Volatile organic compounds play an important role in air quality and climate change, largely because they contribute to the formation of oxidizing compounds and secondary organic aerosol (SOA). In this study, a series of products, including peroxides and carbonyl compounds in both gaseous and particulate phases, were simultaneously detected to investigate the oxidation regime and SOA composition in limonene ozonolysis. The roles of different double bonds (DBs), radicals, and water were also examined. In our first investigation, we focused on representative oxidizing compounds produced in limonene ozonolysis, including stabilized Criegee intermediates (SCIs), OH radicals, and peroxides. The dependence of H2O2 and hydroxymethyl hydroperoxide (HMHP) formation on RH demonstrates that the reaction with water is an important reaction pathway for limonene SCIs, and the lower-limit SCI yields of endocyclic and exocyclic DBs were estimated to be ~0.24 and ~0.43, respectively. The OH yield was determined by adding sufficient amounts of an OH scavenger, and the OH yields of endocyclic and exocyclic DBs were ~0.65 and ~0.24, respectively. These results indicate that in limonene ozonolysis the endocyclic DB is inclined to generate OH radicals through hydroperoxide channel, while the exocyclic DB has a higher fraction of forming SCIs. Besides, other gas-phase and particle-phase peroxides were also studied in this work. The formation of peroxyformic acid (PFA) and peroxyacetic acid (PAA) was promoted significantly by increasing RH and the degree of oxidation, and the discrepancy between the experimental and model results suggested some missing formation pathways. Considerable generation of H2O2 from SOA in the aqueous phase was observed, especially at high [O3]/[limonene], which was mainly attributed to the hydration and decomposition of unstable peroxides in SOA such as peroxycarboxylic acids and peroxyhemiacetals. Different DBs and OH scavengers had a large impact on the particulate peroxides, and their stability indicated that the types of peroxides in SOA changed under different conditions. As for the contribution of peroxides to SOA, the results demonstrated that the mass fraction of particulate peroxides in limonene SOA was less than 0.2 at low [O3]/[limonene], while the mass fraction was 0.4–0.6 at high [O3]/[limonene]. The partitioning behavior of peroxides showed that multi-generation oxidation helped produce more low-volatility peroxides, which partially explained the higher SOA yield. The partitioning behavior of carbonyls was also examined and the experimental partitioning coefficients (Kp) were found to be typically several orders of magnitude higher than the theoretical values. This study provided new insights into the oxidation regime and SOA composition in limonene ozonolysis, and limonene showed its specificity in many aspects when both endocyclic and exocyclic DBs were ozonated. We suggest that the atmospheric implications of terpenes containing more than one DB and the SOA composition, especially particulate peroxides, need further study.
Despite their crucial roles in health and climate concerns, the gas-particle partitioning of carbonyl compounds is poorly characterized in the ambient atmosphere. In this study, we investigate their partitioning by simultaneously measuring six carbonyl compounds (formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal, and methylglyoxal) in gas and particle phase at an urban site in Beijing. The field-derived partitioning coefficients (Kpf) are in the range of 10−5−10−3 m3 µg−1, and corresponding effective Henry’s law coefficients (KHf) should be 107–109 M atm−1. The Pankow’s absorptive partitioning theory and the Henry’s law both significantly underestimate concentrations of particle-phase carbonyl compounds (105–106 times and >103 times, respectively). The observed “salting in” effects only partially explain the enhanced partitioning to particles, approximately one order of magnitude. The measured Kpf values are higher at low relative humidity and the overall effective vapor pressure of these carbonyl species are lower than their hydrates, indicating that carbonyl oligomers potentially formed in highly concentrated particle phase. The reaction kinetics of oligomer formation should be included if applying the Henry’s law to low-to-moderate RH and the high partitioning coefficients observed need further field and laboratory studies. These findings provide deeper insights into the formation of carbonyl secondary organic aerosols in the ambient atmosphere.
Atmospheric peroxides play important roles in atmospheric chemistry, acting as reactive oxidants and reservoirsof HOx and ROx radicals. Field measurements of atmospheric peroxides were conducted over urban Beijing from2015 to 2016, including dust storm days, haze days and different seasons. We employed a box model based onRACM2 mechanism to conduct concentration simulation and budget analysis of hydrogen peroxide (H2O2) andperoxyacetic acid (PAA). In this study, heterogeneous reaction is found to be a significant sink for atmosphericH2O2 and PAA in urban Beijing. Here, we recommend a suitable uptake coefficient formula considering thewater effect for model research of peroxides. It is found that H2O2 and PAA unexpectedly maintained considerableconcentrations on haze days, even higher than that on non-haze days. This phenomenon is mainlyascribed to relatively high levels of volatile organic compounds and ozone on haze days. In addition, high levelsof water vapor in pollution episode can promote not only the heterogeneous uptake to aerosol phase but also theproduction of H2O2. Atmospheric PAA formation is suggested to be sensitive to alkenes and NOx in urbanBeijing. In particular, with the help of peroxides, sulfate formation rate from heterogeneous uptake could increaseby ∼4 times on haze days, indicating the potential effect of peroxides on enhancement of aerosol oxidativeproperty and secondary sulfate formation.
Methacrolein (MACR) is an abundant multifunctional carbonyl compound with highreactivity in the atmosphere. In this study, we investigated the hydroxyl radical initiatedoxidation of MACR at various NO/MACR ratios (0 to 4.04) and relative humidities (< 3% to80%) using a flow tube. Meanwhile, a box model based on the Master Chemical Mechanismwas performed to test our current understanding of the mechanism. In contrast to thereasonable predictions for hydroxyacetone production, the modeled yields of formaldehyde(HCHO) were twice higher than the experimental results. The discrepancy was ascribed tothe existence of unconsidered non-HCHO forming channels in the chemistry of CH3C(=CH2)OO., which account for approx. 50%. In addition, the production of hydroxyacetoneand HCHO were affected by water vapor as well as the initial NO/MACR ratio. The yields ofHCHO were higher under humid conditions than that under dry condition. The yields ofhydroxyacetone were higher under humid conditions at low-NOx level, while lower athigh-NOx level. The reasonable explanation for the lower hydroxyacetone yield underhumid conditions at high-NOx level is that water vapor promotes the production ofmethacrolein nitrate in the reaction of HOCH2C(CH3)(OO.)CHO with NO due to the peroxyradical-water complex formation, which was evidenced by calculational results. And theminimum equilibrium constant of this water complex formation was estimated to be1.89 × 10−18 cm3/molecule. These results provide new insights into the MACR oxidationmechanismand the effects of water vapor.