Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 394 | 23 JULY 1998 353sites. Clearly, differences in catalyst loading must also be taken into account when assessing the relative activities of the catalytic sites.The technique described here could be extended to monitor multiple reaction products, and thus could also be used to acquire information on catalyst selectivity. This could be accomplished by using different laser frequencies to sequentially generate the REMPI signals of different products. The REMPI signals could then be converted into absolute concentrations, using calibration standards, for the determination of selectivities. In addition, the technique developed should be useful in the study of issues related to the operational lifetimes of catalysts, their resistance to poisoning, their regeneration and their loss during operation. Ⅺ

Bromine atoms are believed to play a central role in the depletion of surface-level ozone in the Arctic at polar sunrise. Br2, BrCl, and HOBr have been hypothesized as bromine atom precursors, and there is evidence for chlorine atom precursors as well, but these species have not been measured directly. We report here measurements of Br2, BrCl, and Cl2 made using atmospheric pressure chemical ionization-mass spectrometry at Alert, Nunavut, Canada. In addition to Br2 at mixing ratios up to approximately 25 parts per trillion, BrCl was found at levels as high as approximately 35 parts per trillion. Molecular chlorine was not observed, implying that BrCl is the dominant source of chlorine atoms during polar sunrise, consistent with recent modeling studies. Similar formation of bromine compounds and tropospheric ozone destruction may also occur at mid-latitudes but may not be as apparent owing to more efficient mixing in the boundary layer.

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Abstract. The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx-free system using atmospheric pressure ionization-mass spectrometry (API-MS) as well as GC-MS. 1-chloro-3-methyl-3-butene-2-one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9 + 5 %), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2-methylene-3-butenal, is also tentatively identified using API-MS. Assuming that similar chemistry occurs at the ppb-ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NO x environments where there are also sources of isoprene.

Abstract. The chemical composition of layers of trace gas mixtures within the lower troposphere and their relationship to surface trace gas concentrations are investigated using airborne chemistry and meteorological measurements made over eastern Connecticut and central Massachusetts. Layers of photochemically aged matehal were identified by maxima above the surface stable layer in the profiles of 03, NOy, CO, aerosols, peroxyacetyl nitrate (PAN), and the ratio of highly to lesser reactive hydrocarbons (e.g., benzene and toluene). Observations suggest that strong diurnal variations in surface-ozone mixing ratios are associated with elevated layers of ozone and other trace gases. The elevated layers were also associated with strong gradients of NOx both vertically, across the mixed layer, and horizontally, between urban and rural regions, providing evidence for the dominant role of localized anthropogenic NOx emissions over North America on ozone production in urban regions. On days when elevated layers were detected, isoprene's late-morning propeneequivalent mixing ratio (a measure of nonmethane hydrocarbon reactivity with OH) was an order of magnitude greater than that of the next most reactive species up to an altitude of 650 rn mean sea level. Four-dimensional data assimilation was used within a mesoscale model to study the formation mechanism and history of these layers, with a key result being that no unique "age" or source region could generally be attributed to these layers as a consequence of turbulent mixing and transport leading to their formation.