The Spin-plane double probe instrument (SDP) is part of the FIELDS instrument suite of the Magnetospheric Multiscale mission (MMS). Together with the Axial double probe instrument (ADP) and the Electron Drift Instrument (EDI), SDP will measure the 3-D electric field with an accuracy of 0.5 mV/m over the frequency range from DC to 100 kHz. SDP consists of 4 biased spherical probes extended on 60 m long wire booms 90• apart in the spin plane, giving a 120 m baseline for each of the two spin-plane electric field components. The mechanical and electrical design of SDP is described, together with results from ground tests and calibration of the instrument.Keywords Magnetospheric Multiscale Mission · MMS · Electric field instrument · Spin-plane double probe · SDP

Measurements of cloud condensation nuclei (CCN), aerosol size distribution and chemical composition were obtained at the UNH‐AIRMAP Thompson Farms site, during the ICARTT 2004 campaign. This work focuses on the analysis of a week of measurements, during which semiurban and continental air were sampled. Predictions of CCN concentrations were carried out using “simple” Köhler theory; the predictions are subsequently compared with CCN measurements at 0.2%, 0.3%, 0.37%, 0.5% and 0.6% supersaturation. Using size‐averaged chemical composition, CCN are substantially overpredicted (by 35.8 ± 28.5%). Introducing size‐dependent chemical composition substantially improved closure (average error 17.4 ± 27.0%). CCN closure is worse during periods of changing wind direction, suggesting that the introduction of aerosol mixing state into CCN predictions may sometimes be required. Finally, knowledge of the soluble salt fraction is sufficient for description of CCN activity.

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Abstract. Measurements of gas phase soluble bromide in the boundary layer and in firn air, and Br − in aerosol and snow, were made at Summit, Greenland (72.5 • N, 38.4 • W, 3200 m a.s.l.) as part of a larger investigation into the influence of Br chemistry on HO x cycling. The soluble bromide measurements confirm that photochemical activation of Br − in the snow causes release of active Br to the overlying air despite trace concentrations of Br − in the snow (means 15 and 8 nmol Br − kg −1 of snow in 2007 and 2008, respectively). Mixing ratios of soluble bromide above the snow were also found to be very small (mean < 1 ppt both years, with maxima of 3 and 4 ppt in 2007 and 2008, respectively), but these levels clearly oxidize and deposit long-lived gaseous elemental mercury and may perturb HO x partitioning. Concentrations of Br − in surface snow tended to increase/decrease in parallel with the specific activities of the aerosol-associated radionuclides 7 Be and 210 Pb. Earlier work has shown that ventilation of the boundary layer causes simultaneous increases in 7 Be and 210 Pb at Summit, suggesting there is a pool of Br in the free troposphere above Summit in summer time. Speciation and the source of this free tropospheric Br − are not well constrained, but we suggest it may be linked to extensive regions of active Br chemistry in the Arctic basin which are known to cause ozone and mercury depletion events shortly after polar sunrise. If this hypothesis is correct, it implies persistence of the free troposphere Br − for several months after peak Br activation in March/April. Alternatively, there may be a ubiquitous pool of Br − in the free troposphere, sustained by currently unknown sources and processes.

[1] Nitric oxide (NO) and nitrogen dioxide (NO 2 ) were monitored at the University of New Hampshire Atmospheric Observing Station at Thompson Farm (TF) during the ICARTT campaign of summer 2004. Simultaneous measurement of ozone (O 3 ), temperature, and the photolysis rate of NO 2 (j NO2 ) allow for assessment of the O 3 photostationary state (Leighton ratio, F). Leighton ratios that are significantly greater than unity indicate that peroxy radicals (PO 2 ), halogen monoxides, nitrate radicals, or some unidentified species convert NO to NO 2 in excess of the reaction between NO and O 3 . Deviations from photostationary state occurred regularly at TF (1.0 F 5.9), particularly during times of low NO x (NO x = NO + NO 2 ). Such deviations were not controlled by dynamics, as indicated by regressions between F and several meteorological parameters. Correlation with j NO2 was moderate, indicating that sunlight probably controls nonlinear processes that affect F values. Formation of PO 2 likely is dominated by oxidation of biogenic hydrocarbons, particularly isoprene, the emission of which is driven by photosynthetically active radiation. Halogen atoms are believed to form via photolysis of halogenated methane compounds. Nitrate radicals are believed to be insignificant. Higher F values are associated with lower mixing ratios of isoprene and chloroiodomethane and lower ratios of NO x to total active nitrogen, indicating that photochemical aging may very well lead to increased F values. PO 2 levels calculated using a zero-dimensional model constrained by measurements from TF can account for 71% of the observed deviations on average. The remainder is assumed to be associated with halogen atoms, most likely iodine, with necessary mixing ratios up to 0.6 or 1.2 pptv, for chlorine and iodine, respectively.