G. Schuster

Distribution, speciation, and budget of atmospheric mercury

Total gaseous mercury (TGM) concentrations over the Atlantic Ocean and over Central Europe were measured repeatedly in the years 1978–1981. The latitudinal TGM distribution showed a pronounced and reproducible interhemispherical difference with higher TGM concentrations in the Northern Hemisphere. TGM was found to be vertically well mixed within the troposphere. The TGM concentration seems to increase with time at a rate of 10±8%/yr in the Northern and 8±3%/yr in the Southern Hemisphere. Measurements of mercury speciation showed that elemental mercury is the main TGM component contributing more than 92% and 83% of TGM in marine and continental air, respectively. The tropospheric mercury burden was calculated to be 6×109g. The interhemispheric distribution and temporal and spatial variability of TGM imply a tropospheric residence time of TGM of about 1 yr. Sink strengths calculated independently from the measured mercury concentration on particles and in rainwater are consistent with the above figures.

Direct Kinetic Study of OH and O3 Formation in the Reaction of CH3C(O)O2 with HO2

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

Atmospheric trace gas analysis using matrix isolation-fourier transform infrared spectroscopy

A novel cryogenic sampling method combining the matrix isolation technique with FTIR spectroscopy has been developed for atmospheric trace gas analysis. It is applicable to a wide range of molecules with detection limits typically in the 10–50 ppt range. The method is described along with some measurements of N2O, CFCl3, CF2Cl2, OCS, CS2, SO2 and PAN from samples collected at ground level and from an aircraft between 9 and 14 km.

Determination of H2O2 and organic peroxides in cloud‐and rain‐water on the Kleiner Feldberg during FELDEX

This letter reports high-pressure liquid chromatography (HPLC) measurements of H2O2 and organic hydroperoxides in cloud- and rain-water. Observations were conducted on the Kleiner Feldberg (Germany) from late October to late November 1995, within the FELDEX-campaign. It was the first campaign ever in which organic peroxides were determined in cloud-water. H2O2 was found with concentrations of <0.008–3.19 µmol · 1−1. Hydroxymethyl hydroperoxide (HOCH2OOH, HMHP) and 1-hydroxyethyl hydroperoxide (CH3CH(OH)OOH, 1-HEHP) were observed with concentrations in the range of <0.008–1.0 µmol · 1−1 (HMHP) and <0.008–0.28 µmol · 1−1 (1-HEHP). The ratio of the sum of organic peroxides over the total amount of peroxides (H2O2+organic peroxides) was 0–81%. The concentrations of S(IV) and H2O2 in the samples were anticorrelated. The results indicate that organic peroxides, such as HMHP and 1-HEHP, should be included into modelling studies of the aqueous phase oxidation of S(IV).

Peroxy radical reactions in the photo-oxidation of CH3CHO

The behaviour of reactants, products and radical intermediates during the photo-oxidation of acetaldehyde at room temperature and 700 Torr pressure has been studied using a novel apparatus. The apparatus consists of a ‘double multi-path’ spectrometer, combining both i.r. and u.v. absorption spectrometry with the additional capability of modulated photolysis for transient detection. Photo-oxidation of CH3CHO gave CO, CO2, H2O, CH2O, HCOOH, CH3OH, CH3OOH, CH3COOH and CH3COOOH as identifiable products by long-path F.t.i.r. spectrometry. The effect of variation of the O2 and CH3CHO concentration upon the different photolysis products was studied, and computer simulation of experimental data was used to test a proposed mechanism containing 50 elementary reactions. Absorption vs. time profiles at selected wavelengths in the 210–275 nm region were recorded in the modulated photolysis of CH3CHO–air mixtures. Transient absorptions were assigned to peroxy radicals, and kinetic analysis of the data assisted by computer simulation resulted in a value k10= 4.8 × 10–12 cm3 molecule–1s–1for the reaction HO2+ CH3O2→ products. This value is somewhat higher than the value k10a= 3.5 × 10–12 cm3 molecule–1s–1 for the reaction HO2+ CH3O2→ CH3OOH + O2 obtained from the computer simulations of the methyl hydroperoxide formation, indicating that an additional product channel may occur.

Heterogeneous reactions of HOI, ICl and IBr on sea salt and sea salt proxies

The heterogeneous chemistry of HOI, ICl and IBr on sea salt and sea salt proxies has been studied at 274 K using two experimental approaches: a wetted wall flow tube coupled to an electron impact mass spectrometer (WWFT-MS) and an aerosol flow tube (AFT) coupled to a differential mobility analyser (DMA) and a chemical ionisation mass spectrometer (CIMS). Uptake of all three title molecules into bulk aqueous halide salt films was rapid and controlled by gas phase diffusion. Uptake of HOI gave rise to gas-phase ICl and IBr, with the latter being the predominant product whenever Br(-) was present. Only partial release of IBr was observed due to high solubility of dihalogens in the film. ICl uptake gave the same yield of IBr as HOI uptake. Uptake of ICl on NaBr aerosol was accommodation limited with alpha = 0.018 +/- 0.004 and gas phase IBr product has a yield of 0.6 +/- 0.3. The results show that HOI can act as a catalyst for activation of bromine from sea-salt aerosols in the marine boundary layer, via the reactions: HOI(aq) + Cl + H--> ICl(aq) + H(2)O(l) and ICl(aq) + Br--> IBr(aq) + Cl.

Measurement of ambient NO 3 reactivity: design, characterization and first deployment of a new instrument

This paper describes the first demonstration of NO3 radical reactivity and its use in ambient air. Reactivity measurements for HOx radicals have become a routine feature of instruments that measure ambient HOx concentrations, yet no analogous instrument has been developed for NO3 until now. As such, the paper represents an important contribution that will be of interest to the readership of AMT and that may serve as a seminal work that stimulates a new generation of atmospheric instrumentation. With one exception (see below), the development is thorough and convincing. It should be published in AMT. C1

Coxy’s in the Atmosphere

A brief overview on the origin of the halogenated carbonyls COCl2, COClF and COF2 in the atmosphere is given. A description of the measurement technique, matrix-isolation-spectroscopy, which was used for the determination of these compounds, and results obtained from several flights over northern Europe will be discussed. While COCl2 appears to be present in both troposphere and stratosphere, the two other compounds were detected only in the stratosphere.

Atmospheric Trace Gas Measurements Using Matrix Isolation - FTIR Spectroscopy

The technique of matrix isolation combined with Fourier transform infrared spectroscopy has been developed as a sensitive method for atmospheric trace gas analysis. Matrices of solid CO2 are grown at 77K using natural CO2 as the matrix material, in which the atmospheric trace gases are embedded. Analysis of the matrix by FTIR absorption spectroscopy yields simultaneous measurements of a wide range of atmospheric trace gas species, including unstable and reactive ones, with detection limits typically around 101i molecules or 10-11 mixing ratio from a 20-40 litre air sample.