Significantly Accelerated Hydroxyl Radical Generation by Fe(III)–Oxalate Photochemistry in Aerosol Droplets

Abstract

Fe(III)–oxalate complexes are ubiquitous in atmospheric environments, which can release reactive oxygen species (ROS) such as H2O2, O•2–, and OH• under light irradiation. Although Fe(III)–oxalate photochemistry has been investigated extensively, the understanding of its involvement in authentic atmospheric environments such as aerosol droplets is far from enough, since the current available knowledge has mainly been obtained in bulk-phase studies. Here, we find that the production of OH• by Fe(III)–oxalate in aerosol microdroplets is about 10-fold greater than that of its bulk-phase counterpart. In addition, in the presence of Fe(III)–oxalate complexes, the rate of photo-oxidation from SO2 to sulfate in microdroplets was about 19-fold faster than that in the bulk phase. The availability of efficient reactants and mass transfer due to droplet effects made dominant contributions to the accelerated OH• and SO4 2– formation. This work highlights the necessary consideration of droplet effects in atmospheric laboratory studies and model simulations.

Significantly Accelerated Photosensitized Formation of Atmospheric Sulfate at the Air–Water Interface of Microdroplets

“…In our early studies, the rapid transfer of gasphase reactants (O 2 and SO 2 ) in the microdroplet system is a potential driving force that promotes the fast sulfate production in the ferric oxalate reaction system. 26 The stirring process essentially enhances the supply and mass transfer of the reactants in bulk-phase reactions, thus largely overcoming their solubility and diffusion limitations in the bulk solution. However, in the HULIS reaction system, the sulfate production in the bulk solution with the stirring process was only increased about 1.6 times, much lower than that in the microdroplets (Figure 3b, 0.81 vs 123.3 mg L −1 ∼152 times).…”

“…Additionally, investigation of size-dependent reaction kinetics of T* with S(IV) in HULIS microdroplets is challenging because the strong fluorescence signal in such brown carbon of diverse chromophores cannot be directly analyzed using the confocal Raman technique that essentially enables the direct measurement of size-dependent kinetics. 43,44 Fortunately, an advanced microdroplet-printing device, recently employed for revealing the significantly accelerated photochemical reactions within size-dependent microdroplets by our research group, 26,35,36,45 allows us to tackle this fair challenge and elucidate the great significance of the interface-initiated SO 2 photosensitized reaction channel.…”

“…Sulfate is one of the key constituents in atmospheric fine particulate matter, thus largely regulating the air quality, climate, and human health. , Well-documented strong correlation between the sulfate concentration and aerosol liquid water content during haze events essentially reveals the key role of multiphase oxidation of sulfur dioxide (SO 2 ) in atmospheric sulfate formation. , In recent years, to close the sulfate concentration gap between the numerical models and field observations, several new aerosol-mediated pathways have been proposed, including interfacial SO 2 oxidation on acidic microdroplets, SO 2 triplet-state chemistry, − fast oxidation mediated by nitrate photolysis, − and photosensitized oxidation of S­(IV) by the excited triplet states of photosensitizers . Some traditional SO 2 oxidation mechanisms have been well investigated in the aerosol phase or interfacial microenvironment, such as uncatalyzed SO 2 auto-oxidation, NO 2 , ,− HONO, organic peroxides, H 2 O 2 , and transition metal ions (TMIs), − e.g., Fe­(III) and Mn­(II), thus highlighting the important contribution of multiphase SO 2 oxidation to sulfate in the (sub)­micron aqueous aerosol.…”

Study of Electron Collisions with Isoprene, 1,2-Butadiene, and Their Isomers

“…In addition to deteriorating air quality, SOA affects climate by scattering and absorbing light and aerosol-cloud interactions . Many researchers have found that the oxidation of isoprene by nitrate ( NO 3 ) radicals is perhaps the most important critical dynamic interaction among natural and anthropogenic emissions, which results in the formation of SOA …”

“…8 In addition to deteriorating air quality, SOA affects climate by scattering and absorbing light 9 and aerosol-cloud interactions. 10 Many researchers have found that the oxidation of isoprene 11 by nitrate (NO 3 ) radicals 12 is perhaps the most important critical dynamic interaction among natural and anthropogenic emissions, which results in the formation of SOA. 13 T h i s c o n t e n t i s In recent years, SOA production from the oxidation of isoprene has been the focus of numerous investigations; however, the photooxidation of other dienes emitted from natural (forest fires) and anthropogenic origins (diesel machine exhaust, industrial processes, etc.)…”