Simultaneous measurements of peroxy and nitrate radicals at Schauinsland

We present simultaneous field measurements of NO₃ and peroxy radicals made at night in a forested area (Schauinsland, Black Forest, 48° N, 8° N, 1150 ASL), together with measurements of CO, O₃, NO _x , NO _y , and hydrocarbons, as well as meteorological parameters. NO₂, NO₃, HO₂, and (RO₂) radicals are detected with matrix isolation/electron spin resonance (MIESR). NO₃ and HO₂ were found to be present in the range of 0–10 ppt, whilst organic peroxy radicals reached concentrations of 40 ppt. NO₃, RO₂, and HO₂ exhibited strong variations, in contrast to the almost constant values of the longer lived trace gases. The data suggest anticorrelation between NO₃ and RO₂ radical concentrations at night.The measured trace gas set allows the calculation of NO₃ and peroxy radical concentrations, using a chemical box model. From these simulations, it is concluded that the observed anthropogenic hydrocarbons are not sufficient to explain the observed RO₂ concentrations. The chemical budget of both NO₃ and RO₂ radicals can be understood if emissions of monoterpenes are included. The measured HO₂ can only be explained by the model, when NO concentrations at night of around 5 ppt are assumed to be present. The presence of HO₂ radicals implies the presence of hydroxyl radicals at night in concentrations of up to 10⁵ cm^–3.     

Measurements of stratospheric HO2 and NO2 by matrix isolation and ESR spectroscopy

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.     

NO<Subscript>3</Subscript> Vertical Profile Measurements from Remote Sensing Balloon-Borne Spectrometers and Comparison with Model Calculations

Eleven vertical profiles of stratospheric NO₃ have been obtained since 1992 using the AMON and SALOMON balloon-borne UV-visible spectrometers. The measurements are compared to the SLIMCAT 3D model and calculations based on the steady-state hypothesis for NO₃. The calculations cannot reproduce some parts of the profiles which exhibit strong concentration fluctuations over few kilometres, as a consequence of the dependence of NO₃ on local temperature variations. A statistical use of the data allows us to estimate the influence of the temperature dependence of the absorption cross-section on the data analysis, and the validity of the recommended reaction rates available in the literature. Discrepancies exist between the model based on recommended kinetics and observations at warmer temperatures. Nevertheless, the analysis is biased by local temperature inhomogeneities, and only a low-resolution vertical shape of the NO₃ profiles can be retrieved.     

Measurement of the photodissociation coefficient of NO2 in the atmosphere: II,stratospheric measurements

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.     

Mass transfer at the air/water interface: Mass accommodation coefficients of SO2, HNO3, NO2 and NH3

We present here experimental determinations of mass accommodation coefficients using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase.We report in this article measurements of _exp=(6.0±0.8)×10^–2 at 298 K and with a total pressure of 38 Torr for SO₂, (5.0±1.0)×10^–2 at 297 K and total pressure of 52 Torr for HNO₃, (1.5±0.6)×10^–3 at 298 K and total pressure of 50 Torr for NO₂, (2.4±1.0)×10^–2 at 290 K and total pressure of 70 Torr for NH₃.These values are corrected for mass transport limitations in the gas phase leading to =(1.3±0.1)×10^–1 (298 K) for SO₂, (1.1±0.1)×10^–1 (298 K) for HNO₃, (9.7±0.9)×10^–2 (290 K) for NH₃, (1.5±0.8)×10^–3 (298 K) for NO₂ but this last value should not be considered as the true value of for NO₂ because of possible chemical interferences.Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.     

Ballon-borne and aircraft infrared measurements of ethane (C2H6) in the upper troposphere and lower stratosphere

Quantitative infrared measurements of ethane (C₂H₆) in the upper troposphere and lower stratosphere are reported. The results have been obtained from the analysis of absorption features of the ₉ band at 12.2 m, which have been identified in high-resolution ballon-borne and aircraft solar absorption spectra. The ballon-borne spectral data were recorded at sunset with the 0.02 cm^-1 resolution University of Denver interferometer system from a float altitude of 33.5 km near Alamogordo, New Mexico, on 23 March 1981. The aircraft spectra were recorded at sunset in July 1978 with a 0.06 cm^-1 resolution interferometer aboard a jet aircraft at 12 km altitude, near 35°N, 96°W. The balloon analysis indicates the C₂H₆ mixing ratio decreased from 3.5 ppbv near 8.8 km to 0.91 ppbv near 12.1 km. The results are consistent with the colum value obtained from the aircraft data.     

Atmospheric and Oceanic Variability Associated with Growing Season Droughts and Pluvials on the Canadian Prairies

Abstract

The importance of measurements of the vertical distribution of odd nitrogen in studies of ozone chemistry and climate change has long being recognized. In this paper, we use the optimal estimation method developed by Rodgers (1976, 1990) to retrieve NO₂ vertical profiles from slant column observations made with a portable ultraviolet (UV)‐visible zenith‐sky spectrometer operated on the ground during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) balloon campaign carried out at Vanscoy, Saskatchewan, Canada (52°N, 107°W), from 18 to 25 August 1998. Late summer was chosen for the campaign because the stratospheric zonal wind velocity changes sign at that time. Under such conditions the stratospheric winds are at a minimum, leaving the stratosphere in a dynamically quiescent state and closer to photochemical control (Fahey et al., 2001; Fioletov and Shepherd, 2003). The NO₂ profile retrieved from the ground‐based observations is compared with the co‐located and simultaneous NO₂ profile measured by a balloon‐borne UV‐visible spectrometer during sunrise on 24 August. Good agreement is observed, giving us confidence in the retrieval technique adopted. The retrieved NO₂ profiles are also compared with the output of the Model for Evaluating oZONe trends (MEZON) 3D stratospheric chemical transport model. It is observed that, for altitudes below the peak concentration, the model underestimates the NO₂ amount, and at the altitude of peak concentration, the model values lie between the values measured from the balloon and those retrieved from the ground‐based measurements. Nevertheless, the model reproduces the general shape of the retrieved profiles, including the altitude of the NO₂ maximum, for both sunrise and sunset conditions.     

Intercomparison of instruments for tropospheric measurements using differential optical absorption spectroscopy

The results of an intercomparison campaign of eight different long path UV-visible DOAS instruments measuring NO₂, O₃ and SO₂ concentrations in a moderately polluted urban site are presented. For effective optical path lengths of 230 and 780 m the overall spread of these measurements (±1) are 5×10¹⁰, 6×10¹⁰ and 1×10¹⁰ molec·cm^-3 (2.0, 2.4, and 0.4 ppb) for these molecules respectively when all instruments used a common set of absorption cross sections. The remaining differences are not completely random and the systematic differences are attributed to the different retrieval methods used for each instrument.     

Field measurements of NO and NO2 emissions from fertilized and unfertilized soils

Field measurements of NO and NO₂ emissions from soils have been performed in Finthen near Mainz (F.R.G.) and in Utrera near Seville (Spain). The applied method employed a flow box coupled with a chemiluminescent NO _x detector allowing the determination of minimum flux rates of 2 g N m^-2 h^-1 for NO and 3 g m^-2 h^-1 for NO₂.The NO and NO₂ flux rates were found to be strongly dependent on soil surface temperatures and showed strong daily variations with maximum values during the early afternoon and minimum values during the early morning. Between the daily variation patterns of NO and NO₂, there was a time lag of about 2 h which seem to be due to the different physico-chemical properties of NO and NO₂. The apparent activation energy of NO emission calculated from the Arrhenius equation ranged between 44 and 103 kJ per mole. The NO and NO₂ emission rates were positively correlated with soil moisture in the upper soil layer.The measurements carried out in August in Finthen clearly indicate the establishment of NO and NO₂ equilibrium mixing ratios which appeared to be on the order of 20 ppbv for NO and 10 ppbv for NO₂. The soil acted as a net sink for ambient air NO and NO₂ mixing ratios higher than the equilibrium values and a net source for NO and NO₂ mixing ratios lower than the equilibrium values. This behaviour as well as the observation of equilibrium mixing ratios clearly indicate that NO and NO₂ are formed and destroyed concurrently in the soil.Average flux rates measured on bare unfertilized soils were about 10 g N m^-2 h^-1 for NO₂ and 8 g N m^-2 h^-1 for NO. The NO and NO₂ flux rates were significantly reduced on plant covered soil plots. In some cases, the flux rates of both gases became negative indicating that the vegetation may act as a sink for atmospheric NO and NO₂.Application of mineral fertilizers increased the NO and NO₂ emission rates. Highest emission rates were observed for urea followed by NH₄Cl, NH₄NO₃ and NaNO₃. The fertilizer loss rates ranged from 0.1% for NaNO₃ to 5.4% for urea. Vegetation cover substantially reduced the fertilizer loss rate.The total NO _x emission from soil is estimated to be 11 Tg N yr^-1. This figure is an upper limit and includes the emission of 7 Tg N yr^-1 from natural unfertilized soils, 2 Tg N yr^-1 from fertilized soils as well as 2 Tg N yr^-1 from animal excreta. Despite its speculative character, this estimation indicates that NO _x emission by soil is important for tropospheric chemistry especially in remote areas where the NO _x production by other sources is comparatively small.     

A modified profile method for determining the vertical fluxes of NO,NO2, ozone,and HNO3 in the atmospheric surface layer

A modified profile method for determining the vertical deposition (or/and exhalation) fluxes of NO, NO₂, ozone, and HNO₃ in the atmospheric surface layer is presented. This method is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these trace gases. The analysis (aerodynamic profile method) includes a detailed determination of the micrometeorological quantities (such as the friction velocity, the fluxes of sensible and latent heat, the roughness length and the zero plane displacement), and of the height-invariant fluxes of the composed chemically conservative trace gases with group concentrations c ₁=[NO]+[NO₂]+[HNO₃], c ₂=[NO₂]+[O₃]+3/2·[HNO₃], and c ₃=[NO]–[O₃]–1/2·[HNO₃]. The fluxes of the individual species are finally determined by the numerical solution of a system of coupled nonlinear ordinary differential equations for the concentrations of ozone and HNO₃ (decoding method). The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The model requires only the vertical profile data of wind velocity, temperature and humidity and concentrations of NO, NO₂, ozone, and HNO₃.The method has been applied to vertical profile data obtained at Jülich (September 1984) and collected in the BIATEX joint field experiment LOVENOX (Halvergate, U.K., September 1989).     

Modelling of the vertical fluxes of nitric acid,ammonia, and ammonium nitrate

Results from numerical investigations regarding the exchange of HNO₃, NH₃, and NH₄NO₃ between the atmosphere and the biosphere are presented. The investigations were performed with a modified inferential method which is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these nitrogen compounds. This modified inferential method calculates the micrometeorological quantities (such as the friction velocity and the fluxes of sensible and latent heat), the height-invariant fluxes of the composed chemically conservative trace species with group concentrationsc ₁=[HNO₃]+[NH₄NO₃] (total nitrate),c ₂=[NH₃]+[NH₄NO₃] (total ammonia), andc ₃=[HNO₃]-[NH₃] as well as the fluxes of the individual nitrogen compounds. The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The modified inferential method requires only the data of wind velocity, temperature, humidity and concentrations (HNO₃, NH₃, and NH₄NO₃) measured at a reference height by stations of a monitoring network.     

Intercomparison of NO,NO2 and HNO3 measurements with photochemical theory

The simultaneous measurements of NO, NO₂ and HNO_A mixing‐ratio profiles carried out on the Stratoprobe balloon flight of 22 July 1974 have been simulated with a time‐dependent model using the measured temperature and ozone profiles. The calculated ratios of NO/NO₂, HNO₃/NO₂ using currently accepted photochemistry are consistent with the measured ratios within the experimental errors of the measurements. The measured NO₂/NO ratio is almost a factor of two smaller than predicted, although the discrepancy is still within the experimental errors. A remarkable proportionality in the NO₂ and O₃ profiles has been noted and is unexplained. A time‐dependent simulation has been employed to convert the measurements into diurnally‐averaged profiles suitable for intercomparison with two‐dimensional stratospheric models and a comparison with constituent profiles from Prinn et al. (1975) is carried out as an example. The NO_V mixing ratio, formed from the sum of the NO, NO₂ and HNO₂ measurements is similar to the NO_V mixing ratio from several one‐ and two‐dimensional models used to predict the effects of SST's on the ozone layer. The odd nitrogen mixing ratio is roughly constant from 20 to 35 km at 11 ppbv.     

On the scavenging of gaseous nitrogen compounds by large and small rain drops: II. Wind tunnel and theoretical studies of the simultaneous uptake of NH3, SO2 and CO2 by water drops

An experimental investigation of the simultaneous absorption of NH₃ and SO₂ from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO₂, NH₃ and CO₂ gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this plume-model show that the SO₂ uptake is strongly dependent on the NH₃ concentration in the atmosphere and on the rainrate. We also find that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH₃ and SO₂, desorption of these gases is negligible.     

An improved method of measuring tropospheric NO2 and RO2 by matrix isolation and electron spin resonance

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.     

Determination of heterogeneous reaction probability using deposition profile measurement in an annular reactor: Application to the N2O5/H2O reaction

A method for the estimation of the reaction probability of the heterogeneous N₂O₅+H₂O 2HNO₃ reaction using the deposition profile in a laminar flow tube, in which the walls are coated with the condensed aqueous phase of interest, is presented. The production of gas phase nitric acid on the surface followed by its absorption complicates the deposition profiles and hence the calculation of the reaction probability. An estimation of the branching ratio for this process enables a more appropriate calculation to be carried out. Reaction probabilities of N₂O₅ on substances including some normally constituting atmospheric aerosols, NaCl, NH₄HSO₄, as well as Na₂CO₃ are estimated and found to depend on relative humidity and characteristics of the coating used. These fell within the range (0.04–2.0)×10^–2.     

The NO2 Absorption Spectrum. IV: The 200–400 nm Region at 220 K

Previous experiments in the 400–500 nm region (Coquart et al., 1995) have been extended to the 200–400 nm region to determine the absorption cross-sections of NO₂ at 220 K. The NO₂ and N₂O₄ cross-sections are obtained simultaneously from a calculation applied to the data resulting from measurements at low pressures. A comparison between the NO₂ cross-sections at 220 K and at ambient temperature shows that the low temperature cross-sections are generally lower, except in the region of the absorption peaks. Comparisons are also made with previous data at temperature close to 220 K.     

The NO2 absorption spectrum. I: Absorption cross-sections at ambient temperature in the 300–500 nm region

New laboratory measurements of NO₂ absorption cross-sections have been performed between 300 and 500 nm at ambient temperature with improved experimental conditions: low gas pressures, long absorption paths, suitable absorbance values, narrow spectral bandwidths. The data, stored at 0.01 nm intervals, have been compared to those of the more recent studies and some reasons of disagreement are discussed.In the photolysis region below 400 nm, our absorption cross-sections are larger than those previously published, suggesting that the photodissociation coefficient calculated from the current data sets is underestimated. In the structured region of the spectrum above 400 nm, improvement of the resolution gives more precise values useful for optical measurements in atmosphere.Unité de Recherche Associée au CNRS.     

The NO2 absorption spectrum. III: The 200–300 nm region at ambient temperature

Absorption cross-section measurements of NO₂ performed in our laboratory have been extended to the 200–300 nm region at ambient temperature. Low pressures have been used, limiting the effects of the dimer N₂O₄ which has an absorption cross-section from one to two orders of magnitude larger than that of NO₂ in this region. The results have been compared to those of previous authors and are now available for atmospheric purposes at 0.01 nm intervals.Unité de Recherche Associée au CNRS.     

Kinetics of the reaction of nitrogen dioxide with ozone

The kinetics of the reaction of NO₂ with O₃ have been investigated at 296 K, using UV absorption spectroscopy to monitor decay of NO₂ or O₃ and infrared laser absorption spectroscopy to monitor formation of the reaction product N₂O₅. The results both for the rate coefficient at 296 K (k ₁=3.5×10^-17 cm³ molecule^-1 s^-1) and the reaction stoichiometry (NO₂/O₃=1.85±0.09) are in good agreement with previous studies, confirming that the two step mechanism involving formation of symmetrical NO₃ as an intermediate is predominant.% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOtaiaab+% eadaWgaaWcbaGaaeOmaaqabaGccqGHRaWkcaqGpbWaaSbaaSqaaiaa% bodaaeqaaOWaa4ajaSqaaaqabOGaayPKHaGaaeOtaiaab+eadaWgaa% WcbaGaae4maaqabaGccqGHRaWkcaqGpbWaaSbaaSqaaiaabkdaaeqa% aaaa!41D7!\[{\text{NO}}_{\text{2}} + {\text{O}}_{\text{3}} \xrightarrow{{}}{\text{NO}}_{\text{3}} + {\text{O}}_{\text{2}} \]% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOtaiaab+% eadaWgaaWcbaGaae4maaqabaGccqGHRaWkcaqGobGaae4tamaaBaaa% leaacaqGYaaabeaakiabgUcaRiaab2eadaGdKaWcbaaabeGccaGLsg% cacaqGobWaaSbaaSqaaiaabkdaaeqaaOGaae4tamaaBaaaleaacaqG% 1aaabeaakiabgUcaRiaab2eaaaa!4464!\[{\text{NO}}_{\text{3}} + {\text{NO}}_{\text{2}} + {\text{M}}\xrightarrow{{}}{\text{N}}_{\text{2}} {\text{O}}_{\text{5}} + {\text{M}}\]A possible minor role for the unsymmetrical ONOO species is suggested to account for the lower-than-expected stoichiometry factor. The importance of this reaction in the oxidation of atmospheric NO₂ is discussed.