Thermal decomposition and oxidation of bastnaesite concentrate in inert and oxidative atmosphere

To clearly elucidate the oxidative roasting behaviors of the bastnaesite, the thermal decomposition and oxidation of the bastnaesite concentrate in inert and oxidative atmosphere have been investigated in detail. Experimental data indicated that the initial decomposition temperature of the concentrate under N2 atmosphere is 150 ℃ higher than that under O2 atmosphere,most likely because the oxidation of the cerium induces the decomposition of the concentrate. For the roasted samples under N2 atmosphere at500 ℃ and above,the oxidation efficiency of the cerium is 19.8%-26.8% because of the fact that rareearth fluorocarbonate is first decomposed to form rare-earth oxyfluoride and CO2, and the cerium oxyfluoride is then partially oxidized by the CO2 gas. The rest cerium in these samples can be further oxidized in air at room temperature, with the oxidation efficiency of the cerium gradually increasing to above 80% in 7 d. This can be attributed to the obvious changes in the inner morphology of the roasted samples under N2 atmosphere at high temperatures, which largely induce the diffusion of the air and improves the oxidation activity of CeOF, and further induces the oxidation of CeOF by the air. XRD and XPS techniques were used to further verify the significant differences in the thermal decomposition behaviors of the bastnaesite concentrate under N2 and O2 atmosphere. Moreover, no oxidation of Pr^（3＋） to Pr^（4＋） in the roasted samples under both N2 and O2 atmosphere is observed. This gives an overall understanding of the oxidative roasting of the bastnaesite concentrate without additives.

Himalayas as a global hot spot of springtime stratospheric intrusions:Insight from isotopic signatures in sulfate aerosols

Downward transport of stratospheric air into the troposphere(identified as stratospheric intrusions)could potentially modify the radiation budget and chemical of the Earth's surface atmosphere.As the highest and largest plateau on earth,the Tibetan Plateau including the Himalayas couples to global climate,and has attracted widespread attention due to rapid warming and cryospheric shrinking.Previous studies recognized strong stratospheric intrusions in the Himalayas but are poorly understood due to limited direct evidences and the complexity of the meteorological dynamics of the third pole.Cosmogenic^(35)S is a radioactive isotope predominately produced in the lower stratosphere and has been demonstrated as a sensitive chemical tracer to detect stratospherically sourced air mass in the planetary boundary layer.Here,we report 6-month(April–September 2018)observation of^(35)S in atmospheric sulfate aerosols(^(35)SO_(4)^(2-))collected from a remote site in the Himalayas to reveal the stratospheric intrusion phenomenon as well as its potential impacts in this region.Throughout the sampling campaign,the^(35)SO_(4)^(2-)concentrations show an average of 1,070±980 atoms/m^(3).In springtime,the average is 1,620±730 atoms/m^(3),significantly higher than the global existing data measured so far.The significant enrichments of^(35)SO_(4)^(2-)measured in this study verified the hypothesis that the Himalayas is a global hot spot of stratospheric intrusions,especially during the springtime as a consequence of its unique geology and atmospheric couplings.In combined with the ancillary evidences,e.g.,oxygen-17 anomaly in sulfate and modeling results,we found that the stratospheric intrusions have a profound impact on the surface ozone concentrations over the study region,and potentially have the ability to constrain how the mechanisms of sulfate oxidation are affected by a change in plateau atmospheric properties and conditions.This study provides new observational constraints on stratospheric intrusions in the Himalayas,which would further provide additional information for a deeper understanding on the environment and climatic changes over the Tibetan Plateau.

Role of Hydrogen Migrations in Carbonyl Peroxy Radicals in the Atmosphere

Carbonyl peroxy radicals (RC(O)O2) are the ubiquitous radical intermediates in the atmospheric oxidation of volatile organic compounds. In this work, theoretical studies are carried out to explore the role of the unimolecular H-migration in the carbonyl peroxy radicals by using quantum chemistry and kinetics calculations. The results showed that H-migration could be significant in the atmosphere at least in CH3CH2CH2C(O)O2 and (CH3)2CHCH2C(O)O2 with rates of ∽0.012 and -0.58^-1 at 298 K. Subsequent reactions of CH3CHCH2C(O)OOH would lead to the products with multi-functional groups, which might affect the aerosol formation process;while (CH3)2CCH2C(O)OOH would transform to formaldehyde and acetone in a few steps. These processes would be important for the atmospheric modelling of volatile organic compounds under low-NOx conditions.

Reactions with Criegee intermediates are the dominant gas-phase sink for formyl fluoride in the atmosphere

Atmospheric oxidation processes are of central importance in atmospheric climate models.It is often considered that volatile organic molecules are mainly removed by hydroxyl radical;however,the kinetics of some reactions of hydroxyl radical with volatile organic molecules are slow.Here we report rate constants for rapid reactions of formyl fluoride with Criegee intermediates.These rate constants are calculated by dual-level multistructural canonical variational transition state theory with small-curvature tunneling(DL-MS-CVT/SCT).The treatment contains beyond-CCSD(T)electronic structure calculations for transition state theory,and it employs validated density functional input for multistructural canonical variational transition state theory with small-curvature tunneling and for variable-reaction-coordinate variational transition state theory.We find that the M11-L density functional has higher accuracy than CCSD(T)/CBS for the HC(O)F+CH2OO and HC(O)F+anti-CH_(3)CHOO reactions.We find significant negative temperature dependence in the ratios of the rate constants for HC(O)F+CH2OO/anti-CH_(3)CHOO to the rate constant for HC(O)F+OH.We also find that different Criegee intermediates have different rate-determining-steps in their reactions with formyl fluoride,and we find that the dominant gas-phase removal mechanism for HC(O)F in the atmosphere is the reaction with CH2OO and/or anti-CH_(3)CHOO Criegee intermediates.

Photooxidation of Methacrolein in Fe(III)-Oxalate Aqueous System and Its Atmospheric Implication

Iron and oxalic acids are widely distributed in the atmosphere and easily form ferric oxalate complex(Fe(III)-Ox).The tropospheric aqueous-phase could provide a medium to enable the photo-Fenton reaction with Fe(III)-Ox under solar irradiation.Although the photolysis mechanisms of Fe(III)-Ox have been investigated extensively,information about the oxidation of volatile organic compounds(VOC),specifically the potential for Secondary Organic Aerosol(SOA)formation in the Fe(III)-Ox system,is lacking.In this study,a ubiquitous VOC methacrolein(MACR)is chosen as a model VOC,and the oxidation of MACR with Fe(III)-Ox is investigated under typical atmospheric water conditions.The effects of oxalate concentration,Fe(III)concentration,MACR concentration,and pH on the oxidation of MACR are studied in detail.Results show that the oxidation rate of MACR greatly accelerates in the presence of oxalate when compared with only Fe(III).The oxidation rate of MACR also accelerates with increasing concentration of oxalate.The effect of Fe(III)is found to be more complicated.The oxidation rate of MACR first increases and then decreases with increasing Fe(III)concentration.The oxidation rate of MACR increases monotonically with decreasing pH in the common atmospheric water pH range or with decreasing MACR concentration.The production of ferrous and hydrogen peroxide,pH,and aqueous absorbance are monitored throughout the reaction process.The quenching experiments verify that·OH and O_(2)^(+)are both responsible for the oxidation of MACR.MACR is found to rapidly oxidize into small organic acids with higher boiling points and oligomers with higher molecular weight,which contributes to the yield of SOA.These results suggest that Fe(III)-Ox plays an important role in atmospheric oxidation.

NO_x Release from Snow and Ice Covered Surface in Polar Regions and the Tibetan Plateau

The exchanges of NOx between snow and air have significant impact on the atmospheric components and photochemical processes in the overlying boundary layer. Such exchanges increase the oxidizing capacity of the atmosphere and may have a crucial impact on the air signals that are retrieved from ice cores. In the recent years, sunlit snow and ice have been demonstrated to be important NOx sources in the polar atmospheric boundary layer. This paper makes a thorough review on the release of NOx from snow and ice, including field observations and experimental evidences, release mechanisms and influential parameters that affect such a release process, polar NOx concentrations and fluxes, and environmental impacts of the chemical processes of NOx in the polar atmospheric boundary layer. In the Tibetan Plateau, the released NOx observed recently in the sunlit snow/ice-cover is 1-order magnitude more than that in polar regions, but further scientific research is still needed to reveal its impact on the atmospheric oxidizing capacity.

Significant contribution of carbonyls to atmospheric oxidation capacity(AOC)during the winter haze pollution over North China Plain

Atmospheric carbonyl compounds play significant roles in the cycling of radicals and have exhibited surprisingly high levels in winter that were well correlated to particulate matter,for which the reason have not been clearly elucidated.Here we measured carbonyl compounds and other trace gasses together with PM_(2.5)over urban Jinan in North China Plain during the winter.Markedly higher carbonyl concentrations(average:14.63±4.21 ppbv)were found during wintertime haze pollution,about one to three-times relative to those on nonhaze days,with slight difference in chemical composition except formaldehyde(HCHO).HCHO(3.68 ppbv),acetone(3.17 ppbv),and acetaldehyde(CH_3CHO)(2.83 ppbv)were the three most abundant species,accounting for~75% of the total carbonylson both haze and non-haze days.Results from observational-based model(OBM)with atmospheric oxidation capacity(AOC)indicated that AOC significantly increased with the increasing carbonyls during the winter haze events.Carbonyl photolysis have supplied key oxidants such as RO_(2) and HO_(2),and thereby enhancing the formation of fine particles and secondary organic aerosols,elucidating the observed haze-carbonyls inter-correlation.Diurnal variation with carbonyls exhibiting peak values at early-noon and night highlighted the combined contribution of both secondary formation and primary diesel-fuel sources.1-butene was further confirmed to be the major precursor for HCHO.This study confirms the great contribution of carbonyls to AOC,and also suggests that reducing the emissions of carbonyls would be an effective way to mitigate haze pollution in urban area of the NCP region.

Atmospheric oxidation capacity and O_(3) formation in a coastal city of southeast China:Results from simulation based on four-season observation

The pollution of atmospheric ozone in China shows an obvious upward trend in the past decade.However,the studies on the atmospheric oxidation capacity and O_(3)formation in four seasons in the southeastern coastal region of China with the rapid urbanization remain limited.Here,a four-season field observation was carried out in a coastal city of southeast China,using an observation-based model combining with the Master Chemical Mechanism,to explore the atmospheric oxidation capacity(AOC),radical chemistry,O_(3)formation pathways and sensitivity.The results showed that the average net O_(3)production rate(14.55 ppbv/hr)in summer was the strongest,but the average O_(3)concentrations in autumn was higher.The AOC and ROx levels presented an obvious seasonal pattern with the maximum value in summer,while the OH reactivity in winter was the highest with an average value of 22.75 sec^(-1).The OH reactivity was dominated by oxygenated VOCs(OVOCs)(30.6%-42.8%),CO(23.2%-26.8%),NO_(2)(13.6%-22.0%),and alkenes(8.4%-12.5%)in different seasons.HONO photolysis dominated OH primary source on daytime in winter,while in other seasons,HONO photolysis in the morning and ozone photolysis in the afternoon contributed mostly.Sensitivity analysis indicated that O_(3)production was controlled by VOCs in spring,autumn and winter,but a VOC-limited and NOx-limited regime in summer,and alkene and aromatic species were the major controlling factors to O_(3)formation.Overall,the study characterized the atmospheric oxidation capacity and elucidated the controlling factors for O_(3)production in the coastal area with the rapid urbanization in China.

大气细颗粒物中环境持久性自由基的分子特征与形成机理的研究

Aerosol-bound organic radicals,including environmentally persistent free radicals(EPFRs),are key components that affect climate,air quality,and human health.While putative structures have been proposed,the molecular characteristics of EPFRs remain unknown.Here,we report a surrogate method to characterize EPFRs in real ambient samples using mass spectrometry.The method identifies chemically relevant oxygenated polycyclic aromatic hydrocarbons(OxPAH)that interconvert with oxygen-centered EPFR(OC-EPFR).We found OxPAH compounds most relevant to OC-EPFRs are structurally rich and diverse quinones,whose diversity is strongly associated with OC-EPFR levels.Both atmospheric oxidation and combustion contributed to OC-EPFR formation.Redundancy analysis and photochemical aging model show pyrolytic sources generated more oxidized OC-EPFRs than photolytic sources.Our study reveals the detailed molecular characteristics of OC-EPFRs and shows that oxidation states can be used to identify the origins of OC-EPFRs,offering a way to track the development and evolution of aerosol particles in the environment.

Implications of 1.5 K climate warming on warm-season ozone exposure and atmospheric oxidation capacity in China

Surface ozone(O_(3))poses significant threats to public health,agricultural crops,and plants in natural ecosystems.Global warming is likely to increase future O_(3)mainly by altering atmospheric photochemical reactions and enhancing biogenic volatile organic compound(BVOC)emissions.To assess the impacts of the future 1.5 K climate target on O_(3)concentrations and ecological O_(3)exposure in China,numerical simulations were conducted using the CMAQ(Community Multiscale Air Quality)model during April-October 2018.Ecological O_(3)exposure was estimated using six indices(i.e.,M7,M24,N100,SUM60,W126,and AOT40f).The results show that the temperature rise increases the MDA8 O_(3)(maximum daily eight-hour average O_(3))concentrations by∼3 ppb and the number of O_(3)exceedance days by 10-20 days in the North China Plain(NCP),Yangtze River Delta(YRD),and Sichuan Basin(SCB)regions.All O_(3)exposure indices show substantial increases.M24 and M7 in eastern and southern China will rise by 1-3 ppb and 2-4 ppb,respectively.N100 increases by more than 120 h in the surrounding regions of Beijing.SUM60 increases by greater than 9 ppm h^(−1),W126 increases by greater than 15 ppm h^(−1)in Shaanxi and SCB,and AOT40f increases by 6 ppm h^(−1)in NCP and SCB.The temperature increase also promotes atmospheric oxidation capacity(AOC)levels,with the higher AOC contributed by OH radicals in southern China but by NO_(3)radicals in northern China.The change in the reaction rate caused by the temperature increase has a greater influence on O_(3)exposure and AOC than the change in BVOC emissions.

Enhanced nitrous acid(HONO)formation via NO_(2)uptake and its potential contribution to heavy haze formation during wintertime

full understanding of the sources of atmospheric nitrous acid(HONO)in the polluted urban atmosphere re-mains a challenge.In this study,ambient HONO and relevant species were measured during January 2019 at an urban site in Beijing,China,and a budget analysis of HONO was conducted using a box model combined with field observations.Large nighttime“missing sources”of HONO were identified on heavily polluted days based on traditional sources,which had a significant correlation with the relative humidity,ammonia(NH_(3)),and aerosol surface area,and the promotional effect of NH_(3)for nitrogen dioxide(NO_(2))uptake on the wet aerosol surface was discussed.Then,an updated parameterization scheme for quantifying the enhanced heterogeneous reactions of NO_(2)on aerosol surfaces is proposed,and the missing nighttime sources of HONO could be substantially com-pensated after the new scheme was incorporated.Further evaluation on the contributions of HONO to hydroxyl radicals was conducted,and the authors found that the photolysis of HONO played a dominant role in the primary OH production on the polluted days(78%-90%).The study reveals great potential of an NH3-enhanced uptake coefficient of NO_(2)on the aerosol surface in the nocturnal HONO budget,and highlights the significance of HONO in the strong atmospheric oxidation capability during episodes with a heavily polluted atmosphere.

Atmospheric oxidizing capacity in autumn Beijing:Analysis of the O_(3) and PM_(2.5) episodes based on observation-based model

Atmospheric oxidizing capacity(AOC)is the fundamental driving factors of chemistry process(e.g.,the formation of ozone(O_(3))and secondary organic aerosols(SOA))in the troposphere.However,accurate quantification of AOC still remains uncertainty.In this study,a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing,where O_(3) and PM_(2.5) episodes had been experienced successively.The observation-based model(OBM)is used to quantify the AOC at O_(3) and PM_(2.5) episodes.The strong intensity of AOC is found at O_(3) and PM2.5 episodes,and hydroxyl radical(OH)is the dominating daytime oxidant for both episodes.The photolysis of O_(3) is main source of OH at O_(3) episode;the photolysis of nitrous acid(HONO)and formaldehyde(HCHO)plays important role in OH formation at PM_(2.5) episode.The radicals loss routines vary according to precursor pollutants,resulting in different types of air pollution.O_(3) budgets and sensitivity analysis indicates that O_(3) production is transition regime(both VOC and NOx-limited)at O3 episode.The heterogeneous reaction of hydroperoxy radicals(HO_(2))on aerosol surfaces has significant influence on OH and O_(3) production rates.The HO_(2) uptake coefficient(γHO_(2))is the determining factor and required accurate measurement in real atmospheric environment.Our findings could provide the important bases for coordinated control of PM_(2.5) and O_(3) pollution.

Response of organic aerosol characteristics to emission reduction in Yangtze River Delta region

Organic aerosol(OA)is a major component of atmospheric particulate matter(PM)with complex composition and formation processes influenced by various factors.Emission reduction can alter both precursors and oxidants which further affects secondary OA formation.Here we provide an observational analysis of secondary OA(SOA)variation properties in Yangtze River Delta(YRD)of eastern China in response to large scale of emission reduction during Chinese New Year(CNY)holidays from 2015 to 2020,and the COVID-19 pandemic period from January to March,2020.We found a 17%increase of SOA proportion during the COVID lockdown.The relative enrichment of SOA is also found during multi-year CNY holidays with dramatic reduction of anthropogenic emissions.Two types of oxygenated OA(OOA)influenced by mixed emissions and SOA formation were found to be the dominant components during the lockdown in YRD region.Our results highlight that these emission-reduction-induced changes in organic aerosol need to be considered in the future to optimize air pollution control measures.

Progress in quantitative research on the relationship between atmospheric oxidation and air quality

Atmospheric oxidizing capacity(AOC)is an essential driving force of troposphere chemistry and self-cleaning,but the definition of AOC and its quantitative representation remain uncertain.Driven by national demand for air pollution control in recent years,Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research.This paper will give a brief review of these developments.First,AOC indexes were established that represent apparent atmospheric oxidizing ability(AOIe)and potential atmospheric oxidizing ability(AOIp)based on aspects of macrothermodynamics and microdynamics,respectively.A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing,and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country.In addition,the detection of ground or vertical profiles for atmospheric OH·,HO_(2)·,NO_(3)·radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments.Moreover,laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O_(3)and NO_(2),which are typical oxidants in the surface/interface atmosphere,and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies,multiphase and multi-interface conditions were obtained.Finally,based on the GRAPES-CUACE adjoint model improved by Chinese scholars,simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized.Normalized numerical simulations of AOIe and AOIp were performed,and regional coordination of AOC was adjusted.An optimized plan for controlling O_(3)and PM2.5was analyzed by scenario simulation.

Migration of harmful elements in municipal sludge under hightemperature oxidation conditions

Municipal sludge is produced in large amounts and is difficult to treat.Incineration is the most direct and thorough treatment method.In order to study the feasibility of sintering for municipal sludge treatment,the municipal sludge reforming process was studied under high-temperature oxidation conditions.The results showed that the sludge reforming process could be divided into four stages:the precipitation and evaporation of adsorbed water,the precipitation and combustion of the volatile,the combustion of the residual volatile and solid carbon,and the decomposition of salts and the melting of sludge.An increase in the heating rate resulted in more intense sludge combustion and improved the sludge reaction capacity and combustion performance.After burning at 1300℃,Si,Ca,Mg,Al,K,Na,and Cu formed new phases and entered the slag.75%of P remained in the slag.80%of the S formed SO_(2) and entered the flue gas.Cl formed gaseous chlorides like HCl upon combustion and entered the flue gas.As sintering is a feasible method for treating municipal sludge,care must be taken to limit the amount of P that ends up in the ore.

Identification of close relationship between atmospheric oxidation and ozone formation regimes in a photochemically active region

Understanding ozone(O_(3))formation regime is a prerequisite in formulating an effective O_(3)pollution control strategy.Photochemical indicator is a simple and direct method in identifying O_(3)formation regimes.Most used indicators are derived from observations,whereas the role of atmospheric oxidation is not in consideration,which is the core driver of O_(3)formation.Thus,it may impact accuracy in signaling O_(3)formation regimes.In this study,an advanced three-dimensional numerical modeling system was used to investigate the relationship between atmospheric oxidation and O_(3)formation regimes during a long-lasting O_(3)exceedance event in September 2017 over the Pearl River Delta(PRD)of China.We discovered a clear relationship between atmospheric oxidative capacity and O_(3)formation regime.Over eastern PRD,O_(3)formation was mainly in a NO x-limited regime when HO_(2)/OH ratio was higher than 11,while in a VOC-limited regime when the ratio was lower than 9.5.Over central and western PRD,an HO_(2)/OH ratio higher than 5 and lower than 2 was indicative of NO x-limited and VOC-limited regime,respectively.Physical contribution,including horizontal transport and vertical transport,may pose uncertainties on the indication of O_(3)formation regime by HO_(2)/OH ratio.In comparison with other commonly used photochemical indicators,HO_(2)/OH ratio had the best performance in differentiating O_(3)formation regimes.This study highlighted the necessities in using an atmospheric oxidative capacity-based indicator to infer O_(3)formation regime,and underscored the importance of characterizing behaviors of radicals to gain insight in atmospheric processes leading to O_(3)pollution over a photochemically active region.

Mechanism and kinetic properties of NO_3-initiated atmospheric degradation of DDT

In this article, the NO3 radical-initiated atmospheric oxidation degradation of DDT was theoretically investigated using molecular orbital theory calculations. All the calculations of intermediates, transition states and products were performed at the MPWB1K/6-311＋G（3df,2p）//MPWB1K/6- 31＋G（d,p） level of theory. Several energetically favorable reaction pathways were revealed. The formation mechanisms of secondary pollutants were presented and discussed. The rate constants were deduced over the temperature range of 273-333 K using canonical variational transition-state （CVT） theory with the small curvature tunneling （SCT） method. Our study shows that H abstraction from the alkyl group and NO3 addition to the Ca atom of the benzene ring are the dominant reaction pathways. The rate-temperature formula of the overall rate constants is k（T）（DDT＋NO3） = （7.21 ~ 10-15）exp（-153.81/T） cm3/（mol.sec） over the possible atmospheric temperature range of 273-333 K. The atmospheric lifetime of DDT determined by NO3 radical is about 52.5 days, which indicates that it can be degraded in the gas phase within several months.