Konrad Stemmler

Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid

Nitrous acid is a significant photochemical precursor of the hydroxyl radical, the key oxidant in the degradation of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atmospheric measurements revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that reduction of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces containing humic acid exhibit an organic surface photochemistry that produces reductive surface species, which react selectively with nitrogen dioxide. The observed rate of nitrous acid formation could explain the recently observed high daytime concentrations of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60 per cent of the integrated hydroxyl radical source strengths. We suggest that this photo-induced nitrous acid production on humic acid could have a potentially significant impact on the chemistry of the lowermost troposphere.

Photoenhanced uptake of gaseous NO2 on solid organic compounds: a photochemical source of HONO?

In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present laboratory study, the effect of light (in the range 300-500 nm) on the uptake kinetics of NO2 on various surfaces taken as proxies for organic surfaces encountered in the troposphere (as organic aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing aromatic compounds was significantly enhanced when irradiated with light of 300-420 nm, and HONO was formed with high yield when the gas was humidified. Especially organic substrates containing a combination of electron donors, such as phenols, and of compounds yielding excited triplet states, such as aromatic ketones, showed a high reactivity towards NO2. Based on the results reported a mechanism is suggested, in which photosensitised electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atmosphere at much longer wavelengths compared to the recently proposed nitrate photolysis.

Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol

The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO 2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn <10−7 (in the dark) andγrxn=6×10 were observed. The observed uptake coefficients decreased with increasing NO 2concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity ( <20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONOformation rates observed near the ground in rural and urban environments by the conversion of NO 2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles. Correspondence to: M. Ammann (markus.ammann@psi.ch)

Low European methyl chloroform emissions inferred from long-term atmospheric measurements

Methyl chloroform (CH3CCl3, 1,1,1,-trichloroethane) was used widely as a solvent before it was recognized to be an ozone-depleting substance and its phase-out was introduced under the Montreal Protocol. Subsequently, its atmospheric concentration has declined steadily and recent European methyl chloroform consumption and emissions were estimated to be less than 0.1 gigagrams per year. However, data from a short-term tropospheric measurement campaign (EXPORT) indicated that European methyl chloroform emissions could have been over 20 gigagrams in 2000 (ref. 6), almost doubling previously estimated global emissions. Such enhanced emissions would significantly affect results from the CH3CC13 method of deriving global abundances of hydroxyl radicals (OH) (refs 7–12)—the dominant reactive atmospheric chemical for removing trace gases related to air pollution, ozone depletion and the greenhouse effect. Here we use long-term, high-frequency data from Mace Head, Ireland and Jungfraujoch, Switzerland, to infer European methyl chloroform emissions. We find that European emission estimates declined from about 60 gigagrams per year in the mid-1990s to 0.3–1.4 and 1.9–3.4 gigagrams per year in 2000–03, based on Mace Head and Jungfraujoch data, respectively. Our European methyl chloroform emission estimates are therefore higher than calculated from consumption data, but are considerably lower than those derived from the EXPORT campaign in 2000 (ref. 6).

OH radical-initiated oxidation of organic compounds in atmospheric water phases: part 1. Reactions of peroxyl radicals derived from 2-butoxyethanol in water

The aim of this series of two publications is to characterise important radical reactions of organic molecules in atmospheric water phases, such as cloud, fog, and rain droplets, or on wetted aerosols. In this first study the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the absence of trace compounds typically found in such natural waters. Using gamma-radiation emitted from a Co-60 source, We have produced steady-state concentrations of O-2(-)/HO2- and OH radicals comparable to those predicted for daytime conditions in cloud or fog droplets. The reactions of the peroxyl radicals derived from 2-butoxyethanol (C4H9OCH2CH2OH) in oxygenated aqueous solution have been investigated by measuring detailed product distributions resulting from the oxidation process. The observed and quantified products are mainly esters, ketones, aldehydes and hydroperoxides and accounted for 83% of the removal of 2-butoxyethanol (expressed as a carbon balance). A mechanism for the formation of the reaction products is proposed. It includes the OH attack at four carbon centers of the molecule producing carbon centered radicals, the formation of peroxyl radicals by addition of oxygen to these radicals, the decay of the peroxyl radicals in a reaction system of six different peroxyl radicals and superoxide via bimolecular and unimolecular reactions and the reactions of intermediary produced oxyl radicals

OH radical-initiated oxidation of organic compounds in atmospheric water phases: part 2. Reactions of peroxyl radicals with transition metals

The aim of this series of two publications is to characterise radical reactions of organic molecules, occurring in atmospheric water phases. In this second study, the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the presence of O-2(-)/HO2 radicals, H2O2, and of copper (I and II)- and iron (II and III) species. A general mechanism of the reactions occurring in such systems is discussed and compared to the formation of organic hydroperoxides and hydrogen peroxide from this oxidation reactions. The main features of the mechanism are as follows: (i) production of peroxyl radicals following OH-attack on the model compound; (ii) cycling of the transition metals between the oxidation states given above, arising from a reduction reaction with superoxide, O-2(-) and oxidation reactions by organic peroxyl radicals, organic hydroperoxides, superoxide, and oxygen; (iii) reactions of organic peroxyl radicals, in contrast to superoxide radicals, occur only with the reduced forms of the transition metals and lead to the formation of organic hydroperoxides: (iv) the reaction of organic peroxyl radicals with Cu (I) is fast (k > 10(8) M-1 s(-1)); (v) the reaction of organic hydroperoxides with Cu (I) can he much faster (2 orders of magnitude) than the analogous reaction of hydrogen peroxide and leads to the production of oxyl radicals: and (vi) oxalate, a strong ligand for Cu (II), reduces the activity of copper, owing to its stabilisation of the copper (II) complex. The mechanism is discussed in relation to atmospheric conditions

Oxidation of metal-diethylenetriamine-pentaacetate (DTPA) - Complexes during drinking water ozonation

This study investigates the oxidative transformation of diethylenetriaminepentaacetate (DTPA), a synthetic ligand, during drinking water ozonation. The rate coefficients for the reactions of CaDTPA3- and ZnDTPA3- with ozone were determined to be 6200 and 3500 +/- 150 M-1 s-1, respectively. The reactivity of Fe(III)DTPA2- towards ozone was found to be much lower (< 10 M-1 s-1), but near neutral pH the reactivity of the Fe(III)-complexes is dominated by [Fe(III)(OH)]DTPA3-. For the reaction of Fe[(III)(OH)]DTPA3- with ozone a rate coefficient of 2.4 +/- 0.2 x 10(5) M-1 s-1 was measured. The rate coefficients of the reactions of the ZnDTPA- and Fe(III)DTPA with OH radicals have been determined by a competitive method as 2.4 +/- 0.4 x 10(9) and 1.5 +/- 0.1 x 10(9) M-1 s-1, respectively at pH = 7. The degradation of low concentrations of DTPA complexes during ozonation was investigated in natural waters under drinking water relevant conditions. Based on our findings CaDTPA3- and ZnDTPA3- are judged as easily degradable. Fe(III)DTPA complexes showed a somewhat lower reactivity, but were still typically degraded by one order of magnitude at ozone dosages of approximately 20 microM (1 mg L-1) in the three natural waters tested. Molecular ozone was found to be the major oxidant for the metal-DTPA complexes during ozonation.

Hydroxyl-radical-initiated oxidation of isobutyl isopropyl ether under laboratory conditions related to the troposphere Product studies and proposed mechanism

The products formed by the hydroxyl-radical-initiated oxidation of the model ether, isobutyl isopropyl ether [(CH 3 ) 2 CHCH 2 OCH(CH 3 ) 2 ], have been investigated by irradiating synthetic air mixtures containing the substrate, methyl nitrite, and nitric oxide at ppm levels in a Teflon bag reactor at room temperature. The decay of reactant and formation of products were monitored by gas chromatography, mass spectrometry and by HPLC. The molar yields of the major products (mol of product formed/mol of isobutyl isopropyl ether consumed) were as follows: acetone, 0.56 ± 0.04; isopropyl formate, 0.48 ± 0.03; isobutyl acetate, 0.28 ± 0.02; 2-hydroxy-2-methylpropyl acetate [CH 3 C(O)OCH 2 C(OH)(CH 3 ) 2 ], 0.25 ± 0.1. The molar yields of the minor products were as follows: isobutyraldehyde, 0.06 ± 0.05; isopropyl nitrate, 0.09 ± 0.06; 1,1,4-trimethyl-3-oxapentyl nitrate [(CH 3 ) 2 CHOCH 2 C(CH 3 ) 2 (ONO 2 )], 0.07 ± 0.02; isopropyl isobutyrate [(CH 3 ) 2 CHC(O)OCH(CH 3 ) 2 ] ca. 0.01; and isobutyl formate, ca. 0.01. The major products are explained by a mechanism involving initial OH attack at the –CH– and –CH 2 – groups in the alkyl side chains of the ether followed by the subsequent reactions of the resulting carbon-centred, organic peroxy, and organic oxy radicals. The observed products, in conjunction with the proposed reaction pathways, account for a total yield of about 1.15, indicating that all the main routes are accounted for in the degradation of this ether. The major reaction pathways of the three principal organic oxy radicals are summarised as follows (percentage of overall reaction in brackets):(CH3)2C(O˙)OCH2CH(CH3)2 → CH3C(O)OCH2CH(CH3)2]C˙H3 (28%)(CH3)2CHOCH(O˙)CH(CH3)2 → (CH3)2CHOC(O)H]C˙H(CH3)2 (⩽48%)(CH3)2CHOCH2C(O˙)(CH3)2 → (CH3)2C˙OCH2C(OH)(CH3)2 (25%)This study supports the finding that organic oxy radicals generated from ethers and containing the structure RCH(O˙)OR undergo mainly decomposition by C–C bond cleavage, whereas those oxy radicals with the structure RCH(O˙)CH 2 OR undergo preferential 1,5-H-atom transfer isomerisation reactions. The following rate coefficients (10 −12 cm 3 molecule −1 s −1 ) at room temperature for the reactions of OH radicals with the reactant and products have been determined by the relative rate technique: isobutyl isopropyl ether, 19.5 ± 0.4; isobutyl acetate, 6.0 ± 0.5; isobutyraldehyde, 25.8 ± 0.7; isopropyl formate, 2.1 ± 0.1; isopropyl isobutyrate, 6.5 ± 0.4; 1,1,4-trimethyl-3-oxapentyl nitrate, 16.5 ± 0.7; and 2-hydroxy-2-methylpropyl acetate, 9.5 ± 1.6.

Regional emissions of anthropogenic halocarbons derived from continuous measurements of ambient air in Switzerland

Man-made halocarbons have a considerable impact on the environment. Chlorine- and bromine-containing organic compounds such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are responsible for the decline of stratospheric ozone and their use has therefore been regulated in the Montreal Protocol. Fluoro-containing halocarbons are not regulated within this treaty, but as all halocarbons act as greenhouse gases, this group of compounds was included in the Kyoto Protocol. Halogenated ozone destroying CFCs and chlorinated solvents were continuously measured in Switzerland at a suburban site near Zurich (Dubendorf) from 1994 until 2000 and since 2000 at the high alpine site of Jungfraujoch together with all greenhouse-active halocarbons. Furthermore, all anthropogenic halocarbons were measured in a campaign in Dubendorf in 2002. The analysis of the measurements confirms that a substantial decline of their emissions took place as a consequence of the Montreal Protocol. On the other hand, the concentration of substitutes such as hydrofluorocarbons (HFCs) increased dramatically. Linking continuously measured time series with meteorological information allows an estimation of regional emissions of the regulated compounds for Switzerland. In contrast to trade survey statistics or inventories, this method is an efficient tool to track the development of diffusive emissions of substances in long term usage such as foams and refrigerant systems.