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1.
Improvements of the matrix isolation/electron spin resonance technique for the measurement of NO2, NO3, and RO2 radicals in the atmosphere are described. The use of D2O instead of H2O as the matrix yields a better spectral resolution and, as a consequence, larger a signal-to-noise ratio as well as better identification of the different species. Reference spectra of the different radicals in H2O and D2O matrices are compared. While a large degree of correlation exists amongst the spectra of the different (unsubstituted and substituted) alkylperoxy radicals, the spectra of HO2, CH3C(O)O2, and NO3 show significant differences that allow their distinction in atmospheric samples.A numerical procedure for the analysis of the composite ESR spectra obtained from atmospheric samples was developed. Subtraction of the dominant NO2 signal is performed by matching a reference NO2 spectrum with respect to amplitude, spectral position, and line width to the sample spectrum. The manipulations are performed with the virtually noise-free reference spectrum and are based on physical information. The residual spectrum is then analyzed for RO2 (and/or NO3) by simultaneously fitting up to six different reference spectra.The method was applied to laboratory samples as well as to atmospheric samples in order to demonstrate the ability of retrieving small amounts of HO2 in the presence of large amounts of NO2 and other peroxy radicals. The new algorithm allowed, for the first time, the identification of the HO2 and CH3C(O)O2 radical in tropospheric air at concentrations ranging up to 40 ppt.  相似文献   

2.
A cryogenic system for the airborne and ground based sampling of ambient radicals by matrix isolation is described. The trapped radicals, e.g., NO2 and RO2, are analyzed by ESR. The technique has been improved, mainly by addition of water vapor to the sampled air, to yield a collection efficiency of (90±10)% and a lower detection limit of about 20 ppt, but it still does not distinguish between the different RO2. Careful calibration reduced the measurement error (1 ) to ±10% for NO2 and ±15% for HO2. Two diurnal variations of RO2 and NO2 at ground level and vertical profiles in the lower troposphere are presented.  相似文献   

3.
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4.
Measurements of total peroxy radicals (HO2?+?RO2) and nitrate radical (NO3) were made on the NOAA research vessel R/V?Brown along the U.S. Gulf Coast during the TexAQS 2006 field campaign. The measurements were modelled using a constrained box-model based upon the Master Chemical Mechanism (MCM). The agreement between modelled and measured HO2?+?RO2 was typically within ??40% and, in the unpolluted regions, within 30%. The analysis of the model results suggests that the MCM might underestimate the concentrations of some acyl peroxy radicals and other small peroxy radicals. The model underestimated the measurements of NO3 by 60?C70%, possibly because of rapid heterogeneous uptake of N2O5. The MCM model results were used to estimate the composition of the peroxy radical pool and to quantify the role of DMS, isoprene and alkenes in the formation of RO2 in the different regions. The measurements of HO2?+?RO2 and NO3 were also used to calculate the gas-phase budget of NO3 and quantify the importance of organic peroxy radicals as NO3 sinks. RO2 accounted, on average, for 12?C28% of the total gas-phase NO3 losses in the unpolluted regions and for 1?C2% of the total gas-phase NO3 losses in the polluted regions.  相似文献   

5.
Simultaneous measurements of ozone and ozoneprecursors were made during a field campaign atSchauinsland in the Black Forest and in the valleynorth of Schauinsland that channels the flow ofpolluted air from the city of Freiburg to the site.From the decay of hydrocarbons and NOx between the twomeasuring sites and the known rate coefficients, theconcentration of OH radicals was calculated. From abudget analysis of OH and HOx it is concluded that therelatively high OH concentrations (5–8 ×106cm-3) in the presence of high NO2concentrations cannot be explained by the knownprimary sources. The budget can be closed if efficientrecycling of OH via HO2 is assumed to occur andthat, based on the measured hydrocarbons, 2 HO2molecules are formed for each OH radical that reactswith a hydrocarbon molecule. This assumption is inaccordance with the budget of Ox obtained from ourmeasurements and with results from earliermeasurements of alkylnitrates and peroxy radicals atSchauinsland. A possible conclusion is that the decayof precursors and production of photooxidants in urbanplumes proceeds at a faster rate than is currentlyassumed. The potential role of biogenichydrocarbons for the radical budget is alsodiscussed.  相似文献   

6.
Chemical amplification, CA, a method commonly used for the detection of peroxy radicals, HO2 and RO2, was found to be sensitive towards ClOx (Cl+ClO+OClO) as well. ClOx is reduced by NO to Cl atoms which react with carbon monoxide in the presence of O2. The reaction sequence thus initiated oxidizes CO to CO2 and NO to NO2, with a chain length of 300 ± 60. This allows the atmospheric ClOx content to be measured under ambient conditions with a detection limit of better than 1 ppt. In parallel peroxy radicals are indicated with a chain length of 160 ± 15. Chemical amplification is not specific and does not indicate which radical chain it is seeing. Identification relies solely on plausibility. During the ARCtic Tropospheric Ozone Chemistry (ARCTOC) campaign in spring 1995 and 1996 the CA technique was used at Ny-Ålesund. ClOx at mixing ratios of up to 2 ppt were found in the boundary layer under certain conditions. The low concentrations of ClOx indicate that the arctic boundary ozone depletion is mainly driven by bromine.  相似文献   

7.
Atmospheric hydroxyl (OH), hydroperoxy (HO2), total peroxy (HO2 and organic peroxy radicals, RO2) mixing ratios and OH reactivity (first order OH loss rate) were measured at a rural site in central Pennsylvania during May and June 2002. OH and HO2 mixing ratios were measured with laser induced fluorescence (LIF); HO2 + RO2 mixing ratios were measured with chemical ionization mass spectrometry (CIMS). The daytime maximum mixing ratios were up to 0.6 parts per trillion by volume (pptv) for OH, 30 pptv for HO2, and 45 pptv for HO2 + RO2. A parameterized RACM (Regional Atmospheric Chemistry Mechanism) box model was used to predict steady state OH, HO2 and HO2 + RO2 concentrations by constraining the model to the measured OH reactivity and previously measured volatile organic compound (VOC) distributions. The averaged model calculations are generally in good agreement with the observations. For OH, the model matched the observations for day and night, with an average observed-to-modeled ratio of 0.80. In previous studies such as PROPHET98, nighttime NO was near 0 pptv and observed nighttime OH was significantly larger than modeled OH. In this study, nighttime observed and modeled OH agree to within measurement and model uncertainties because the main source of the nighttime OH was the reaction HO2 + NO → OH + NO2, with the NO being continually emitted from the surrounding fertilized corn field. The observed-to-modeled ratio for HO2 is 1.0 on average, although daytime HO2 is underpredicted by a factor of 1.2 and nighttime HO2 is over-predicted by a factor of ∼2. The average measured and modeled HO2 + RO2 agree well during daytime, but the modeled value is about twice the measured value during nighttime. While measured HO2 + RO2 values agree with modeled values for NO mixing ratios less than a few parts per billion by volume (ppbv), it increases substantially above the expected value for NO greater than a few ppbv. This observation of the higher-than-expected HO2 + RO2 with the CIMS technique confirms the observed increase of HO2 above expected values at higher NO mixing ratios in HO2 measurements with the LIF technique. The maximum instantaneous O3 production rate calculated from HO2 and RO2 reactions with NO was as high as 10–15 ppb h−1 at midday; the total daily O3 production varied from 13 to 113 ppbv d−1 and was 48 ppbv d−1 on average during this campaign.  相似文献   

8.
Vertical profiles of stratospheric HO2 and NO2 concentrations were determined using matrix isolation and ESR. Up to 10 different samples per flight were collected in situ by a balloon borne cryosampler. Free radicals and trace constituents which are condensable at 68 K are trapped in a polycristalline H2O or D2O matrix. After collection, the samples are stored at a temperature below 83 K until they are analysed in the laboratory by X-band ESR spectroscopy at 4 K. The HO2 and NO2 were identified and calibrated by comparison with standard samples collected in the laboratory under typical stratospheric sampling conditions. From several flights over Southern France (44°N) we obtained two profiles of the stratospheric NO2 mixing ratio. One, from 21 October 1982, agrees well with previous measurements. The other, from 8 October 1981, is lower by one order of magnitude. The few HO2 data obtained around 35 km altitude agree with previous measurements. An isolated measurement at 17 km altitude is one order of magnitude higher than the model predicted HO2 concentration.  相似文献   

9.
An instrument is developed for the measurement of peroxy radical using chemical amplification coupled with NO2-luminol chemiluminescence detection. The chain length of 147 ± 10 (1σ) is determined by an HO2 source that uses the photolysis of water vapor under 184.9 nm in air. A Nafion system equipped with a Nafion tube of ~2.2 mm external diameter and 350 mm length is employed in the PERCA instrument (Nafion-PERCA system). When flowing an air sample containing HO2 through the Nafion system, it is found that - 94.6 % of HO2 is removed. In contrast, only 17.8 % of RO2 radicals (a mixtures of CH3O2 and CH3C(O)O2 with a ratio of 1.1:0.9) are removed. The results indicate the Nafion system has a good selective removing performance of HO2 radical during the PERCA measurement. Therefore, the method could be applied to ambient and laboratory measurements of absolute concentrations of RO2 as well as the sum of HO2 and RO2.  相似文献   

10.
High levels of uncertainty in non-methane volatile organic compound （NMVOC） emissions in China could lead to significant variation in the budget of the sum of hydroxyl （OH） and peroxy （HO2,RO2） radicals （ROx =OH ＋ HO2 ＋ RO2） and the ozone production rate [P（O3）],but few studies have investigated this possibility,particularly with three-dimensional air quality models.We added diagnostic variables into the WRF-Chem model to assess the impact of the uncertainty in anthropogenic NMVOC （AVOC） emissions on the ROx budget and P（O3） in the Beijing-Tianjin-Hebei region,Yangtze River Delta,and Pearl River Delta of China.The WRF-Chem simulations were compared with satellite and ground observations,and previous observation-based model studies.Results indicated that 68％ increases （decreases） in AVOC emissions produced 4％-280％ increases （2％-80％ decreases） in the concentrations of OH,HO2,and RO2 in the three regions,and resulted in 35％-48％ enhancements （26％-39％ reductions） in the primary ROx production and ～ 65％ decreases （68％-73％ increases） of the P（O3） in Beijing,Shanghai,and Guangzhou.For the three cities,the two largest contributors to the ROx production rate were the reaction of O1D ＋ H2O and photolysis of HCHO,ALD2,and others; the reaction of OH ＋ NO2 （71％-85％） was the major ROx sink; and the major contributor to P（O3） was the reaction of HO2 ＋ NO （～ 65％）.Our results showed that AVOC emissions in 2006 from Zhang et al.（2009） have been underestimated by ～ 68％ in suburban areas and by ＞ 68％ in urban areas,implying that daily and hourly concentrations of secondary organic aerosols and inorganic aerosols could be substantially underestimated,and cloud condensation nuclei could be underestimated,whereas local and regional radiation was overestimated.  相似文献   

11.
Free Radicals and Fast Photochemistry during BERLIOZ   总被引：4，自引：0，他引：4  
The free radicals OH, HO2, RO2, and NO3 are known to be the driving force for most chemical processes in the atmosphere. Since the low concentration of the above radicals makes measurements particularly difficult, only relatively few direct measurements of free radical concentrations have been reported to date.We present a comprehensive set of simultaneous radical measurements performed by Laser Induced Fluorescence (LIF), Matrix Isolation –Electron spin Resonance (MI-ESR), Peroxy Radical Chemical Amplification (PERCA), and Differential Optical Absorption Spectroscopy (DOAS) during the BERLIner OZonexperiment (BERLIOZ) during July and August of 1998 near Berlin, Germany. Most of the above radical species were measured by more than one technique and an intercomparison gave good agreement. This data set offered the possibility to study and quantify the role of each radical at a rural, semi-polluted site in the continental boundary layer and to investigate interconnections and dependencies among these free radicals.In general (box) modelled diurnal profiles of the different radicals reproduced the measurements quite well, however measured absolute levels are frequently lower than model predictions. These discrepancies point to disturbing deficiencies in our understanding of the chemical system in urban air masses.In addition considerable night-time peroxy radical production related to VOC reactions with NO3 and O3 could be quantified.  相似文献   

12.
Measurements of the sum of peroxy radicals [HO2 + RO2],NOx (NO + NO2) and NOy (the sum of oxidisednitrogen species) made at Mace Head, on the Atlantic coast of Ireland in summer 1996 and spring 1997 are presented. Together with a suite of ancillary measurements, including the photolysis frequencies of O3 O(1D)(j(O1D)) and NO2 (j(NO2)), the measured peroxy radicals are used to calculate meandailyozone tendency (defined as the difference of the in-situphotochemical ozone production and loss rates); these values are compared with values derived from the photochemical stationary state (PSS) expression. Although the correlation between the two sets of values is good, the PSS values are found to be significantly larger than those derived from the peroxy radical measurements, on average, in line with previous published work. Possible sources of error in these calculations are discussed in detail. The data are further divided up into five wind sectors, according to the instantaneous wind direction measured at the research station. Calculation of mean ozone tendencies by wind sector shows that ozone productivity was higher during spring (April–May) 1997 than during summer (July–August) 1996across all airmasses, suggesting that tropospheric photochemistry plays an important role in the widely-reported spring ozone maximum in the Northern Hemisphere. Ozone tendencies were close to zero for the relatively unpolluted south-west, west and north-west wind sectors in the summer campaign, whereas ozone productivity was greatest in the polluted south-east sector for both campaigns. Daytime weighted average ozone tendencies were +(0.3± 0.1) ppbv h–1 for summer 1996 and +(1.0± 0.5) ppbvh–1 for spring 1997. These figures reflect the higher mixing ratios of ozone precursors in spring overall, as well as the higher proportion of polluted air masses from the south-east arriving at the site during the spring campaign. The ozone compensation point, where photochemical ozone destruction and production processes are in balance, is calculated to be ca. 14 pptv NO for both campaigns.  相似文献   

13.
Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO y , called NO z , was neither NO nor NO2. This NO z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO z to NO3 - in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO3 and N2O5 by aerosol of surface area proportional to the NO2 mixing ratio, shows that NO3 and N2O5 may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N2O5 forming NO3 - in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N2O5 was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO2 and the cloud liquid water content were related to each other. Also, indications of conversion of NO x to NO z were found. To explain these observations, scavenging of NO x and HNO2 by cloud droplets and/or aqueous-phase oxidation of NO2 - by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO x or SO2, NO3 - which had entered the cloud as aerosol particles was liberated as HNO3 vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO3 to particulate NO3 -, was observed.  相似文献   

14.
The photodissociation coefficient of NO2, J NO 2, has been measured from a balloon platform in the stratosphere. Results from two balloon flights are reported. High Sun values of J NO 2 measured were 10.5±0.3 and 10.3±0.3×10-3 s-1 at 24 and 32 km respectively. The decrease in J NO 2 at sunset was monitored in both flights. The measurements are found to be in good agreement with calculations of J NO 2 using a simplified isotropic multiple scattering computer routine.  相似文献   

15.
A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H2SO4 in reaction with SO2 and detection of H2SO4 using an ion molecule reaction with NO3─, the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO3─ ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 105 molecule cm-3 for OH and 30% and 1 × 105 molecule cm-3 for HO2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 105 molecules cm-3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO2.  相似文献   

16.
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17.
Accurate OH and HO2 (collectively called HOx) measurements by laser-induced fluorescence (LIF) may be contaminated by spurious signals from interfering atmospheric chemicals or from the instrument itself. Interference tests must be conducted to ensure that observed OH signal originates solely from ambient OH and is not due to instrument artifacts. Several tests were performed on the Penn State LIF HOx instrument, both in the laboratory and in the field. Theseincluded measurements of the instrument's zero signal by using either zero air or perfluoropropylene to remove OH, examination of spectral interferences from naphthalene, sulfur dioxide, and formaldehyde, and tests of interferences by addition of suspected interfering atmospheric chemicals, including ozone, hydrogen peroxide, nitrous acid, formaldehyde, nitric acid, acetone, and organic peroxy radicals (RO2). All tests lacked evidence ofsignificant interferences for measurements in the atmosphere, including highly polluted urban environments.  相似文献   

18.
A novel method has been examined for monitoring tropospheric hydroxyl radicals (OH), the most important oxidant in tropospheric chemistry. Aqueous phase salicylic acid reacts with atmospheric OH to produce 2,5-dihydroxy benzoic acid (2,5-DHBA) and other products. High Performance Liquid Chromatography (HPLC) is used to separate the post-reaction solution and the products are quantified using fluorescence detection. Unlike other methods, it has been reported to be inexpensive, portable and relatively simple. Although the sensitivity was sufficient to measure typical daytime OH concentrations of 0.04–0.4 ppt., the method was hindered by numerous interferences. Successive identification and elimination of these still resulted in a signal that was much larger than expected. Tests showed that this was not likely to be due to ozone, HO2, NOx, H2O2, aerosols, light or bacteria. Experimental and numerical studies suggest that the interference could be due to methyl peroxy radicals. The effect of many other components in the atmosphere, both individual and combined, must also be tested before the method can be used reliably in the field. The validity of previous reports of ambient hydroxyl measurements using this technique is therefore brought into question.  相似文献   

19.
Using long path UV absorption spectroscopy we have measured OH concentrations close to the earth's surface. The OH values observed at two locations in Germany during 1980 through 1983 range from 0.7×106 to 3.2×106 cm-3. Simultaneously we measured the concentrations of O3, H2O, NO, NO2, CH4, CO, and the light non methane hydrocarbons. We also determined the photolysis rates of O3 and NO2. This allows calculations of OH using a zero dimensional time depdendent model. The modelled OH concentrations significantly exceed the measured values for low NO x concentrations. It is argued that additional, so far unidentified. HO x loss reactions must be responsible for that discrepancy.  相似文献   

20.
We show that photochemical processes in the lower half of the troposphere are strongly affected by the presence of liquid water clouds. Especially CH2O, an important intermediate of CH4 (and of other hydrocarbon) oxidation, is subject to enhanced breakdown in the aqueous phase. This reduces the formation of HO x -radicals via photodissociation of CH2O in the gas phase. In the droplets, the hydrated form of CH2O, its oxidation product HCO2 –, and H2O2 recycle O2 – radicals which, in turn, react with ozone. We show that the latter reaction is a significant sink for O3. Further O3 concentrations are reduced as a result of decreased formation of O3 during periods with clouds. Additionally, NO x , which acts as a catalyst in the photochemical formation of O3, is depleted by clouds during the night via scavenging of N2O5. This significantly reduces NO x -concentrations during subsequent daylight hours, so that less NO x is available for O3 production. Clouds thus directly reduce the concentrations of O3, CH2O, NO x , and HO x . Indirectly, this also affects the budgets of other trace gases, such as H2O2, CO, and H2.  相似文献   

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