D. Mihelcic

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, NOx, NOy, 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.

Calibration source for peroxy radicals with built-in actinometry using H2O and O2 photolysis at 185 nm

A radical source was developed and tested that allows one to generate HO2 at atmospheric concentrations. It is based on H2O photolysis at 185 nm with subsequent conversion of OH and H to HO2 for radical production and photolysis of O2 at the same wavelength to produce a reference ozone concentration. The source can be used as an absolute calibration method for chemical amplifiers by linking the produced HO2 concentration to the ozone concentration. It can also be used to produce organic alkyl peroxy radicals, if CO is replaced by the respective hydrocarbons. The source was calibrated with matrix isolation-electron spin resonance spectroscopy. The measured concentrations are in good agreement with those calculated from first principles if losses due to the self reaction of HO2 are taken into account. These losses are less than 10% at HO2 concentrations below 100 parts per trillion by volume (pptv) and increase to about 40% at an HO2 concentration of 250 pptv. Without addition of CO, the source can also be used for OH calibration, but chemical losses are then more important.

Free radicals and fast photochemistry during BERLIOZ

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.

Numerical analysis of ESR spectra from atmospheric samples

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.

Comparison of tropospheric NO3 radical measurements by differential optical absorption spectroscopy and matrix isolation electron spin resonance

Despite the importance of NO 3 in the nighttime atmosphere only two techniques, Differential Optical Absorption Spectroscopy (DOAS) and Matrix Isolation Electron Spin Resonance (MIESR) have been applied to its detection in ambient air to date. Here we report the results of the first intercomparisons of these techniques in the atmosphere carried out at rural sites in Germany, at Deuselbach in 1983 and near Berlin in 1998. The simultaneously measured NO 3 mixing ratios, which were in the range from 9 to 20 ppt and at one measurement near 100 ppt, were in good agreement within the error limits. A regression analysis yields a linear relationship between the DOAS and MIESR data with a correlation coefficient of R = 0.99 and a slope of 0.83 ± 0.03 (1σ error) at a negligible intercept of 0.33 ± 0.73 ppt (1σ error). The deviation from unity is within the total systematic error of both measurement techniques. This result shows the reliability of the two techniques over the past 15 years.

On the exchange of NO3 radicals with aqueous solutions : Solubility and sticking coefficient

The exchange of NO3 radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO3 was determined on 0.1 M NaCl solutions and was found to be γ(NO3) ≥ 2 × 10-3 in good agreement with recent studies. The Henry coefficient of NO3 was estimated to be KH(NO3) = 1.8 M · atm-1, with a (2σ) uncertainty of ±3 M · atm-1. From the upper limit for the Henry coefficient (KH = 5 M · atm-1) and available thermodynamic data, the redox potential of dissolved NO3/NO3− is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient KH and the rate coefficient k10 of the potential reaction NO3 + H2O → HNO3 + OH. For KH = 0.6 M · atm-1, we find k10 < 0.05 s-1 · atm-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO3 is only important under conditions where the dissolved NO3 is removed quickly from equilibrium, for example by reactions with Cl− or HSO3− ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N2O5.

Formation of hydroxyl and hydroperoxy radicals in the gas-phase ozonolysis of ethene

Abstract Ozonolysis of alkenes is thought be a significant source of free radicals (OH and HO 2 ) in the atmosphere. Although studied for many years, the reaction mechanism and the product yields are still very much under discussion. We report measurements of the production of HO 2 radicals from the reaction of O 3 with ethene using matrix isolation and electron-spin–resonance spectroscopy (MIESR). Formation of OH radicals was established by conversion of OH to HO 2 via reaction with CO. The OH yield is 20±2%, the HO 2 yield is 39±3%. Our measurements suggest that the Criegee intermediate in the ground state is not a radical.

Chemical Mechanism Development: Laboratory Studies and Model Applications

Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out:• oxygenated VOC, • aromatic VOC, • biogenic VOC, •short-lived intermediates, such as alkoxy and alkylperoxy radicals.At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.