[1] The impact of iodine oxide chemistry upon OH and HO 2 concentrations in the coastal marine boundary layer has been evaluated using data from the NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, conducted at Mace Head, Ireland during the summer of 2002. Observationally constrained calculations show that under low NO x conditions experienced during NAMBLEX (NO 50 pptv), the reaction IO + HO 2 ! HOI + O 2 accounted for up to 40% of the total HO 2 radical sink, and the subsequent photolysis of HOI to form OH + I comprised up to 15% of the total midday OH production rate. The XO + HO 2 (X = Br, I) reactions may in part account for model overestimates of measured HO 2 concentrations in previous studies at Mace Head, and should be considered in model studies of HO x chemistry at similar coastal locations.Citation: Bloss, W. J., et al. (2005), Impact of halogen monoxide chemistry upon boundary layer OH and HO 2 concentrations at a coastal site, Geophys. Res. Lett., 32, L06814,
Abstract. OH and HO 2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×10 5 molecule cm −3 (5 min acquisition period; signal-tonoise ratio = 1). HO 2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×10 6 molecule cm −3 . The diurnal variation of OH was measured on 24 days, and that of HO 2 on 17 days. The local solar noon OH concentrations ranged between (3-8)×10 6 molecule cm −3 , with a 24 h mean concentration of 9.1×10 5 molecule cm −3 . The local solar noon HO 2 concentrations were (0.9-2.1)×10 8 molecule cm −3 (3.5-8.2 pptv), with a 24 h mean concentration of 4.2×10 7 molecule cm −3 (1.6 pptv). HO 2 radicals in the range (2-3)×10 7 molecule cm −3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HO x radicals under primarily clean marine conditions. Steady state expressions are used to calculate OH and HO 2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afternoon/evening, and greater than 1 in the middle of the day. For HO 2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly Correspondence to: D. E. Heard (d.e.heard@leeds.ac.uk) over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
Abstract.A field instrument utilising the artificial generation of OH radicals in a sliding injector flow-tube reactor with detection by laser-induced fluorescence spectroscopy has been developed to measure the rate of decay of OH by reaction with its atmospheric sinks. The OH reactivity instrument has been calibrated using known concentrations of CO, NO 2 and single hydrocarbons in a flow of zero air, and the impact of recycling of OH via the reaction HO 2 +NO→OH+NO 2 on the measured OH reactivity has been quantified. As well as a detailed description of the apparatus, the capabilities of the new instrument are illustrated using representative results from deployment in the semi-polluted marine boundary layer at the Weybourne Atmospheric Observatory, UK, and in a tropical rainforest at the Bukit Atur Global Atmospheric Watch station, Danum Valley, Borneo.
A tuneable, high pulse-repetition-frequency, solid state Nd:YAG pumped titanium sapphire laser capable of generating radiation for the detection of OH, HO2, NO and IO radicals in the atmosphere by laser induced fluorescence (LIF) has been developed. The integration of the laser system operating at 308 nm into a field measurement apparatus for the simultaneous detection of hydroxyl and hydroperoxy radicals is described, with detection limits of 3.1 x 10(5) molecule cm(-3) (0.012 pptv in the boundary layer) and 2.6 x 10(6) molecule cm(-3) (0.09 pptv) achieved for OH and HO2 respectively (30 s signal integration, 30 s background integration, signal-to-noise ratio = 1). The system has been field tested and offers several advantages over copper vapour laser pumped dye laser systems for the detection of atmospheric OH and HO2 radicals by LIF, with benefits of greater tuning range and ease of use coupled with reduced power consumption, instrument footprint and warm-up time. NO has been detected in the atmosphere at approximately 1 ppbv by single photon LIF using the Alpha 2Sigma+ <-- Chi 2Pi1/2 (0,0) transition at 226 nm, with absolute concentrations in good agreement with simultaneous measurements made using a chemiluminescence analyser. With some improvements in performance, particularly with regard to laser power, the theoretical detection limit for NO is projected to be approximately 2 x 10(6) molecule cm(-3) (0.08 pptv). Whilst operating at 445 nm, the laser system has been used to readily detect the IO radical in the laboratory, and although it is difficult to project the sensitivity in the field, an estimate of the detection limit is < 1 x 10(5) molecule cm(-3) (< 0.004 pptv), well below previously measured atmospheric concentrations of IO.
Abstract. The North Atlantic Marine Boundary Layer Experiment (NAMBLEX), involving over 50 scientists from 12 institutions, took place at Mace Head, Ireland (53.32 • N, 9.90 • W), between 23 July and 4 September 2002. A wide range of state-of-the-art instrumentation enabled detailed measurements of the boundary layer structure and atmospheric composition in the gas and aerosol phase to be made, providing one of the most comprehensive in situ studies of Correspondence to: D. E. Heard (dwayneh@chem.leeds.ac.uk) the marine boundary layer to date. This overview paper describes the aims of the NAMBLEX project in the context of previous field campaigns in the Marine Boundary Layer (MBL), the overall layout of the site, a summary of the instrumentation deployed, the temporal coverage of the measurement data, and the numerical models used to interpret the field data. Measurements of some trace species were made for the first time during the campaign, which was characterised by predominantly clean air of marine origin, but more polluted air with higher levels of NO x originating from continental regions was also experienced. ThisPublished by Copernicus GmbH on behalf of the European Geosciences Union. paper provides a summary of the meteorological measurements and Planetary Boundary Layer (PBL) structure measurements, presents time series of some of the longer-lived trace species (O 3 , CO, H 2 , DMS, CH 4 , NMHC, NO x , NO y , PAN) and summarises measurements of other species that are described in more detail in other papers within this special issue, namely oxygenated VOCs, HCHO, peroxides, organohalogenated species, a range of shorter lived halogen species (I 2 , OIO, IO, BrO), NO 3 radicals, photolysis frequencies, the free radicals OH, HO 2 and (HO 2 + RO 2 ), as well as a summary of the aerosol measurements. NAMBLEX was supported by measurements made in the vicinity of Mace Head using the NERC Dornier-228 aircraft. Using ECMWF windfields, calculations were made of the air-mass trajectories arriving at Mace Head during NAMBLEX, and were analysed together with both meteorological and trace-gas measurements. In this paper a chemical climatology for the duration of the campaign is presented to interpret the distribution of air-mass origins and emission sources, and to provide a convenient framework of air-mass classification that is used by other papers in this issue for the interpretation of observed variability in levels of trace gases and aerosols.
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