Catalytic role of formaldehyde in particulate matter formation

Abstract

Formaldehyde (HCHO), the simplest and most abundant carbonyl in the atmosphere, contributes to particulate matter (PM) formation via two in-cloud processing pathways. First, in a catalytic pathway, HCHO reacts with hydrogen peroxide (H2O2) to form hydroxymethyl hydroperoxide (HMHP), which rapidly oxidizes dissolved sulfur dioxide (SO2,aq) to sulfate, regenerating HCHO. Second, HCHO reacts with dissolved SO2,aq to form hydroxymethanesulfonate (HMS), which upon oxidation with the hydroxyl radical (OH) forms sulfate and also reforms HCHO. Chemical transport model simulations using rate coefficients from laboratory studies of the reaction rate of HMHP with SO2,aq show that the HMHP pathways reduce the SO2 lifetime by up to a factor of 2 and contribute up to ∼18% of global sulfate. This contribution rises to >50% in isoprene-dominated regions such as the Amazon. Combined with recent results on HMS, this work demonstrates that the one-carbon molecules HMHP and HCHO contribute significantly to global PM, with HCHO playing a crucial catalytic role.

Water enhances the formation of fragmentation products via the cross-reactions of RO<sub>2</sub> and HO<sub>2</sub> in the photooxidation of isoprene

“…HMHP is previously known as the product of gas-phase ozonolysis of alkene (Huang et al, 2013;Li et al, 2016;Gong et al, 2018) and in-cloud reactions of H2O2 and HCHO (Dovrou et al, 2022). Our study implies that isoprene photooxidation is also a potential source of HMHP in the atmosphere.…”

“…Besides MACR, MVK, and β-ISOPOOH, a variety of low-molecular-weight OVOCs were also observed in our experiments, among which HMHP is the most special as a first-generation product. HMHP, as a prevailing organic hydroperoxide in the atmosphere (Hua et al, 2008;Qin et al, 2018), contributes to the regional sulfate burden considerably (Dovrou et al, 2022).…”

Persistence of Monoterpene-Derived Organic Peroxides in the Atmospheric Aqueous Phase

“…A recent work has shown that the rapid hydrolysis of α-HHs in the aqueous phase can outcompete their Fenton-like reactions with TMIs . The aqueous hydrolysis of α-substituted ROOHs also seems to be competitive with their reactions with S­(IV) (for which the lifetime of peroxides is estimated to be several minutes to a few hours at 1 ppb SO 2 with the recently reported ROOH + S­(IV) rate constants) , and with their aqueous-phase OH oxidation in deliquesced aerosols and cloud droplets (for which the peroxide lifetimes range between a few minutes to a few days, see Section S2 for details). In contrast, longer lifetimes measured for non-α-substituted peroxides in this study indicate that they may have significantly higher chance to react with OH radicals, TMIs, and dissolved SO 2 in the aqueous phase.…”

“…Organic peroxides (ROOH and ROOR) are key reactive species formed during atmospheric oxidation of organic compounds , and contribute significantly to the formation of secondary organic aerosol (SOA), − which accounts for a substantial fraction (20–70%) of atmospheric fine particulate matter. , Organic peroxides play an essential role in determining the life cycle of aerosol. First, organic peroxides formed via autoxidation of organic peroxy radicals (RO 2 ), , also called highly oxygenated organic molecules, can effectively contribute to atmospheric particle nucleation and growth. − Second, similar to hydrogen peroxide (H 2 O 2 ), organic peroxides are particularly reactive toward sulfur dioxide and play an important role in atmospheric sulfate formation. − Third, many organic peroxides are chemically labile and can degrade on timescales of minutes to hours in atmospheric condensed phases ,,− or surrogate lung fluids, leading to the generation of various organic products or reactive oxygen species (ROS) such as OH radicals, H 2 O 2 , superoxide radicals (O 2 – ), and organic radicals (R and RO), which have profound effects on aerosol aging and human health. ,,, …”

“…The aqueous-phase chemical behaviors of organic peroxides have been of great interest to the atmospheric chemistry community in recent years. The aqueous reactions of various biogenic and anthropogenic organic peroxides with SO 2 ,,− and transition-metal ions (TMIs) ,,,− have been studied with focus on the kinetics and mechanisms of sulfate and ROS production, respectively. However, studies on the aqueous reactivity of peroxides in the absence of SO 2 and TMIs have focused on the aqueous hydrolysis of a few types of α-substituted ROOHs such as α-acyloxyalkyl hydroperoxides (α-AAHPs), α-hydroxyalkyl hydroperoxides (α-HHs), ,, and α-alkoxy hydroperoxides (α-AHs), , which are reaction products from the bimolecular interactions of stabilized Criegee intermediates (SCIs) with carboxylic acids, water, and alcohols, respectively, during ozonolysis of alkenes .…”