[1] The nocturnal conversion of dinitrogen pentoxide (N 2 O 5 ) to nitryl chloride (ClNO 2 ) on chloride-containing aerosol can be a regionally important NO x (= NO + NO 2 ) recycling and halogen activation pathway that affects oxidant photochemistry the following day. Here we present a comprehensive measurement data set acquired at Pasadena, California, during the CalNex-LA campaign 2010 that included measurements of odd nitrogen and its major components (NO y = NO x + NO 3 + 2N 2 O 5 + ClNO 2 + HNO 3 + HONO + peroxyacyl, alkyl, and aerosol nitrates) and aerosol size distribution and composition. Nitryl chloride was present during every night of the study (median mixing ratio at sunrise 800 pptv) and was usually a more significant nocturnal NO x and odd oxygen (O x = O 3 + NO 2 + 3N 2 O 5 + ClNO 2 ) reservoir species than N 2 O 5 (whose concentrations were calculated from its equilibrium with NO 2 and NO 3 ). At sunrise, ClNO 2 accounted for 21% of NO z (=NO y À NO x ), 4% of NO y , and 2.5% of O x , respectively (median values). Kinetic parameters for the N 2 O 5 to ClNO 2 conversion were estimated by relating ClNO 2 concentrations to their time-integrated heterogeneous production from N 2 O 5 and were highly variable between nights. Production of ClNO 2 required conversion of N 2 O 5 on submicron aerosol with average yield (ϕ) and N 2 O 5 reactive uptake probability (γ) of γϕ = 0.008 (maximum 0.04), scaled with submicron aerosol chloride content, and was suppressed by aerosol organic matter and liquid water content. Not all of the observed variability of ClNO 2 production efficiency could be rationalized using current literature parameterizations.

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Abstract. Reactive halogens, and in particular bromine oxide (BrO), have frequently been observed in regions with large halide reservoirs, for example during bromine catalyzed coastal polar ozone depletion events. Much less is known about the presence and impact of reactive halogens in areas without obvious halide reservoirs, such as the polar ice sheets or continental snow.We report the first LP-DOAS measurements of BrO at Summit research station in the center of the Greenland ice sheet at an altitude of 3200 m. BrO mixing ratios in May 2007 and June 2008 were typically between 1-3 pmol mol −1 , with maxima of up to 5 pmol mol −1 . These measurements unequivocally show that halogen chemistry is occurring in the remote Arctic, far from known bromine reservoirs, such as the ocean. During periods when FLEXPART retroplumes show that airmasses resided on the Greenland ice sheet for 3 or more days, BrO exhibits a clear diurnal variation, with peak mixing ratios of up to 3 pmol mol −1 in the morning and at night. The diurnal cycle of BrO can be explained by a changing boundary layer height combined with photochemical formation of reactive bromine driven by solar radiation at the snow surface. The shallow stable boundary layer in the morning and night leads to an accumulation of BrO at the surface, leading to elevated BrO despite the expected smaller release from the snowpack during these times of low solar radiation. During the day when photolytic formation of reacCorrespondence to: J. Stutz (jochen@atmos.ucla.edu) tive bromine is expected to be highest, efficient mixing into a deeper neutral boundary layer leads to lower BrO mixing ratios than during mornings and nights.The extended period of contact with the Greenland snowpack combined with the diurnal profile of BrO, modulated by boundary layer height, suggests that photochemistry in the snow is a significant source of BrO measured at Summit during the 2008 experiment.

[1] Multiaxis differential optical absorption spectroscopy (MAX-DOAS) has many advantages that make it ideally suited for trace gas monitoring. MAX-DOAS instruments can be relatively small, are easy to operate, and allow long-term automated operation. The MAX-DOAS technique also allows derivation of vertical aerosol and trace gas profiles in the troposphere and thus the monitoring of pollution aloft. However, this relatively new technique still requires validation to determine uncertainties, for example, introduced by the radiative transfer modeling needed to derive trace gas concentrations. Here we present MAX-DOAS measurements of NO 2 and HCHO performed in the Gulf of Maine during the ICARTT campaign in summer 2004. O 4 measurements were performed to gain information on the vertical distribution of aerosols, which were used in radiative transfer calculations to convert the measured trace gas slant column densities to concentrations. The successful identification of an aerosol layer between 1 and 2 km altitude illustrates the potential for aerosol remote sensing. The comparison with LP DOAS measurements performed simultaneously shows that MAX-DOAS accurately measures trace gases that are well mixed within the boundary layer. MAX-DOAS measurements also provide valuable information on the vertical distribution of pollutants in and above the boundary layer, as demonstrated by the identification of elevated HCHO levels in a layer between 1 km and 2 km altitude. This phenomenon, which is associated with outflow of continental air from the American northeast, was not detected by in situ ground-based instruments.Citation: Pikelnaya, O., S. C. Hurlock, S. Trick, and J. Stutz (2007), Intercomparison of multiaxis and long-path differential optical absorption spectroscopy measurements in the marine boundary layer,