OH Exposure Research Articles

Investigation of levoglucosan decay in wood smoke smog-chamber experiments: The importance of aerosol loading, temperature, and vapor wall losses in interpreting results

Levoglucosan has been extensively used as a molecular marker of biomass burning in source-apportionment studies over the last few decades. However, recent studies suggest that the atmospheric lifetime of levoglucosan may be in the order of only 1–2 days under summertime conditions relevant to prescribed/wildfires. Implications of levoglucosan reactivity to wintertime conditions, however, remain uncertain despite significant contributions of domestic wood combustion to wintertime air quality. This study presents smog chamber experiments to investigate levoglucosan decay during photo-oxidation of wood smoke over a temperature range between −8 and 10 °C.Significant decay in particle wall-loss corrected levoglucosan is only observed around 10 °C in these experiments. Theoretical analysis shows that the apparent chemical lifetime of levoglucosan increases at lower temperatures as well as at higher organic aerosol mass concentrations as a result of smaller vapor fractions. The chemical lifetime of a molecular marker is commonly interpreted by a relationship between relative decay in particle wall-loss corrected marker concentrations versus integrated OH exposure. However, this relationship strongly depends on vapor wall-loss rates in addition to temperature, organic aerosol concentrations, and OH concentrations. Therefore, inferred lifetimes from a small set of experiments from a single chamber cannot easily be generalized for the full range of atmospheric smoke conditions.

Abatement of ozone-recalcitrant micropollutants during municipal wastewater ozonation: Kinetic modelling and surrogate-based control strategies

Although ozonation is nowadays recognized as one of the most efficient technologies for micropollutants abatement in municipal wastewater effluents, several of the compounds potentially present in those waters exhibit a strong resistance to direct ozone oxidation. In addition, the real-time control of the removal process is still challenging. In this work, the abatement of ozone-recalcitrant micropollutants during wastewater ozonation of six different wastewater effluents was explored using the pesticide acetamiprid as hydroxyl radical (OH) probe. By means of this data, the oxidation efficiency (i.e., hydroxyl radical exposure per consumed ozone) could be described by means of a two-stage model based on the ROHO3 concept. This was possible using a semi-continuous bubbling ozone contactor in all experiments, which permitted the inclusion of the ozone mass balance in the model. ROHO3 values of (1.53–7.60)·10−7 s for initial ozonation stage and (0.61–2.95)·10−6 s for the secondary stage were obtained allowing the characterization and comparison of the process performance in a wide range of effluent qualities, including water matrices with a high content of dissolved and particulate organic matter (total organic carbon (TOC), dissolved organic carbon (DOC) and turbidity tested ranges: 6.7–50 mg C L−1, 6.6–27.6 mg C L−1 and 0.3–28.6 NTU, respectively). Finally, a surrogate strategy involving OH exposure estimation by means of ultraviolet absorbance at 254 nm (UVA254) measurements was proposed based on the ROHO3 concept, and by means of its application the removal of atrazine and ibuprofen in six different wastewaters could be rightly predicted (R2 > 0.98).

Influence of relative humidity on the heterogeneous oxidation of secondary organic aerosol

Abstract. Secondary organic aerosol (SOA) is a complex mixture of hundreds of semi-volatile to extremely low-volatility organic compounds that are chemically processed in the atmosphere, including via heterogeneous oxidation by gas-phase radicals. Relative humidity (RH) has a substantial impact on particle phase, which can affect how SOA evolves in the atmosphere. In this study, SOA from dark α-pinene ozonolysis is heterogeneously aged by OH radicals in a flow tube at low and high RH. At high RH (RH =89 %) there is substantial loss of particle volume (∼60 %) at an equivalent atmospheric OH exposure of 3 weeks. In contrast, at low RH (RH =25 %) there is little mass loss (<20 %) at the same OH exposure. Mass spectra of the SOA particles were measured as a function of OH exposure using a vacuum ultraviolet aerosol mass spectrometer (VUV-AMS). The mass spectra observed at low RH overall exhibit minor changes with oxidation and negligible further changes above an OH exposure =2×1012 molecule cm−3 s suggesting limited impact of oxidation on the particle composition. In contrast, the mass spectra observed at high RH exhibit substantial and continuous changes as a function of OH exposure. Further, at high RH clusters of peaks in the mass spectra exhibit unique decay patterns, suggesting different responses of various species to oxidation. A model of heterogeneous oxidation has been developed to understand the origin of the difference in aging between the low- and high-RH experiments. Differences in diffusivity of the SOA between the low- and high-RH experiments alone can explain the difference in compositional change but cannot explain the difference in mass loss. Instead, the difference in mass loss is attributable to RH-dependent differences in the OH uptake coefficient and/or the net probability of fragmentation, with either or both larger at high RH compared to low RH. These results illustrate the important impact of relative humidity on the fate of SOA in the atmosphere.

Heterogeneous OH oxidation of secondary brown carbon aerosol

Abstract. Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate; for example, oxidation in the atmosphere may alter its optical properties, leading to absorption enhancement or bleaching. Here, we investigate for the first time the effects of heterogeneous OH oxidation on the optical properties of a laboratory surrogate of aqueous, secondary BrC in a series of photo-oxidation chamber experiments. The BrC surrogate was generated from aqueous resorcinol, or 1,3-dihydroxybenzene, and H2O2 exposed to >300 nm radiation that is atomized, passed through trace gas denuders, and injected into the chamber, which was conditioned to either 15 % or 60 % relative humidity (RH). Aerosol absorption and scattering coefficients and single scattering albedo (SSA) at 405 nm were measured using a photoacoustic spectrometer. At 60 % RH, upon OH exposure, absorption first increased, and the SSA decreased sharply. Subsequently, absorption decreased faster than scattering, and SSA increased gradually. Comparisons to the modelled trend in SSA, based on Mie theory calculations, confirm that the observed trend is due to chemical evolution, rather than slight changes in particle size. The initial absorption enhancement is likely due to molecular functionalization and/or oligomerization and the bleaching to fragmentation. By contrast, at 15 % RH, slow absorption enhancement was observed without appreciable bleaching. A multi-layer kinetics model, consisting of two surface reactions in series, was constructed to provide further insights regarding the RH dependence of the optical evolution. Candidate parameters suggest that the oxidation is efficient, with uptake coefficients on the order of unity. The parameters also suggest that, as RH decreases, reactivity decreases and aerosol viscosity increases, such that particles are well-mixed at 60 % RH but not at 15 % RH. These results further the current understanding of the complex processing of BrC that may occur in the atmosphere.

Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation.

Bicarbonate scavenges OH to form CO3- that enhances the bromate formation by ozonation. However, the role of CO3- in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO3- based on the detection of CO3--involved bromate and CO3- exposure. Experiments demonstrated that the CO3--involved bromate was responsible for 33.7-69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO3- exposure was two orders of magnitude higher than the corresponding OH exposure during ozonation. These results demonstrate that CO3- plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than OH. A model was developed based on the ratio of bromine oxidized by CO3-, which could predict the CO3--involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO3--involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of humic acid led to an increase in this contribution during ozonation. The results of this study provide a more in-depth understanding of the mechanism of bromate formation during ozonation.

Ratios of Volatile Organic Compounds in Ambient Air of Various Cities of China

Volatile organic compounds (VOC) ratios in urban atmosphere could provide important information on VOC data quality, sources, and options for removal processes. Based on seven VOC measurement datasets, useful VOC ratios were calculated using four methods, including orthogonal distance(ODR), linear regression, geometric mean, mean, and frequency distribution. Ambient levels of m,p-xylene and o-xylene from the seven datasets showed good correlation with r ranging from 0.975 to 0.997, and their ratios (m,p-xylene/o-xylene) were close, with values of 2.78-3.05, indicating therefore that the ratio of m,p-xylene/o-xylene could be used to evaluate the quality of ambient measurement data in urban atmospheres. In addition, ratios of toluene versus benzene (T/B) and propane versus ethane (P/E) in different cities were compared, to investigate VOC sources in China. The highest T/B ratios were found in Shanghai and Guangzhou City, with values of 2.37 and 1.78, respectively, higher than the result from a tunnel study (1.52), suggesting the important influence of paint and solvent use. The T/B value for Beijing City during summer was close to that from the tunnel study, while the T/B ratios for Chengdu, Beijing during winter, and Chongqing were lower, with values ranging from 0.744 to 1.36, suggesting possible influences from biomass burning and coal combustion. The P/E value in Guangzhou was 1.27, significantly higher than the range of 0.270 to 0.645 found in other cities. The OH exposure in different cities were calculated based on o-xylene/ethylbenzene (X/E), with values ranging from 2.70×1010 to 4.45×1010 molecule·cm-3·s.

Gas-phase composition and secondary organic aerosol formation from standard and particle filter-retrofitted gasoline direct injection vehicles investigated in a batch and flow reactor

Abstract. Gasoline direct injection (GDI) vehicles have recently been identified as a significant source of carbonaceous aerosol, of both primary and secondary origin. Here we investigated primary emissions and secondary organic aerosol (SOA) from four GDI vehicles, two of which were also retrofitted with a prototype gasoline particulate filter (GPF). We studied two driving test cycles under cold- and hot-engine conditions. Emissions were characterized by proton transfer reaction time-of-flight mass spectrometry (gaseous non-methane organic compounds, NMOCs), aerosol mass spectrometry (sub-micron non-refractory particles) and light attenuation measurements (equivalent black carbon (eBC) determination using Aethalometers) together with supporting instrumentation. Atmospheric processing was simulated using the PSI mobile smog chamber (SC) and the potential aerosol mass oxidation flow reactor (OFR). Overall, primary and secondary particulate matter (PM) and NMOC emissions were dominated by the engine cold start, i.e., before thermal activation of the catalytic after-treatment system. Trends in the SOA oxygen to carbon ratio (O : C) for OFR and SC were related to different OH exposures, but divergences in the H : C remained unexplained. SOA yields agreed within experimental variability between the two systems, with a tendency for higher values in the OFR than in the SC (or, vice versa, lower values in the SC). A few aromatic compounds dominated the NMOC emissions, primarily benzene, toluene, xylene isomers/ethylbenzene and C3-benzene. A significant fraction of the SOA was explained by those compounds, based on comparison of effective SOA yield curves with those of toluene, o-xylene and 1,2,4-trimethylbenzene determined in our OFR, as well as others from literature. Remaining discrepancies, which were smaller in the SC and larger in the OFR, were up to a factor of 2 and may have resulted from diverse reasons including unaccounted precursors and matrix effects. GPF retrofitting significantly reduced primary PM through removal of refractory eBC and partially removed the minor POA fraction. At cold-started conditions it did not affect hydrocarbon emission factors, relative chemical composition of NMOCs or SOA formation, and likewise SOA yields and bulk composition remained unaffected. GPF-induced effects at hot-engine conditions deserve attention in further studies.

Prediction of micropollutant abatement during homogeneous catalytic ozonation by a chemical kinetic model

Prediction of micropollutant abatements by catalytic ozonation is critical for its process design and optimization in water treatment. In this study, a chemical kinetic model based on ozone (O3) and hydroxyl radical (OH) rate constants (kO3 and kOH) and O3 and OH exposures is proposed for the generalized prediction of micropollutant abatement by homogeneous catalytic ozonation. Several micropollutants with kO3 ranging from <0.15 to 1.0 × 106 M−1 s−1 were spiked in water matrices (deionized water and surface water) and then treated by ozonation alone and homogeneous catalytic ozonation with varying transition metals (Ti2+, Co2+, Ni2+, Zn2+, Cu2+, Mn2+, Fe2+, and Fe3+). The addition of the varying catalysts enhanced the kinetics and yield of OH formation from O3 decomposition to different extent. Consequently, for the same applied O3 doses, higher OH exposures can generally be obtained at the expense of lower O3 exposures during catalytic ozonation with the varying catalysts compared to ozonation alone. The changes in O3 and OH exposures did not considerably influence the abatement of micropollutants with high and moderate O3 reactivities (diclofenac, gemfibrozil, and bezafibrate), whose abatement efficiencies were generally >90% during both ozonation alone and catalytic ozonation with the varying catalysts. In contrast, ozone-resistant micropollutants (2,4-dichlorophenoxyacetic acid, clofibric acid, and ibuprofen) were less effectively abated during ozonation (∼40–60% abatement), and the addition of the varying catalysts could enhance their absolute abatement efficiencies to various extent (∼0–10% in the deionized water and ∼0–22% in the surface water) during catalytic ozonation. Despite the differing catalytic mechanisms of the varying transition metals, the abatement efficiencies of micropollutants by catalytic ozonation could be satisfactorily predicted by the chemical kinetic model using the O3 and OH rate constants of the micropollutants reported in literature and the O3 and OH exposures determined during the treatment processes. These results demonstrate that the chemical kinetic model can provide a useful tool for the generalized prediction of micropollutant abatement by homogeneous catalytic ozonation.

SOA and gas phase organic acid yields from the sequential photooxidation of seven monoterpenes

We investigate secondary organic aerosol (SOA) and gas phase organic acid yields from the sequential photooxidation of seven monoterpene isomers (α-pinene, β-pinene, limonene, sabinene, terpinolene, α-terpinene, and γ-terpinene) using an Oxidative Flow Reactor under dry conditions. SOA yields were highest for terpinolene (33% at 5.7 days of aging), followed by sabinene, β-pinene, α-pinene, limonene, γ-terpinene, and α-terpinene. Isomers with exocyclic double bonds (i.e. immediately adjacent to a ring) had higher SOA yields than those with endocyclic double bonds, or with double bonds that are unattached to a ring structure. SOA yields increased with OH exposure, highlighting the limitation of using single values for SOA yield in modeling studies, and the need for isomer-specific SOA parameterizations. SOA yields were adequately fit by a one-product model and in broad agreement with previous studies. SOA yields linearly increased with organic aerosol mass concentration, possibly the result of high OH loading and short residence times in the flow reactor. Gas phase yields of formic, acetic, butyric, and methacrylic acid (or their isomers) followed very different patterns as a function of OH exposure from SOA yields, and were poorly correlated with OH rate constants. These observations suggest that higher volatility (fragmentation) products of monoterpene photooxidation were produced and lost on different timescales from the production and condensation of lower-volatility (functionalization) products. Formic acid yields ranged from 0.06 to 9.3% across all OH exposures. Formic acid yields from γ-terpinene (0.36–3%) monotonically increased with OH exposure, unlike the other monoterpene isomers, which exhibited initial increases in formic acid yields with OH exposure, followed by decreases at higher OH concentrations. This difference in organic acid yield trends is consistent with the distribution of carbon-containing species identified by chemical ionization mass spectrometry.

Secondary Organic Aerosol Formation from Ambient Air at an Urban Site in Beijing: Effects of OH Exposure and Precursor Concentrations.

Secondary organic aerosol (SOA) is an important component of atmospheric fine particles (PM2.5), while the key factors controlling SOA formation in ambient air remain poorly understood. In this work, the SOA formation in Beijing urban ambient air was investigated using an oxidation flow reactor (OFR) with high concentrations of OH radicals. The SOA formation potential increased significantly with the increase of ambient PM2.5 concentration during the observation. The optimum ambient exposure time, which is the aging time equivalent to atmospheric oxidation (with similar OH exposure) associated with the peak SOA formation, varied between 2 and 4 days in this study. The OA enhancement in this study was much higher than that of developed countries under different environmental conditions. The higher OA enhancement is probably due to the higher concentrations of volatile organic compounds (VOCs) in the urban air of Beijing. This might also have occurred because fragmentation did not dominate in the oxidation of OA, and did not result in negative OA enhancement on highly polluted days compared to relatively clean days with similar exposure time. These results suggested that under typical ambient conditions, high concentrations of VOC precursors might contribute to sustained organic aerosol growth and long duration haze events in Beijing.

Particle-bound reactive oxygen species (PB-ROS) emissions and formation pathways in residential wood smoke under different combustion and aging conditions

Abstract. Wood combustion emissions can induce oxidative stress in the human respiratory tract by reactive oxygen species (ROS) in the aerosol particles, which are emitted either directly or formed through oxidation in the atmosphere. To improve our understanding of the particle-bound ROS (PB-ROS) generation potential of wood combustion emissions, a suite of smog chamber (SC) and potential aerosol mass (PAM) chamber experiments were conducted under well-determined conditions for different combustion devices and technologies, different fuel types, operation methods, combustion regimes, combustion phases, and aging conditions. The PB-ROS content and the chemical properties of the aerosols were quantified by a novel ROS analyzer using the DCFH (2′,7′-dichlorofluorescin) assay and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). For all eight combustion devices tested, primary PB-ROS concentrations substantially increased upon aging. The level of primary and aged PB-ROS emission factors (EFROS) were dominated by the combustion device (within different combustion technologies) and to a greater extent by the combustion regimes: the variability within one device was much higher than the variability of EFROS from different devices. Aged EFROS under bad combustion conditions were ∼ 2–80 times higher than under optimum combustion conditions. EFROS from automatically operated combustion devices were on average 1 order of magnitude lower than those from manually operated devices, which indicates that automatic combustion devices operated at optimum conditions to achieve near-complete combustion should be employed to minimize PB-ROS emissions. The use of an electrostatic precipitator decreased the primary and aged ROS emissions by a factor of ∼ 1.5 which is however still within the burn-to-burn variability. The parameters controlling the PB-ROS formation in secondary organic aerosol were investigated by employing a regression model, including the fractions of the mass-to-charge ratios m∕z 44 and 43 in secondary organic aerosol (SOA; f44−SOA and f43−SOA), the OH exposure, and the total organic aerosol mass. The regression model results of the SC and PAM chamber aging experiments indicate that the PB-ROS content in SOA seems to increase with the SOA oxidation state, which initially increases with OH exposure and decreases with the additional partitioning of semi-volatile components with lower PB-ROS content at higher OA concentrations, while further aging seems to result in a decay of PB-ROS. The results and the special data analysis methods deployed in this study could provide a model for PB-ROS analysis of further wood or other combustion studies investigating different combustion conditions and aging methods.

Photochemical aging of aerosol particles in different air masses arriving at Baengnyeong Island, Korea

Abstract. Atmospheric aerosol particles are a serious health risk, especially in regions like East Asia. We investigated the photochemical aging of ambient aerosols using a potential aerosol mass (PAM) reactor at Baengnyeong Island in the Yellow Sea during 4–12 August 2011. The size distributions and chemical compositions of aerosol particles were measured alternately every 6 min from the ambient air or through the highly oxidizing environment of a potential aerosol mass (PAM) reactor. Particle size and chemical composition were measured by using the combination of a scanning mobility particle sizer (SMPS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Inside the PAM reactor, O3 and OH levels were equivalent to 4.6 days of integrated OH exposure at typical atmospheric conditions. Two types of air masses were distinguished on the basis of the chemical composition and the degree of aging: air transported from China, which was more aged with a higher sulfate concentration and O : C ratio, and the air transported across the Korean Peninsula, which was less aged with more organics than sulfate and a lower O : C ratio. For both episodes, the particulate sulfate mass concentration increased in the 200–400 nm size range when sampled through the PAM reactor. A decrease in organics was responsible for the loss of mass concentration in 100–200 nm particles when sampled through the PAM reactor for the organics-dominated episode. This loss was especially evident for the m∕z 43 component, which represents less oxidized organics. The m∕z 44 component, which represents further oxidized organics, increased with a shift toward larger sizes for both episodes. It is not possible to quantify the maximum possible organic mass concentration for either episode because only one OH exposure of 4.6 days was used, but it is clear that SO2 was a primary precursor of secondary aerosol in northeast Asia, especially during long-range transport from China. In addition, inorganic nitrate evaporated in the PAM reactor as sulfate was added to the particles. These results suggest that the chemical composition of aerosols and their degree of photochemical aging, particularly for organics, are also crucial in determining aerosol mass concentrations.

Comparison of secondary organic aerosol formation from toluene on initially wet and dry ammonium sulfate particles at moderate relative humidity

Abstract. The formation of secondary organic aerosol (SOA) has been widely studied in the presence of dry seed particles at low relative humidity (RH). At higher RH, initially dry seed particles can exist as wet particles due to water uptake by the seeds as well as the SOA. Here, we investigated the formation of SOA from the photooxidation of toluene using an oxidation flow reactor in the absence of NOx under a range of OH exposures on initially wet or dry ammonium sulfate (AS) seed particles at an RH of 68 %. The ratio of the SOA yield on wet AS seeds to that on dry AS seeds, the relative SOA yield, decreased from 1.31 ± 0.02 at an OH exposure of 4.66 × 1010 molecules cm−3 s to 1.01 ± 0.01 at an OH exposure of 5.28 × 1011 molecules cm−3 s. This decrease may be due to the early deliquescence of initially dry AS seeds after being coated by highly oxidized toluene-derived SOA. SOA formation lowered the deliquescence RH of AS and resulted in the uptake of water by both AS and SOA. Hence the initially dry AS seeds contained aerosol liquid water (ALW) soon after SOA formed, and the SOA yield and ALW approached those of the initially wet AS seeds as OH exposure and ALW increased, especially at high OH exposure. However, a higher oxidation state of the SOA on initially wet AS seeds than that on dry AS seeds was observed at all levels of OH exposure. The difference in mass fractions of m ∕ z 29, 43 and 44 of SOA mass spectra, obtained using an aerosol mass spectrometer (AMS), indicated that SOA formed on initially wet seeds may be enriched in earlier-generation products containing carbonyl functional groups at low OH exposures and later-generation products containing acidic functional groups at high exposures. Our results suggest that inorganic dry seeds become at least partially deliquesced particles during SOA formation and hence that ALW is inevitably involved in the SOA formation at moderate RH. More laboratory experiments conducted with a wide variety of SOA precursors and inorganic seeds under different NOx and RH conditions are warranted.

Optimization of ozonation and peroxone process for simultaneous control of micropollutants and bromate in wastewater.

The study aims to simultaneously control micropollutants and bromate formations by using ozonation and peroxone process. The batch experiments were run with variations in specific ozone dose (SOD) and hydrogen peroxide-to-ozone (H2O2/O3) ratio. Based on the removal by ozonation and peroxone, micropollutants were categorized into three groups: non-reactive compounds (i.e. amidotrizoate), moderately reactive compounds (i.e. metoprolol, acesulfame potassium, bezafibrate, and benzotriazole), and highly reactive compounds (i.e. carbamazepine and diclofenac). For ozonation and peroxone process, the removals for highly reactive compounds and moderately reactive compounds were 82-99% and 29-99%, respectively. The removal of amidotrizoate was not observed in this study. The effect of ozonation on micropollutant removals was similar to the peroxone process. However, differences in bromate formation were observed. Bromate formation depended on the SOD, while addition of hydrogen peroxide suppressed the bromate formation. The peroxone process at the H2O2/O3 ratio of 0.3 was recommended to bromide-containing water below 100 µg·L-1 for simultaneous control of micropollutants and bromate. Enhancement in micropollutant removals, except for the non-reactive groups, was achieved with either higher SOD or the addition of hydrogen peroxide to ozonation. The micropollutant removal predicted from the second-order kinetic reaction with ozone and •OH exposures was higher than the observed data.

Importance of sulfate radical anion formation and chemistry in heterogeneous OH oxidation of sodium methyl sulfate, the smallest organosulfate

Abstract. Organosulfates are important organosulfur compounds present in atmospheric particles. While the abundance, composition, and formation mechanisms of organosulfates have been extensively investigated, it remains unclear how they transform and evolve throughout their atmospheric lifetime. To acquire a fundamental understanding of how organosulfates chemically transform in the atmosphere, this work investigates the heterogeneous OH radical-initiated oxidation of sodium methyl sulfate (CH3SO4Na) droplets, the smallest organosulfate detected in atmospheric particles, using an aerosol flow tube reactor at a high relative humidity (RH) of 85 %. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer showed that neither functionalization nor fragmentation products are detected. Instead, the ion signal intensity of the bisulfate ion (HSO4−) has been found to increase significantly after OH oxidation. We postulate that sodium methyl sulfate tends to fragment into a formaldehyde (CH2O) and a sulfate radical anion (SO4 ⋅ −) upon OH oxidation. The formaldehyde is likely partitioned back to the gas phase due to its high volatility. The sulfate radical anion, similar to OH radical, can abstract a hydrogen atom from neighboring sodium methyl sulfate to form the bisulfate ion, contributing to the secondary chemistry. Kinetic measurements show that the heterogeneous OH reaction rate constant, k, is (3.79 ± 0.19) × 10−13 cm3 molecule−1 s−1 with an effective OH uptake coefficient, γeff, of 0.17 ± 0.03. While about 40 % of sodium methyl sulfate is being oxidized at the maximum OH exposure (1.27 × 1012 molecule cm−3 s), only a 3 % decrease in particle diameter is observed. This can be attributed to a small fraction of particle mass lost via the formation and volatilization of formaldehyde. Overall, we firstly demonstrate that the heterogeneous OH oxidation of an organosulfate can lead to the formation of sulfate radical anion and produce inorganic sulfate. Fragmentation processes and sulfate radical anion chemistry play a key role in determining the compositional evolution of sodium methyl sulfate during heterogeneous OH oxidation.

Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400-650 nm) of β-pinene and p-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NO x-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, n, shows a slight spectral dependence in the visible range. With increased OH exposure, n first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA.

Kinetic evaluation of graphene oxide based heterogenous catalytic ozonation for the removal of ibuprofen

In this study, the performance of graphene oxide (GO) in ozonation process was kinetically evaluated using the modified Rct concept since GO may act as initiator, promoter and inhibitor in ozone radical chain reaction. The applicability of the modified Rct concept was demonstrated using different GO suspensions (GO alone, GO/TiO2, GO/Fe3O4, GO/TiO2/Fe3O4) in ozonation process. Results showed that ozone exposure and •OH exposure were found to be higher for GO/Fe3O4 and GO/TiO2/Fe3O4 compared to other GO suspensions, which was almost equivalent to O3/H2O2 process. The determined initiation and inhibition rate constants of GO alone, were 1 fold higher than GO/Fe3O4 and GO/TiO2/Fe3O4, since the GO alone suspension possesses higher O3 decomposition but lower organic degradation because that GO does not yield •OH. Moreover, GO/Fe3O4 suspension, along with natural organic matter (NOM), was proven to be helpful in degrading ibuprofen in ozonation process, but the effect was minimal when compared to O3/H2O2 process. These results exhibited that the surface modified GO suspensions could be utilized as future alternative AOPs.

Open burning of rice, corn and wheat straws: primary emissions, photochemical aging, and secondary organic aerosol formation

Abstract. Agricultural residues are among the most abundant biomass burned globally, especially in China. However, there is little information on primary emissions and photochemical evolution of agricultural residue burning. In this study, indoor chamber experiments were conducted to investigate primary emissions from open burning of rice, corn and wheat straws and their photochemical aging as well. Emission factors of NOx, NH3, SO2, 67 non-methane hydrocarbons (NMHCs), particulate matter (PM), organic aerosol (OA) and black carbon (BC) under ambient dilution conditions were determined. Olefins accounted for &gt; 50 % of the total speciated NMHCs emission (2.47 to 5.04 g kg−1), indicating high ozone formation potential of straw burning emissions. Emission factors of PM (3.73 to 6.36 g kg−1) and primary organic carbon (POC, 2.05 to 4.11 gC kg−1), measured at dilution ratios of 1300 to 4000, were lower than those reported in previous studies at low dilution ratios, probably due to the evaporation of semi-volatile organic compounds under high dilution conditions. After photochemical aging with an OH exposure range of (1.97–4.97) × 1010 molecule cm−3 s in the chamber, large amounts of secondary organic aerosol (SOA) were produced with OA mass enhancement ratios (the mass ratio of total OA to primary OA) of 2.4–7.6. The 20 known precursors could only explain 5.0–27.3 % of the observed SOA mass, suggesting that the major precursors of SOA formed from open straw burning remain unidentified. Aerosol mass spectrometry (AMS) signaled that the aged OA contained less hydrocarbons but more oxygen- and nitrogen-containing compounds than primary OA, and carbon oxidation state (OSc) calculated with AMS resolved O ∕ C and H ∕ C ratios increased linearly (p &lt; 0.001) with OH exposure with quite similar slopes.

Significant Production of Secondary Organic Aerosol from Emissions of Heated Cooking Oils

Cooking emissions have been identified as a major source of primary organic aerosol (POA) in urban environments. Cooking may also be a potential source of secondary organic aerosol (SOA) because of the abundant emissions of non-methane organic gases. We studied SOA formation from the photooxidation of emissions from seven vegetable oils heated at 200 °C under high-NOx conditions in a smog chamber. After the samples had been aged under an OH exposure of 1.0 × 1010 molecules cm–3 s, the SOA formation rate was generally 1 order of magnitude higher than the POA emission rate. We determined that alkenals, which are not traditional SOA precursors in chemical transport models, accounted for 5–34% of the observed SOA. The unexplained SOA may be attributed to the oxidation of primary semivolatile and intermediate-volatility organic compounds (SVOCs and IVOCs, respectively), which were estimated to contribute an additional 9–106% of the observed SOA assuming the volatility distribution of heated cooking oils is the...

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

Abstract. Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxyl functionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.