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To quantify the effects of an advanced iron photochemistry scheme, the chemical aqueous-phase radical mechanism (CAPRAM 3.0i) has been updated with several new Fe(III)-carboxylate complex photolysis reactions. Newly introduced ligands are malonate, succinate, tartrate, tartronate, pyruvate, and glyoxalate. Model simulations show that more than 50% of the total Fe(III) is coordinated by oxalate and up to 20% of total Fe(III) is bound in the newly implemented 1:1 complexes with tartronate, malonate, and pyruvate. Up to 20% of the total Fe(III) is found in hydroxo and sulfato complexes. The fraction of [Fe(oxaIate)2]- and [Fe(pyruvate)]2+ is significantly higher during nighttime than during daytime, which points toward a strong influence of photochemistry on these species. Fe(III) complex photolysis is an important additional sink for tartronate, pyruvate, and oxalate, with a complex photolysis contribution to overall degradation of 46, 40, and 99%, respectively, compared to all possible sink reactions with atmospheric aqueous-phase radicals, such as OH, NO3, and SO4-. Simulated aerosol particles have a much lower liquid water content than cloud droplets, thus leading to high concentrations of species and, consequently, an enhancement of the photolysis sink reactions in the aerosol particles. The simulations showed that Fe(III) photochemistry should not be neglected when considering the fate of carboxylic acids, which constitute a major part of aqueous secondary organic aerosol (aqSOA) in tropospheric cloud droplets and aqueous particles. Failure to consider this loss pathway has the potential to result in a significant overestimate of aqSOA productioa. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) and organic peroxides play an important role in atmospheric chemistry, but knowledge of their abundances, sources, and sinks from heterogeneous processes remains incomplete. Here we report the measurement results obtained in four seasons during 2011-2012 at a suburban site and a background site in Hong Kong. Organic peroxides were found to be more abundant than H2O2, which is in contrast to most previous observations. Model calculations with a multiphase chemical mechanism suggest important contributions from heterogeneous processes (primarily transition metal ion [TMI]-HOx reactions) to the H2O2 budget, accounting for about one-third and more than half of total production rate and loss rate, respectively. In comparison, they contribute much less to organic peroxides. The fast removal of H2O2 by these heterogeneous reactions explains the observed high organic peroxide fractions. Sensitivity analysis reveals that the role of heterogeneous processes depends on the abundance of soluble metals in aerosol, serving as a net H2O2 source at low metal concentrations, but as a net sink with high metal loading. The findings of this study suggest the need to consider the chemical processes in the aerosol aqueous phase when examining the chemical budget of gas-phase H2O2. [ABSTRACT FROM AUTHOR]

T -dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals ( • OH), the sulfate radicals (SO 4 •- ), and the nitrate radicals (NO 3 • ). The obtained Arrhenius expressions for the oxidation by the • OH radical are: k (T,GLYCERALDEHYDE+OH • ) = (3.3 ± 0.1) × 10 10 × exp((-960 ± 80 K)/ T )/L mol -1 s -1 , k (T,GLYCOLALDEHYDE+OH • ) = (4.3 ± 0.1) × 10 11 × exp((-1740 ± 50 K)/ T )/L mol -1 s -1 , k (T,LACTALDEHYDE+OH • ) = (1.6 ± 0.1) × 10 11 × exp((-1410 ± 180 K)/ T )/L mol -1 s -1 ; for the SO 4 •- radical: k (T,GLYCERALDEHYDE+SO 4 •- ) = (4.3 ± 0.1) × 10 9 × exp((-1400 ± 50 K)/ T )/L mol -1 s -1 , k (T,GLYCOLALDEHYDE+SO 4 •- ) = (10.3 ± 0.3) × 10 9 × exp((-1730 ± 190 K)/ T )/L mol -1 s -1 , k (T,LACTALDEHYDE+SO 4 •- ) = (2.2 ± 0.1) × 10 9 × exp((-1030 ± 230 K)/ T )/L mol -1 s -1 ; and for the NO 3 • radical: k (T,GLYCERALDEHYDE+NO 3 • ) = (3.4 ± 0.2) × 10 11 × exp((-3470 ± 460 K)/ T )/L mol -1 s -1 , k (T,GLYCOLALDEHYDE+NO 3 • ) = (7.8 ± 0.2) × 10 11 × exp((-3820 ± 240 K)/ T )/L mol -1 s -1 , k (T,LACTALDEHYDE+NO 3 • ) = (4.3 ± 0.2) × 10 10 × exp((-2750 ± 340 K)/ T )/L mol -1 s -1 , respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous • OH concentration is 2.6 × 10 -14 and 1.8 × 10 -14 mol L -1 , whereas it is 7.9 × 10 -14 and 3.5 × 10 -14 mol L -1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by • OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H 2 O 2 formation through HO 2 recombination. Surprisingly, H 2 O 2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H 2 O 2 , driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H 2 O 2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H 2 O 2 formation rates of about 0.2 ppbv h -1 during the initial irradiation periods are consistent with the H 2 O 2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H 2 O 2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM 2.5  ≥ 75 μg m -3 agree with the observed H 2 O 2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO 2 reaction with OH and HSO 3 - oxidation by H 2 O 2 . Overall, under high pollution, the H 2 O 2 -caused sulfate formation rate is above 250 ng m -3 h -1 , contributing to the sulfate formation by more than 70%.

Organosulfates (OSs), also referred to as organic sulfate esters, are well-known and ubiquitous constituents of atmospheric aerosol particles. Commonly, they are assumed to form upon mixing of air masses of biogenic and anthropogenic origin, that is, through multiphase reactions between organic compounds and acidic sulfate particles. However, in contrast to this simplified picture, recent studies suggest that OSs may also originate from purely anthropogenic precursors or even directly from biomass and fossil fuel burning. Moreover, besides classical OS formation pathways, several alternative routes have been discovered, suggesting that OS formation possibly occurs through a wider variety of formation mechanisms in the atmosphere than initially expected. During the past decade, OSs have reached a constantly growing attention within the atmospheric science community with evermore studies reporting on large numbers of OS species in ambient aerosol. Nonetheless, estimates on OS concentrations and implications on atmospheric physicochemical processes are still connected to large uncertainties, calling for combined field, laboratory, and modeling studies. In this Critical Review, we summarize the current state of knowledge in atmospheric OS research, discuss unresolved questions, and outline future research needs, also in view of reductions of anthropogenic sulfur dioxide (SO 2 ) emissions. Particularly, we focus on (1) field measurements of OSs and measurement techniques, (2) formation pathways of OSs and their atmospheric relevance, (3) transformation, reactivity, and fate of OSs in atmospheric particles, and (4) modeling efforts of OS formation and their global abundance.

Hydrogen peroxide (H2O2) and organic peroxides play an important role in atmospheric chemistry, but knowledge of their abundances, sources, and sinks from heterogeneous processes remains incomplete. Here we report the measurement results obtained in four seasons during 2011-2012 at a suburban site and a background site in Hong Kong. Organic peroxides were found to be more abundant than H2O2, which is in contrast to most previous observations. Model calculations with a multiphase chemical mechanism suggest important contributions from heterogeneous processes (primarily transition metal ion [TMI]-HOx reactions) to the H2O2 budget, accounting for about one-third and more than half of total production rate and loss rate, respectively. In comparison, they contribute much less to organic peroxides. The fast removal of H2O2 by these heterogeneous reactions explains the observed high organic peroxide fractions. Sensitivity analysis reveals that the role of heterogeneous processes depends on the abundance of soluble metals in aerosol, serving as a net H2O2 source at low metal concentrations, but as a net sink with high metal loading. The findings of this study suggest the need to consider the chemical processes in the aerosol aqueous phase when examining the chemical budget of gas-phase H2O2.