Determination of the Tropospheric Hydroxyl Radical by Liquidphase Scrubbing and HPLC: Preliminary Results

A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×10⁵ radicalscm^-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×10⁶ radicals cm^-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×10⁶ radicals cm^-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit.     

Tropospheric Box-Modelling and Analytical Studies of the Hydroxyl (OH) Radical and Related Species: Comparison with Observations

Calculated and observed hydroxyl (OH) fields are presented. Calculated OH was obtained in three ways using (1) a photochemical box-model (2) a simple OH steady state approach and (3) a variant on (2) – the multiple equation steady state approach which assumes steady state for OH, HO2 and RO2 and hence obtains three simultaneous, non linear, equations. All three methods used data collected in June 1995 during the Weybourne Atmospheric Observatory Summer Experiment (WAOSE'95). Julian Days 169, 178, 179 and 180 displayed especially good data capture and were consequently chosen for study. The two steady state methods are essentially driven purely by observations and derive OH from the ratio of the relevant source and sink terms. The box-model was constrained where possible to observations; remaining unmeasured volatile organic compounds (VOCs) were initialised to an arbitrary low value of 10 ppt. Agreement between theory and experiment was usually around 50% and often better than this value, especially on J169, though discrepancies of up to a factor of 3 were occasionally apparent. Despite the inherent scatter, neither the box-model nor the simple steady state method were found to consistently over-estimate OH (a common feature of many numerical approaches) although this did occur to a certain extent using the multiple equation steady state approach, probably due to breakdowns in the steady state approximation. More data spread was evident in the box-model approach compared with the other methods. An analysis of the major sources and sinks of OH is presented for the three methods of calculation. Calculated and observed peroxy radicals are also presented. Calculated peroxy radicals were generally lower than that observed at night yet higher, sometimes by up to a factor of 7, during the day. Possible explanations for this result are explored.     

Product study of the reaction of OH radicals with isoprene in the atmosphere simulation chamber SAPHIR

Atmospheric oxidation of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) have an important impact on the photochemical activity in the boundary layer, in particular in forested areas. The oxidation of isoprene by OH radicals was investigated in chamber experiments conducted under tropospheric conditions in the atmosphere simulation chamber SAPHIR at the Research Center Jülich. The aim was to determine the product yield of MVK and MACR in the OH-induced isoprene oxidation and the rate constant of their reaction with OH under real atmospheric conditions. The recently published updated degradation scheme for isoprene from Geiger et al. (2003) was used to determine rate constants and product yields. The fractional yields in the isoprene peroxy radical reaction with NO were found to be 0.41±0.03 for MVK and 0.27±0.03 for MACR. The rate coefficient for MACR with OH was found to be in very good agreement with the recommended value of IUPAC Atkinson (Atkinson et al., 2005). while the rate coefficient for MVK with OH was 27% lower.     

Hydroxyl Radical Concentrations Estimated from Measurements of Trichloroethylene during the EASE/ACSOE Campaign at Mace Head, Ireland during July 1996

During the EASE/OXICOA campaign of the NERC ACSOE programme, trichloroethylene and a wide range of man-made halocarbons and radiatively-active trace gases were monitored with high precision and high frequency throughout July 1996 at Mace Head on the Atlantic Ocean coast of Ireland. Trichloroethylene concentrations in concert with many other trace gases became elevated as regionally-polluted and photochemically-aged air masses reached Mace Head. However, as the anticyclonic air masses retreated during 19 and 20 July, trace gas concentrations remained elevated for a significant period. During this 2–4 day period, trichloroethylene concentrations decayed significantly, though the concentrations of the other more chemically-inert trace gases did not. A detailed interpretation of this behaviour using a Lagrangian dispersion model has allowed the estimation of average and peak OH radical concentrations of 3 and 9×10⁶ molecule cm^-3, respectively, during the travel from the source areas in the U.K. and the low countries out to Mace Head. Using a simple box model, the available Mace Head measurements, when combined into a detailed chemical mechanism, generated OH radical concentrations which peaked at 7×10⁶ molecule cm^-3, in close agreement with the estimates based on trichloroethylene decay.     

Intercomparison of Two Box Models of the Chemical Evolution in Biomass-Burning Smoke Plumes

Results from two independently developed biomass-burning smoke plume models are compared. Model results were obtained for the temporal evolution of two nascent smoke plumes originating from significantly different fire environments (an Alaskan boreal forest and an African savanna). The two smoke plume models differed by 1%–10% for [O₃], with similar differences for NO _x and formaldehyde (relative percent differences). Smaller intermodel differences were observed for the African savanna smoke plume as compared to the plume from the Alaskan boreal fire. Mechanistic differences between the models are heightened for the Alaskan smoke plume due to the higher VOC emission ratios as compared to the African savanna fire. The largest deviations result from the differences in oxidative photochemical mechanisms, with a smaller contribution attributable to the calculation of photolysis frequencies. The differences between the two smoke plume models are significantly smaller than the uncertainties of available photokinetic data or field measurements. Model accuracy depends most significantly on having the fullest possible VOC data, a requirement that is constrained by currently available instrumentation.     

Hydroxyl and Peroxy Radical Chemistry in a Rural Area of Central Pennsylvania: Observations and Model Comparisons

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.     

Doppler Detection of Hydroxyl Column Abundance in the Middle Atmosphere: 2. Measurement for Three Years and Comparison with a 1D Model

The hydroxyl column abundance in the middle atmosphere is measured for 3 years in Tokyo by ground-based uv absorption spectroscopy utilizing the Doppler shift of the solar spectrum due to the solar rotation. The diurnal and seasonal variations of the measured column abundance are found to be reproduced qualitatively by a one-dimensional photochemical-diffusive model; however, the measured column abundances are smaller than the calculated by 20 to 30% systematically.     

Intercomparison of Tropospheric OH Measurements by Different Laser Techniques during the POPCORN Campaign 1994

In-situ OH measurements by laser-induced fluorescence (LIF) spectroscopy and folded long-path differential optical absorption spectroscopy (DOAS) were carried out in a rural environment in North-East Germany as part of the field experiment POPCORN in August 1994. The large set of OH data obtained allowed an intercomparison of both techniques based on relative diurnal profiles and simultaneously measured absolute concentrations. Most of the time the two OH instruments encountered the same air and agreed well in the measured relative diurnal variations. Only on a few occasions the measurements significantly disagreed due to a perturbation of the DOAS measurements by a local OH source in the north-western wind sector. Excluding data from this wind direction, the statistical analysis of 137 data pairs yields a correlation coefficient of r = 0.90 and a weighted linear fit with a slope of 1.09 ± 0.12. The correlations are carefully analyzed. The comparison of both instruments is discussed in the light of newly published effective absorption cross-sections for H2O and O2 that affect the calibration of LIF.     

Absolute Rate Coefficients for the Gas-Phase Reactions of NO3 Radical with a Series of Monoterpenes at T = 298 to 433 K

The aim of this work is to study the reactivity of some naturally emitted terpenes, 2-carene, sabinene, myrcene, -phellandrene, d-limonene, terpinolene and -terpinene, towards NO3 radical to evaluate the importance of these reactions in the atmosphere and their atmospheric impact. The experiments with these monoterpenes have been carried out under second-order kinetic conditions over the range of temperature 298–433 K, using a discharge flow system and monitoring the NO3 radical by Laser Induced Fluorescence (LIF). This work is the first temperature dependence study for the reactions of the nitrate radical with the above-mentioned monoterpenes. The measured rate constants at 298 K for the reaction of NO3 with such terpenes are as follows: 2-carene, 16.6 ± 1.8, sabinene 10.7 ± 1.6, myrcene 12.8 ± 1.1, -phellandrene 42 ± 10, d-limonene 9.4 ± 0.9, terpinolene 52 ± 9 and -terpinene 24 ± 7, in units of 10-12 cm3 molecule-1 s-1. The proposed Arrhenius expressions, for the reactions of NO3 with 2-carene, sabinene, myrcene and -phellandrene are, respectively k1 = (1.4 ± 0.7) × 10-12 exp[(741 ± 190/T)] (cm3 molecule-1 s-1), k2=(2.3 ± 1.3) × 10-10 exp[–(940 ± 200/T)] (cm3 molecule-1 s-1), k3 = (2.2 ± 0.2) × 10-12 exp[(523 ± 35/T)] (cm3 molecule1 s-1) and k4 = (1.9 ± 1.3) × 10-9 exp[–(1158 ± 270/T)] (cm3 molecule-1 s-1). A decrease in the rate constants when raising the temperature has also been found for the reaction of d-limonene with NO3 while an increase in the rate constant with temperature has been observed for the reactions of terpinolene and -terpinene with NO3. Tropospheric half-lives for these terpenes have been calculated at night and during the day for typical NO3 and OH concentrations showing that both radicals provide an effective tropospheric sink for these compounds and that the night-time reaction with NO3 radical can be an important, if not dominant, loss process for these naturally emitted organics and for NO3 radicals.     

Oxidation of Elemental Mercury in the Atmosphere; Constraints Imposed by Global Scale Modelling

We use the global mercury model published by Bergan et al. (1999) to evaluate the potential role of ozone and the hydroxyl radical as gas phase oxidants for the oxidation of elemental mercury in the atmosphere. The magnitude of natural and man-made mercury emissions are taken from recent literature estimates. We consider only two mercury reservoirs, elemental mercury, Hg⁰, and the more soluble divalent form, HgII. Wet and dry deposition of HgII is explicitly treated.Applying monthly mean fields of ozone for the oxidation of gas phase Hg⁰ and using the reaction rate by Hall (1995) yields a global transformation of Hg⁰ to HgII which is too slow to keep the simulated concentration of Hg⁰ near observed values. This shows that there must be additional important removal processes for Hg⁰ or that the reaction rate proposed by Hall (1995) is too slow. A simulation in which the oxidation rate was artificially increased, so that the global turn-over time of Hg⁰ is one year and the simulated average concentration of Hg⁰ realistic, produces latitudinal and seasonal variations in Hg⁰ that do not support the hypothesis that gas phase reaction with O₃ is the major oxidation process for Hg⁰.Recent studies indicate that OH may be an important gas phase oxidant for Hg⁰ (Sommar et al., 2001). Using OH as the sole oxidantand applying the oxidation rate by Sommar et al., we calculate aconcentration of Hg⁰ well below (about a factor of three) the observations. By prescribing a slower rate, corresponding to a turn-over time of Hg⁰ of one year, we calculate concentrations of both Hg⁰ in surface air and HgII in precipitation which correspond reasonably well, both in magnitude and temporal variation, with seasonal observations in Europe and North America. This result supports the suggestion that the oxidation by OH is an important pathway for the removal of Hg⁰. In view of the uncertainties associated with our calculations, this conclusion should still be regarded as tentative.     

Simulation Chamber Studies of the Atmospheric Oxidation of 2-Methyl-3-Buten-2-ol: Reaction with Hydroxyl Radicals and Ozone Under a Variety of Conditions

This article presents a complete study of the diurnal chemical reactivity of the biogenic volatile organic compound (BVOC), 2-methyl-3-buten-2-ol (MBO) in the troposphere. Reactions of MBO with OH and with ozone were studied to analyse the respective parts of both processes in the global budget of MBO atmospheric reactivity. They were investigated under controlled conditions for pressure (atmospheric pressure) and temperature (298 ± 2 K) using three complementary European simulation chambers. Reaction with OH radicals was studied in the presence of and in the absence of NO _x . The kinetic study was carried out by relative rate study using isoprene as a reference. The rate constant found for this reaction was molecule⁻¹ cm³ s⁻¹. FTIR spectroscopy, DNPH- and PFBHA-derivatisation analyses were performed for reactions with both OH radicals and ozone. In both reactions, the hydroxycarbonyl compound, 2-hydroxy-2-methylpropanal (HMPr) was positively identified and quantified, with a yield of in the reaction with OH, and a yield of and 0.84 ± 0.08 in the reaction with ozone under dry (HR < 1%) and humid conditions (HR = 20%–30%). A primary production of two other carbonyl compounds, acetone , and formaldehyde was found in the case of the dry ozonolysis experiments. Under humid conditions, only formaldehyde was co-produced with HMPr as a primary carbonyl compound, with a yield of . For the reaction with OH, three other carbonyl compounds were detected, acetone , formaldehyde and glycolaldehyde . In addition some realistic photo-oxidation experiments were performed to understand in an overall way the transformations of MBO in the atmosphere. The realistic photo-oxidation experiments were conducted in the EUPHORE outdoor simulation chamber. It was found that this compound is a weak secondary aerosol producer (less than 1% of the carbon balance). But it was confirmed that it is a potentially significant source of acetone, Δ[Acetone]/Δ[MBO] = 0.45. With our experimental conditions ([MBO]₀ = 200 ppb, [NO]o = 50 ppb), an ozone yield of Δ[O₃]/Δ[MBO] = 1.05 was found.     

Evaluation of the Salicylic Acid—Liquid Phase Scrubbing Technique to Monitor Atmospheric Hydroxyl Radicals

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.     

Kinetics of the Reaction of Hydroxyl Radicals with CH2Br2 and its Implications in the Atmosphere

Rate constants for the reaction of hydroxyl radicals with dibromomethanehave been measured by discharge flow-resonance fluorescence technique(DF-RF) over the temperature range 288–368 K. The derived Arrheniusequation is k₁=(1.51 ± 0.37)× 10^-12 exp(-(720 ±60)/T) cm³ molec.^-1 s^-1.The tropospheric lifetime of dibromomethane has been estimated to be 0.29years. An ozone depletion potential (ODP) value of 0.10 for dibromomethanehas been obtained.     

参考大气在中期天气预报和气候模拟中的应用

本文概述了根据应用参考大气概念提出的参考大气谱模式计算格式（或称静力扣除法）在国内外一些气象机构的中期天气预报、后处理和四维同化、以及气候模拟中的应用。计算表明，这种参考大气谱模式计算格式，能有效的减少截谱误差，明显地改进中期天气预报和气候模拟。     

重庆两场森林火灾蔓延特征的数值模拟

为了深入认识同时段内发生的两场森林火灾蔓延特征以及大气、地形等对不同林火行为的影响,进而为多起林火扑灭火工作提供科学支撑,选取 2022年8月17日发生在涪陵两场森林火灾,利用高分辨率地理信息、植被数据和气象数据对两场林火行为进行分析研究,同时运用林火-大气耦合WRF Fire（Weather Research and Forecasting Model with Fire Module）模式进行数值模拟。研究发现：①受深厚高压控制,两场林火是在持续高温、干燥背景下发生的,气象干旱等级达到重旱,火险气象等级高。②模式模拟风向风速变化与实况较为一致,基本体现出火场风向和风速突变,表明WRF Fire模式可以较准确地再现火场蔓延情况。③基于林火蔓延特征时间和空间变化可将大梁山林火发展分为6个阶段、北山坪林火分为4个阶段,造成直线距离相隔4.4 km两个火场蔓延发展程度不同的原因在于地形差异造成的局地风场差异。     

实验室模拟研究大气二次有机气溶胶的形成

二次有机气溶胶（SOA）是大气中重要的气溶胶组分,主要由挥发性有机物（VOCs）经化学转化形成,对天气、气候、大气环境和人体健康有重要影响,但至今其确切的化学成分和形成机制还十分不清楚。研究SOA的方法主要采用实验室单个物种或多物种的化学过程的模拟研究,野外实际大气的SOA化学成分、源汇和多尺度分析的观测研究,以及大气中SOA形成的数值模拟的回报和预报研究。实验室研究是对SOA形成过程中获取基础数据和推究SOA生成机制的最主要手段。在过去的几十年中,特别是近五年,SOA的研究取得了较大的进展,其中包括SOA前体物、SOA形成机制及影响因子的进一步理解。本文就这些方面展开了概要性的综述,重点强调了我国研究人员所做的研究工作。在采用实验室烟雾箱系统模拟研究SOA方面,首先简述了烟雾系统的发展以及表征,讨论了跟烟雾箱箱体相关的壁效应问题,重点综述了萜烯类、芳香烃类、小分子类等化学物种转化形成SOA的研究进展。在采用流动管和其他反应器类模拟研究SOA方面,重点讨论了挥发性有机物在颗粒物表面或在液相中所形成的SOA的主要化学成分及其可能的作用。     

大气低频振荡的数值模拟

本文利用IAP_GCM积分结果中10年候平均资料，研究了模式大气中的低频振荡现象。结果表明，该模式能相当逼真地模拟出低频振荡的地理分布、垂直结构和传播过程的主要特征，因而低频振荡是大气本身所固有的一种现象。而流场与大气加热场之间在低频域内的相关分析则证实了大气加热对低频振荡的产生起重要的作用，其中最重要的因子是水汽潜热的释放。     

Relative-rate study of the gas-phase reaction of hydroxy radicals with difunctional organic nitrates at 298 K and atmospheric pressure

Rate coefficients for the reactions of difunctional nitrates with atmospherically important OH radicals are not currently available in the literature. This study represents the first determination of rate coefficients for a number of C(3) and C(4) carbonyl nitrates and dinitrates with OH radicals in a 38 l glass reaction chamber at 1000 mbar total pressure of synthetic air by 298±2 K using a relative kinetic technique.The following rate coefficients (in units of 10^-12 cm³ molecule^-1 s^-1) were obtained: 1,2-propandiol dinitrate, <0.31; 1,2-butandiol dinitrate, 1.70±0.32; 2,3-butandiol dinitrate, 1.07±0.26; -nitrooxyacetone, <0.43; 1-nitrooxy-2-butanone, 0.91±0.16; 3-nitrooxy-2-butanone, 1.27±0.14; 1,4-dinitrooxy-2-butene, 15.10±1.45; 3,4-dinitrooxy-1-butene, 10.10±0.50.The possible importance of reaction of OH as an atmospheric sink for the compounds compared to other loss processes is considered.     

热带季节内振荡模拟研究的若干进展

大气季节内振荡（ISO）在长期天气和气候变化中有重要作用,它是20世纪70～80年代以来大气科学领域的重要研究课题。本文简要介绍近年来季节内振荡的数值模拟研究的成果和进展,包括数值模式模拟季节内振荡能力的进展; 模式模拟ISO能力对模式中对流参数化方案的敏感性;ISO模拟结果与基本态的关系;外强迫对模拟结果的影响; ENSO与ISO关系的模拟研究,以及全球变暖对ISO影响的模拟研究等。最后,对今后的研究工作提出了一些建议。