Atmos. Chem. Phys., 6, 5525–5536, 2006 www.atmos-chem-phys.net/6/5525/2006/ Atmospheric © Author(s) 2006. This work is licensed Chemistry under a Creative Commons License. and Physics Processes controlling the concentration of hydroperoxides at Jungfraujoch Observatory, Switzerland S. J. Walker1, M. J. Evans1, A. V. Jackson1, M. Steinbacher2, C. Zellweger2, and J. B. McQuaid1 1Institute for Atmospheric Science, School of the Earth and Environment, University of Leeds, Leeds, UK 2Swiss Federal Institute for Materials Science and Technology (Empa), Laboratory for Air Pollution/Environmental Technology, 8600 Dubendorf,¨ Switzerland Received: 6 June 2006 – Published in Atmos. Chem. Phys. Discuss.: 28 July 2006 Revised: 17 October 2006 – Accepted: 30 November 2006 – Published: 7 December 2006 Abstract. An automated, ground-based instrument was used oxy radicals (hydroperoxy, HO2 and organic peroxy, RO2) to measure gas-phase hydroperoxides at the Jungfraujoch (e.g. Lee et al., 2000). These peroxy radicals are intimately High Altitude Research Station as part of the Free Tro- linked to ozone (O3) production and loss, leading to hy- pospheric EXperiment (FREETEX) during February/March droperoxides being a key diagnostic of the chemical state of 2003. A nebulising reflux concentrator sampled ambient air the atmosphere. The chemistry of odd oxygen species (Ox), twice hourly, prior to on-site analysis by HPLC speciation, HOx (the hydroxyl radical, OH and HO2) and ROx (RO and coupled with post-column peroxidase derivatisation and flu- RO2) is summarized in Fig. 1. H2O2 is also a major oxidant orescence detection. Hydrogen peroxide (H2O2) concentra- of sulphur dioxide (SO2) in clouds (Penkett et al., 1979) so tions reached up to 1330 pptv over the 13-day period with a alongside O3 and OH, its measurement is needed to assess mean of 183±233 pptv (± one standard deviation). Methyl the oxidative capacity of the atmosphere. hydroperoxide (CH3OOH) reached up to 379 pptv with a Previous analyses of H2O2 and CH3OOH observations mean of 51±55 pptv. No other organic hydroperoxides were have identified key physical and chemical processes control- detected. The lack of an explicit diurnal cycle suggests that ling their concentration (e.g. Heikes et al., 1987). As shown hydroperoxide concentrations are chiefly influenced by trans- in Fig. 1, the production of H2O2 is controlled by the abun- port processes rather than local photochemistry at this moun- dance of HO2, which shows sensitivity to HOx production, tainous site. There was some evidence that elevated concen- loss and recycling. Formation of peroxy radicals is predomi- trations of H2O2 existed in air-masses originating from the nantly through the photo-oxidation of carbon monoxide (CO) south-west, suggesting higher concentrations of HOx due to and volatile organic compounds (VOC), by the OH radical more active photochemistry. Air which had been recently (detailed in Lightfoot et al., 1992). A second significant polluted exhibited low H2O2 concentration due to a combi- source is from formaldehyde (CH2O), which undergoes pho- nation of suppression of HO2 by NOx and deposition. The tolysis and reaction with OH to produce HO2. concentrations of H2O2 sampled here are consistent with pre- Air-masses rich in nitrogen oxides, NOx (NO and NO2) vious box modelling studies of hydroperoxides, except in pe- tend to show lower H2O2 concentrations as peroxy radicals riods influenced by the boundary layer, where agreement re- oxidise NO to NO2 rather than self-reacting to form H2O2 quired a depositional sink. (Tremmel et al., 1993; Penkett et al., 1995) and total radi- cal loadings are suppressed by nitric acid (HNO3) produc- tion through the reaction of OH with NO2 radicals (Poppe et al., 1993). According to Lee et al. (2000), substantial sup- 1 Introduction pression of hydroperoxide production occurs at NO concen- trations exceeding 100 pptv. It is calculated that NO con- Hydroperoxides play an important role in gas-phase free centrations below 3 to 20 pptv are needed for hydroperoxide radical chemistry of the atmosphere and in the aqueous- production to dominate over the reaction between HO and phase chemistry of acid precipitation. Hydroperoxides such 2 NO (Reeves and Penkett, 2003; Crutzen and Zimmermann, as hydrogen peroxide (H O ) and methyl hydroperoxide 2 2 1991; Finlayson-Pitts and Pitts, 1986). Such low concentra- (CH OOH) are produced through the self-reaction of per- 3 tions of NO can only be found in very remote regions of the Correspondence to: A. V. Jackson troposphere lacking NOx sources. ([email protected]) Published by Copernicus GmbH on behalf of the European Geosciences Union. 5526 S. J. Walker et al.: Hydroperoxides at Jungfraujoch Observatory 2003). The high altitude site is thought to experience few O 3 local emission sources, and therefore can offer ideal condi- hν, H O 2 tions for long-term sampling of free tropospheric air. The VOC OH JFJ is incorporated in the Swiss National Air Pollution Mon- CO O OH or VOC 3 itoring Network (NABEL), which is maintained by EMPA hν H O OH 2 2 HO2 HO2 NO Dubendorf¨ on behalf of the Swiss Federal Office for the En- 2 hν Dry OH or hν, O Dry and and CH O 2 O2 vironment (FOEN). Among other species, NO, NO2,O3 and 2 O3 HNO Wet Wet O2 3 Deposition Deposition O CO are measured routinely and are available with a time res- RO NO 3 olution of 30 min. All meteorological parameters are mea- OH or RONO2 hν, O 2 sured by the Federal Office of Meteorology and Climatol- RO2 ROOH HO2 ogy (MeteoSwiss). Due to the importance of monitoring long-term trends of gaseous and aerosol parameters in the Fig. 1. Tropospheric gas-phase reactions linked to hydroperoxides. free troposphere, the JFJ station has been incorporated into the Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO) as a Global GAW Sta- Lower concentrations of H2O2 are observed in air-masses tion. exposed to the ground or clouds due to dry and wet depo- sition (e.g. Kleinman, 1986; Heikes et al., 1987; Chandler 2.2 Method for hydroperoxide sample collection and anal- et al., 1988; Gallagher et al., 1991). Rapid loss of gaseous ysis H2O2 into clouds is due to a high Henry’s law coefficient of × 4 −1 Sample collection took place at the Sphinx laboratory be- H2O2(HH2O2 is 7.73 10 M atm at 298 K, Sander et al., 2003) and rapid oxidation of S(IV) species to S(VI) within tween the 27 February and the 12 March 2003. Figure 3 shows a schematic diagram of the analytical system. Sam- the aqueous phase. Other important H2O2 and CH3OOH sinks are their reaction with OH radicals and photolysis at ples were obtained from the gas-phase using an amber-glass ultraviolet wavelengths. nebulising reflux concentrator (Fig. 3) based on the design The aim of this paper is to investigate the impact of these by Cofer et al. (1985). Atmospheric air was drawn by a vac- uum pump from the NABEL inlet (previously described in chemical and physical processes on the concentration of 00 Zellweger et al., 2000), through ∼5 m 1 OD Teflon (per- H2O2 and CH3OOH at a remote mountainous site and to un- 4 fluoroalkoxy) tubing into the nebuliser, at a flow rate of derstand the usefulness of H2O2 observations as a diagnostic −1 of the atmospheric compositional system. 2.7 l min . The residence time in the inlet tubing was less than 4 sec. A sampling time of 25 min provided sufficient concentration of the solution for analysis. Collection effi- 2 Experimental ciencies of 75±3% and 40±2% have been previously deter- mined experimentally for H2O2 and CH3OOH respectively 2.1 Site description and were assumed in this study (Jackson and Hewitt, 1996; Morgan, 2004). The observations are from the FREE Tropospheric EXper- Once collected, aqueous samples were injected through iment (FREETEX) that took place in February to March a 0.2 µm filter onto the HPLC where hydroperoxide spe- 2003 at the Sphinx Observatory, Jungfraujoch High Alti- ciation was achieved using a C-18 reversed-phase column. tude Research Station (7.98◦ E, 46.55◦ N) (subsequently re- Helium-degassed mobile phase was supplied at a flow rate ferred to as JFJ) situated at 3580 m above mean sea level of 0.6 ml min−1. Post-column, the hydroperoxide compo- (a.m.s.l.). There have been three previous FREETEX cam- nents were derivatised in a reaction coil in the presence of paigns in 1996, 1998 and 2001 at this site (Zanis et al., horseradish peroxidase to form a stable dimer, based on the 1999, 2000a, b, 2003; Carpenter et al., 2000). The Sphinx technique developed by Lazrus et al. (1985, 1986). The Observatory resides in a saddle between the two alpine derivatisation reagent was raised to pH 5.8 by the addition summits, Jungfrau (4155 m a.m.s.l.) to the south-west and of potassium hydroxide (1 M) and buffered by potassium Monch¨ (4099 m a.m.s.l.) to the north-east, in the Swiss Alps. hydrogen phthalate (0.5 M). A peristaltic pump delivered This leads to mainly north-westerly and southerly air-masses the derivatisation reagent at constant rate of 0.25 ml min−1. reaching the station (see Fig. 2). The site characteristics The pH was then raised above 10 to produce the fluo- are described in Zellweger et al. (2003). The observatory is rescent anionic dimer form via a Nafion membrane sub- usually thought to be located in the lower free troposphere merged in 100 ml ammonium hydroxide (30%). Detection (FT) in winter and early spring but can experience plane- was achieved using a fluorescence spectrophotometer with tary boundary layer (PBL) air (Carpenter et al., 2000), es- excitation and emission wavelengths of 305 and 410 nm re- pecially in the summer when convection is enhanced (Bal- spectively, via a xenon short arc lamp.