(IV) species that can be present in fog and cloud water that has been largely neglected in both chemical models and field measurements of PM composition and has drown significant scientific attention. We investigated the ability of AMS to distinguish sulfate and HMS and provide laboratory evidence of HMS identification under specific conditions.

More information can be found at:

Dovrou, E., Lim, C. Y., Canagaratna, M. R., Kroll, J. H., Worsnop, D. R., and Keutsch, F. N.: Measurement techniques for identifying and quantifying hydroxymethanesulfonate (HMS) in an aqueous matrix and particulate matter using aerosol mass spectrometry and ion chromatography, Atmos. Meas. Tech., 12, 5303–5315,, 2019.

(2) Oxidation of SO2,aq by multifunctional organic hydroperoxides

Schematic representation of in-cloud sulfur dioxide oxidation pathways.

Figure obtained by Dovrou et al., 2019, ES&T

The oxidation of SO2,aq by hydrogen peroxide (H2O2) has been widely investigated, as H2O2 is considered to be the main oxidant of SO2,aq. Previous studies have examined the oxidation of SO2,aq in cloudwater by small organic peroxides with one functional group; however, oxidation by multifunctional organic hydroperoxides, which are expected to have higher water solubility and reactivity, has not been examined. We investigate the aqueous oxidation of SO2,aq by the two main isomers of isoprene hydroxyl hydroperoxide (1,2-ISOPOOH and 4,3-ISOPOOH), the primary low-NOx isoprene oxidation products in the atmosphere revealing the importance their importance and the significant contribution of 1,2-ISOPOOH to regional sulfate formation.

More information can be found at:

(3) Towards a chemical mechanism of the oxidation of SO2,aq via isoprene hydroxyl hydroperoxides (ISOPOOH)

Proposed mechanism of the oxidation of SO2,aq with 1,2-ISOPOOH.

Figure obtained by Dovrou et al., 2021, ACP

In-cloud chemistry has important ramifications for atmospheric particulate matter formation and gas-phase chemistry. Our previous work has shown that, like hydrogen peroxide (H2O2), the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH) oxidize sulfur dioxide dissolved in cloud droplets (SO2,aq) to sulfate. The work revealed that the pathway of SO2,aq oxidation with ISOPOOH differs from that of H2O2. We investigate the chemical mechanisms of oxidation of SO2,aq with ISOPOOH in the cloud-relevant pH range of 3–6 and compare them with the previously reported mechanisms of oxidation of SO2,aq with H2O2, methyl hydroperoxide and peroxyacetic acid.

More information can be found at:

(4) Catalytic role of formaldehyde in particulate matter formation

Overall schematic of the main examined processes

Figure obtained by Dovrou et al., 2022, PNAS

Particulate matter, often formed via cloud processing, strongly influences the Earth’s climate and air quality. Particle composition depends on anthropogenic and biogenic emissions. Thus, in order to understand climate change, knowledge of the difference between preindustrial and current conditions is critical. Under preindustrial conditions, multifunctional organic hydroperoxides, which are strong oxidants and have the ability to contribute to particulate matter formation, are in higher concentrations in the atmosphere. In this work, we focus on the previously unknown importance of hydroxymethyl hydroperoxide, which can be formed by gas-phase reactions and in-cloud reaction of hydrogen peroxide with the simplest aldehyde, formaldehyde, revealing the catalytic role of formaldehyde, and demonstrate that this chemistry is of great importance for particle formation.

More information can be found at:

The research was conducted by Eleni Dovrou. Contact: