Hydrogen peroxide (H2O2) is a vital oxidant in the atmosphere and plays critical roles in the oxidation chemistry of both liquid and aerosol phases. The partitioning of H2O2 between the gas and liquid phase or the aerosol phase could affect its abundance in these condensed phases and eventually the formation of secondary components. However, the partitioning processes of H2O2 in gas-liquid and gas-aerosol phases are still unclear, especially in the ambient atmosphere. In this study, field observations of gas-, liquid-, and aerosol-phase H2O2 were carried out in the urban atmosphere of Beijing during the summer and winter of 2018. The effective field-derived mean value of Henry’s law constant ( , 2.1 × 105 M atm−1) was 2.5 times of the theoretical value in pure water ( , 8.4 × 104 M atm−1) at 298 ± 2 K. The effective derived gas-aerosol partitioning coefficient ( , 3.8 × 10−3 m3 mg−1) was four orders of magnitude higher on average than the theoretical value ( , 2.8 × 10−7 m3 mg−1) at 270 ± 4 K. Beyond following Henry’s law or Pankow’s absorptive partitioning theory, the partitioning of H2O2 in the gas-liquid and gas-aerosol phases in the ambient atmosphere was also influenced by certain physical and chemical reactions. The average concentration of liquid-phase H2O2 in rainwater during summer was 44.12 ± 26.49 mM. In 69 % of the collected rain samples, the measured level of H2O2 was greater than the predicted value in pure water calculated by Henry’s law. In these samples, 41 % of the measured H2O2 was from gas-phase partitioning, while most of the rest may be from residual H2O2 in raindrops. In winter, the level of aerosol-phase H2O2 was 0.093 ± 0.085 ng mg−1, which was much higher than the predicted value based on Pankow’s absorptive partitioning theory. The contribution of partitioning of the gas-phase H2O2 to the aerosol-phase H2O2 formation was negligible. The decomposition/hydrolysis rate of aerosol-phase organic peroxides could account for 11−74 % of the consumption rate of aerosol-phase H2O2, and the value depended on the composition of organic peroxides in the aerosol particles. Furthermore, the heterogeneous uptake of HO2 and H2O2 on aerosols contributed to 22 % and 2 % of the aerosol-phase H2O2 consumption, respectively.
Carbonyl compounds (carbonyls) play important roles in atmospheric photochemistry, serving as reservoirs of radicals (OH, HO2, and RO2) and precursors of secondary organic aerosols (SOA). Field measurements of gaseous and particulate carbonyls were taken over urban Beijing during summer and winter, and field-measured gas-particle partitioning coefficients (Kp ) were determined. Compared with theoretical values, field-measured Kp values were 4–6 orders of magnitude higher for the six detected carbonyls, which underlined the importance of heterogeneous reactions. In winter, the Kp values of carbonyl compounds were one order of magnitude higher than those in summer owing to the effect of temperature. This study applied the positive matrix factorization (PMF) model to the source apportionment of carbonyl compounds. Five factors were identified for both summer and winter, whereas the biogenic factor was only identified in summer and coal burning was only found in winter. In summer, secondary formation was the largest contributor (39%) to the measured total carbonyl compounds levels. In contrast, vehicular exhaust was the largest source of the measured total carbonyl compounds in winter (37%), although secondary formation still had an important contribution of 31%. The contribution of coal burning to ambient carbonyls was reduced by half compared with prior results. As the most abundant carbonyl compound in the atmosphere, formaldehyde in summer mainly came from secondary production (42%) and primary anthropogenic emissions (48%), while biogenic sources had a minor contribution (10%). However, 78% of formaldehyde was attributed to primary anthropogenic emissions in winter, which indicated that these winter emissions were more important sources of carbonyl compounds. Glyoxal was always dominated by secondary formation, with contributions of 56% in summer and 52% in winter.
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.
Peroxyacetic acid (PAA, CH3C(O)OOH) plays an important role in atmospheric chemistry, serving as reactive oxidant and affecting radical recycling. However, previous studies revealed an obvious gap between modelled and observed concentrations of atmospheric PAA, which may be partly ascribed to the uncertainty in the kinetics and mechanism of OH-oxidation. In this study, we measured the rate constant of OH radical reaction with PAA (kPAA+OH) and investigated the products in order to develop a more robust atmospheric PAA chemistry. Using the relative rates technique and employing toluene and metaxylene as reference compounds, the kPAA+OH was determined to be (9.4-11.9)*10-12 cm3 molecule-1 s-1 at 298 K and 1 atm, which is about (2.5-3.2) times larger than that parameter used in Master Chemical Mechanism v3.3.1 (MCM v3.3.1) (3.70*10-12 cm3 molecule-1 s-1). Incorporation of a box model and MCM v3.3.1 with revised PAA chemistry represented a better simulation of atmospheric PAA observed during Wangdu Campaign 2014, a rural site in North China Plain. It is found that OH-oxidation is an important sink of atmospheric PAA in this rural area, accounting for ~30% of the total loss. Moreover, the major terminal products of PAA-OH reaction were identified as formaldehyde (HCHO) and formic acid (HC(O)OH). The modelled results show that both primary and secondary chemistry play an important role in the large HCHO and HC(O)OH formation under experimental conditions. There should exist the channel of methyl H-abstraction for PAA-OH reaction, which may also provide routes to HCHO and HC(O)OH formation.
The heterogeneous oxidation of sulfur dioxide (SO2) on a-Al2O3 particles was investigated using a flow reactor coupled with a transmission-Fourier transform infrared (T-FTIR) spectrometer at different relative humidities (RH) in the absence or presence of hydrogen peroxide (H2O2), with an emphasis on the saturation coverage of SO2 and the timescale on which the reaction reaches saturation. It is found that the saturation coverage of SO2 in the absence of H2O2 increases with rising RH due to the hydrolysis of SO2 by surface adsorbed water. However, the reaction ultimately reaches saturation since the produced sulfite/bisulfite cannot be further converted to sulfate/bisulfate in the absence of oxidants. In addition, the presence of H2O2 can significantly increase the saturation coverage of SO2 by efficiently oxidizing sulfite/bisulfite to sulfate/bisulfate. Under humid conditions, adsorbed water facilitates the hydrolysis of SO2 and mitigates the increase of surface acidity, which can inhibit the hydrolysis of SO2. Hence, in the presence of H2O2, the saturation coverage of SO2 as well as the time of reaction reaching saturation increases with rising RH and the surface is not saturated on the timescale of the experiments (40 h) at 60% RH. Furthermore, the increase of saturation coverage of SO2 in the presence of H2O2 was observed on chemically inactive SiO2 particles, indicating that the hydrolysis of SO2 and subsequent oxidation by H2O2 likely occurs on other types of particles. Our findings are of importance for understanding the role of water vapor and trace gases (e.g., H2O2) in the heterogeneous reaction of SO2 in the atmosphere.
Measurements of atmospheric peroxides were made during Wangdu Campaign 2014 at Wangdu, a rural site in the North China Plain (NCP) in summer 2014. The predominant peroxides were detected to be hydrogen peroxide (H2O2), methyl hydroperoxide (MHP) and peroxyacetic acid (PAA). The observed H2O2 reached up to 11.3 ppbv, which was the highest value compared with previous observations in China at summer time. A box model simulation based on the Master Chemical Mechanism and constrained by the simultaneous observations of physical parameters and chemical species was performed to explore the chemical budget of atmospheric peroxides. Photochemical oxidation of alkenes was found to be the major secondary formation pathway of atmospheric peroxides, while contributions from alkanes and aromatics were of minor importance. The comparison of modeled and measured peroxide concentrations revealed an underestimation during biomass burning events and an overestimation on haze days, which were ascribed to the direct production of peroxides from biomass burning and the heterogeneous uptake of peroxides by aerosols, respectively. The strengths of the primary emissions from biomass burning were on the same order of the known secondary production rates of atmospheric peroxides during the biomass burning events. The heterogeneous process on aerosol particles was suggested to be the predominant sink for atmospheric peroxides. The atmospheric lifetime of peroxides on haze days in summer in the NCP was about 2–3 h, which is in good agreement with the laboratory studies. Further comprehensive investigations are necessary to better understand the impact of biomass burning and heterogeneous uptake on the concentration of peroxides in the atmosphere.
Organic peroxides, important species in the atmosphere, promote secondary organic aerosol (SOA) aging, affect HOx radicals cycling, and cause adverse health effects. However, the formation, gas-particle partitioning, and evolution of organic peroxides are complicated and still unclear. In this study, we investigated in the laboratory the production and gas-particle partitioning of peroxides from the ozonolysis of a-pinene, which is one of the major biogenic volatile organic compounds in the atmosphere and an important precursor for SOA at a global scale. We have determined the molar yields of hydrogen peroxide (H2O2), hydromethyl hydroperoxide (HMHP), peroxyformic acid (PFA), peroxyacetic acid (PAA), and total peroxides (TPOs, including unknown peroxides) and the fraction of peroxides in a-pinene/O3 SOA. Comparing the gas-phase peroxides with the particle-phase peroxides, we find that gas-particle partitioning coefficients of PFA and PAA are 104 times higher than the values from the theoretical prediction, indicating that organic peroxides play a more important role in SOA formation than previously expected. Here, the partitioning coefficients of TPO were determined to be as high as (2–3)*104 m3 mg-1. Even so, more than 80% of the peroxides formed in the reaction remain in the gas phase. Water changes the distribution of gaseous peroxides, while it does not affect the total amount of peroxides in either the gas or the particle phase. Approx. 18% of gaseous peroxides undergo rapid heterogeneous decomposition on SOA particles in the presence of water vapor, resulting in the additional production of H2O2. This process can partially explain the unexpectedly high H2O2 yields under wet conditions. Transformation of organic peroxides to H2O2 also preserves OH in the atmosphere, helping to improve the understanding of OH cycling.
Glyoxal (GL) plays a crucial role in the formation of secondary organic aerosols (SOA), because it is highly water soluble and capable of oligomerization. This is the first study to describe irreversible heterogeneous reactions of GL on clean and acidic gas-aged SiO2, a-Al2O3, and CaCO3 particles, as models of real mineral particles, at various relative humidity and without irradiation and gas phase oxidants. A series of products, including oligomers, organosulfates, and organic acids, which contribute to SOA formation, were produced. GL uptake on SO2-aged a-Al2O3 enabled the oxidation of surface S(IV) to S(VI). The presence of adsorbed water on particles favored GL uptake and the formation of oligomers and organosulfate, but it suppressed organic acid formation. In addition, the aging process enhanced the positive effect of adsorbed water on GL uptake. These findings will further our understanding of the GL sink and SOA sources in the atmosphere.
Carbonyl compounds are important precursors of secondary air pollutants. With the rapid economic development and the implementation of stricter control measures in Beijing, the sources of carbonyls possibly changed. Based on measurement data obtained at an urban site in Beijing between 2005 and 2012, we investigated annual variations in carbonyl levels and sources during these years. In summer, formaldehyde and acetaldehyde levels decreased significantly at a rate of 9.1%/year and 7.2%/year, respectively, while acetone levels increased at a rate of 4.3%/year. In winter, formaldehyde levels increased and acetaldehyde levels decreased. We also investigated the factors driving the variation in carbonyls levels during summer by determination of emission ratios for carbonyls and their precursors, and calculation of photochemical formation of carbonyls. The relative declines for primary formaldehyde and acetaldehyde levels were larger than those for secondary formation. This is possibly due to the increasing usage of natural gas and liquefied petroleum gas which could result in the rise of carbonyl precursor emission ratios. The increase in acetone levels might be related to the rising solvent usage in Beijing during these years. The influences of these sources should be paid more attention in future research.
Carbonyl compounds play an important role in the formation of secondary aerosols and the cycling of free radicals in the atmosphere. We measured carbonyl compounds over urban Beijing, a megacity in the North China Plain, in summer and winter to investigate the relation of carbonyl compounds with haze and the interaction between carbonyl compounds and atmospheric radical cycling. We also determined carbonyl compounds in summer rainwater. Data of carbonyl compounds were analyzed in four cases, i.e., summer haze days (SHD), summer non-haze days (SND), winter haze days (WHD), and winter non-haze days (WND). Interestingly, the level of carbonyl compounds during WHD approached that of summer days. The results of the principal component analysis showed that there was no obvious source difference between SHD and SND. On WHD, however, more carbonyl compounds originated from the “diesel engine exhaust emission” than those on WND. We evaluated the effect of carbonyl compounds on the free radical cycling and the NO consumption potential for OH formation in the photochemical reactions using a novel ratio method. It was found that the production rate of ROx (the sum of OH, HO2 and RO2 radicals) was highest on SND, while the yield of ROx radicals from the reactions of carbonyl compounds was highest on WHD. Further, carbonyl compounds consumed more NO to produce OH radicals on WHD compared to the other three cases.
Heterogeneous reaction of SO2 on mineral dust seems to be an important sink for SO2. However, kinetic data about this reaction on authentic mineral dust are scarce and are mainly limited to low relative humidity (RH) conditions. In addition, little is known about the role of hydrogen peroxide (H2O2) in this reaction. Here, we investigated the uptake kinetics of SO2 on three authentic mineral dusts (i.e., Asian mineral dust (AMD), Tengger desert dust (TDD), and Arizona test dust (ATD)) in the absence and presence of H2O2 at different RHs using a filter-based flow reactor, and applied a parameter (effectiveness factor) to the estimation of the effective surface area of particles for the calculation of the corrected uptake coefficient (γc). We found that with increasing RH, the γc decreases on AMD particles, but increases on ATD and TDD particles. This discrepancy is probably due to the different mineralogy compositions and aging extents of
these dust samples. Furthermore, the presence of H2O2 can promote the uptake of SO2 on mineral dust at different RHs. The probable explanations are that H2O2 rapidly reacts with SO2 on mineral dust in the presence of adsorbed water, and OH radicals, which can be produced from the heterogeneous decomposition of H2O2 on the mineral dust, immediately react with adsorbed SO2 as well. Our results suggest that the removal of SO2 via the heterogeneous reaction on mineral dust is an important sink for SO2 and has the potential to alter the physicochemical properties (e.g., ice nucleation ability) of mineral dust particles in the atmosphere.
The existence and importance of peroxyformic acid (PFA) in the atmosphere has been under controversy. We present here, for the first time, the observation data for PFA from four field measurements carried out in China. These data provided powerful evidence that PFA can stay in the atmosphere, typically in dozens of pptv level. The relationship between PFA and other detected peroxides was examined. The results showed that PFA had a strong positive correlation with its homolog, peroxyacetic acid, due to their similar sources and sinks. Through an evaluation of PFA production and removal rates, we proposed that the reactions between peroxyformyl radical (HC(O)O2) and formaldehyde or the hydroperoxyl radical (HO2) were likely to be the major source and degradation into formic acid (FA) was likely to be the major sink for PFA. Based on a box model evaluation, we proposed that the HC(O)O2 and PFA chemistry was a major source for FA under low NOx conditions. Furthermore, it is found that the impact of the HC(O)O2 and PFA chemistry on radical cycling was dependent on the yield of HC(O)O2 radical from HC(O) + O2 reaction. When this yield exceeded 50%, the HC(O)O2 and PFA chemistry should not be neglected for calculating the radical budget. To make clear the exact importance of HC(O)O2 and PFA chemistry in the atmosphere, further kinetic, field and modeling studies are required.
Hydrogen peroxide (H2O2) and organic peroxides play important roles in the cycle of oxidants and the formation of secondary aerosols in the atmosphere. Recent field observations have suggested that the budget of peroxyacetic acid (PAA, CH3C(O)OOH) is potentially related to the aerosol phase processes, especially to secondary aerosol formation. Here, we present the first laboratory measurements of the uptake coefficient of gaseous PAA and H2O2 onto ambient fine particulate matter (PM2.5) as a function of relative humidity (RH) at 298 K. The results show that the PM2.5, which was collected in an urban area, can take up PAA and H2O2 at the uptake coefficient (r) of 10-4, and bothg rPAA and rH2O2 increase with increasing RH. The value of rPAA at 90%RH is 5.4±1.9 times that at 3%RH, whereas rH2O2 at 90%RH is 2.4±0.5 times that at 3%RH, which suggests that PAA is more sensitive to the RH variation than H2O2 is. Considering the larger Henry’s law constant of H2O2 than that of PAA, the smaller RH sensitivity of the H2O2 uptake coefficient suggests that the enhanced uptake of peroxide compounds on PM2.5 under humid conditions is dominated by chemical processes rather than dissolution. Considering that mineral dust is one of the main components of PM2.5 in Beijing, we also determined the uptake coefficients of gaseous PAA and H2O2 on authentic Asian Dust storm (ADS) and Arizona Test Dust (ATD) particles. Compared to ambient PM2.5, ADS shows a similar value and RH dependence in its uptake coefficient for PAA and H2O2, while ATD gives a negative dependence on RH. The present study indicates that, in addition to the mineral dust in PM2.5, other components (e.g., soluble inorganic salts) are also important to the uptake of peroxide compounds. When the heterogeneous reaction of PAA on PM2.5 is considered, its atmospheric lifetime is estimated to be 3.0 h on haze days and 7.1 h on non-haze days, values that are in good agreement with the field observations.
Heterogeneous oxidation of oxygenated volatile organic compounds (OVOCs) serves as an important sink of OVOCs as well as a source of secondary organic material. However, the roles of gas phase oxidants in these reactions are poorly understood. In this work, we present the first laboratory study of the heterogeneous reactions of methacrolein (MACR) on various mineral dust particles in the presence of gaseous H2O2. It is found that the presence of gaseous H2O2 significantly promotes both the uptake and oxidation of MACR on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, but not on CaCO3. The oxidation of MACR produces organic acids as its major low-molecular-weight product, whose yields are enhanced by a factor of 2−6 in the presence of H2O2. In addition, organic peroxides such as methyl hydroperoxide, peroxyformic acid, and peroxyacetic acid are only formed in the presence of H2O2, and the formation of methyl hydroperoxide indicates that MACR oxidation on the surface involves reaction with OH radicals. A probe reaction using salicylic acid verifies the production of OH radicals from H2O2 decomposition on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, which rationalizes the enhanced MACR oxidation observed on these particles. The uptake coefficients of MACR on kaolinite, α-Fe2O3, and TiO2 in the presence of H2O2 are on the order of 10−5−10−4. Our results provide new insights into the formation and chemical evolution of organic species in the atmosphere.
Field measurements of atmospheric peroxides were obtained during the summer on two consecutive years over urban Beijing, which highlighted the impacts of aerosols on the chemistry of peroxide compounds and hydroperoxyl radicals (HO2). The major peroxides were determined to be hydrogen peroxide (H2O2), methyl hydroperoxide (MHP), and peroxyacetic acid (PAA). A negative correlation was found between H2O2 and PAA in rainwater, providing evidence for a conversion between H2O2 and PAA in the aqueous phase. A standard gas phase chemistry model based on the NCAR Master Mechanism provided a good reproduction of the observed H2O2 profile on non-haze days but greatly overpredicted the H2O2 level on haze days. We attribute this overprediction to the reactive uptake of HO2 by the aerosols, since there was greatly enhanced aerosol loading and aerosol liquid water content on haze days. The discrepancy between the observed and modeled H2O2 can be diminished by adding to the model a newly proposed transition metal ion catalytic mechanism of HO2 in aqueous aerosols. This confirms the importance of the aerosol uptake of HO2 and the subsequent aqueous phase reactions in the reduction of H2O2. The closure of HO2 and H2O2 between the gas and aerosol phases suggests that the aerosols do not have a net reactive uptake of H2O2, because the conversion of HO2 to H2O2 on aerosols compensates for the H2O2 loss. Laboratory studies for the aerosol uptake of H2O2 in the presence of HO2 are urgently required to better understand the aerosol uptake of H2O2 in the real atmosphere.
In this study, 121 daily PM2.5 (aerosol particle with aerodynamic diameter less than 2.5 μm) samples were collected from an urban site in Beijing in four months between April 2009 and January 2010 representing the four seasons. The samples were determined for various compositions, including elements, ions, and organic/elemental carbon. Various approaches, such as chemical mass balance, positive matrix factorization (PMF), trajectory clustering, and potential source contribution function (PSCF), were employed for characterizing aerosol speciation, identifying likely sources, and apportioning contributions from each likely source. Our results have shown distinctive seasonality for various aerosol speciations associated with PM2.5 in Beijing. Soil dust waxes in the spring and wanes in the summer. Regarding the secondary aerosol components, inorganic and organic species may behave in different manners. The former preferentially forms in the hot and humid summer via photochemical reactions, although their precursor gases, such as SO2 and NOx, are emitted much more in winter. The latter seems to favorably form in the cold and dry winter. Synoptic meteorological and climate conditions can overwhelm the emission pattern in the formation of secondary aerosols. The PMF model identified six main sources: soil dust, coal combustion, biomass burning, traffic and waste incineration emission, industrial pollution, and secondary inorganic aerosol. Each of these sources has an annual mean contribution of 16, 14, 13, 3, 28, and 26%, respectively, to PM2.5. However, the relative contributions of these identified sources significantly vary with changing seasons. The results of trajectory clustering and the PSCF method demonstrated that regional sources could be crucial contributors to PM pollution in Beijing. In conclusion, we have unraveled some complex aspects of the pollution sources and formation processes of PM2.5 in Beijing. To our knowledge, this is the first systematic study that comprehensively explores the chemical characterizations and source apportionments of PM2.5 aerosol speciation in Beijing by applying multiple approaches based on a completely seasonal perspective.