[1] During summer 2004, a comprehensive suite of reactive trace gases (including halogen radicals and precursors, ozone, reactive N, soluble acids, and hydrocarbons), the chemical and physical characteristics of size-resolved aerosols, actinic flux, and related physical conditions were measured at Appledore Island, Maine, as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). Sea-salt mass averaged 4 to 8 times lower than that over the open North Atlantic Ocean. Production in association with sea salt was the primary source for inorganic Cl and Br. Acid displacement of sea-salt Cl À primarily by HNO 3 sustained high HCl mixing ratios (often >2000 pptv) during daytime. Median pHs for the larger sea-salt size fractions (geometric mean diameters, GMDs ! 2.9 mm) ranged from 3.1 to 3.4; median pHs for sub-mm size fractions were 1.6. Cl* (including HOCl and Cl 2 ) ranged from <20 to 421 pptv Cl but was less than the detection limit (DL) during most sampling intervals. Periods during which Cl* was consistently detectable corresponded to relatively clean conditions, multiday transport over water, and relatively low actinic flux. At high HCl mixing ratios (>1000 pptv), HCl + OH sustained steady state Cl-atom concentrations in the range of 10 4 cm À3 . When detectable, photolysis of Cl* was generally the dominant source of atomic Cl; steady state concentrations of Cl atoms were frequently in the range of 10 4 to 10 5 cm À3 . At these concentrations, Cl played an important role in the chemical evolution of polluted coastal air. Br radical chemistry was relatively unimportant.

Abstract.Cirrus cloud formation is believed to be dominated by homogeneous freezing of supercooled liquid aerosols in many instances. Heterogeneous ice nuclei such as mineral dust, metallic, and soot particles, and some crystalline solids within partially soluble aerosols are suspected to modulate cirrus properties. Among those, the role of ubiquitous soot particles is perhaps the least understood. Because aviation is a major source of upper tropospheric soot particles, we put emphasis on ice formation in dispersing aircraft plumes. The effect of aircraft soot on cirrus formation in the absence of contrails is highly complex and depends on a wide array of emission and environmental parameters. We use a microphysical-chemical model predicting the formation of internally mixed, soot-containing particles up to two days after emission, and suggest two principal scenarios: high concentrations of original soot emissions could slightly increase the number of ice crystals; low concentrations of particles originating from coagulation of emitted soot with background aerosols could lead to a significant reduction in ice crystal number. Both scenarios assume soot particles to be moderate ice nuclei relative to cirrus formation by homogeneous freezing in the presence of few efficient dust ice nuclei. A critical discussion of laboratory experiments reveals that the ice nucleation efficiency of soot particles depends strongly on their source, and, by inference, on atmospheric aging processes. Mass and chemistry of soluble surface coatings appear to be crucial factors. Immersed soot particles tend to be poor ice nuclei, some bare ones nucleate ice at low supersaturations. However, a fundamental understanding of these studies is lacking, rendering extrapolations to atmospheric conditions speculative. In particular, we cannot yet decide which indirect aircraft effect scenario is more plausible, and options suggested to mitigate the problem remain uncertain.

Abstract. We present results of three field campaigns using active longpath DOAS (Differential Optical Absorption Spectroscopy) for the study of reactive halogen species (RHS) BrO, IO, OIO and I 2 . Two recent field campaigns took place in Spring 2002 in Dagebüll at the German North Sea Coast and in Spring 2003 in Lilia at the French Atlantic Coast of Brittany. In addition, data from a campaign in Mace Head, Ireland in 1998 was partly re-evaluated. During the recent field campaigns volatile halogenated organic compounds (VHOCs) were determined by a capillary gas chromatograph coupled with an electron capture detector and an inductively coupled plasma mass spectrometer (GC/ECD-ICPMS) in air and water. Due to the inhomogeneous distribution of macroalgae at the German North Sea Coast we found a clear connection between elevated levels of VHOCs and the appearance of macroalgae. Extraordinarily high concentrations of several VHOCs, especially CH 3 I and CH 3 Br of up to 1830 pptv and 875 pptv, respectively, were observed at the coast of Brittany, demonstrating the outstanding level of bioactivity there. We found CH 2 I 2 at levels of up to 20 pptv, and a clear anti-correlation with the appearance of IO. The IO mixing ratio reached up to 7.7±0.5 ppt (pmol/mol) during the day, in reasonable agreement with model studies designed to represent the meteorological and chemical conditions in Brittany. For the two recent campaigns the DOAS spectra were evaluated for BrO, OIO and I 2 , but none of these species could be clearly identified (average detection limits around 2 ppt, 3 ppt, 20 ppt, resp., significantly higher in individual cases). Only in the Mace Head spectra evidence was found for the presence of OIO. Since macroalgae under oxidative stress are suggested to be a further source for I 2 in the marine boundary layer, we re-analyzed spectra in the 500-600 nm range taken during the 1998 PARFORCE campaign in Mace Head, Ireland, Correspondence to: C. Peters (christina.peters@iup.uni-heidelberg.de) which had not previously been analyzed for I 2 . We identified molecular iodine above the detection limit (∼20 ppt), with peak mixing ratios of 61±12 ppt. Since I 2 was undetectable during the Brittany campaign, we suggest that iodine may not be released into the atmosphere by macroalgae in general, but only by a special type of the laminaria species under oxidative stress. Only during periods of extraordinarily low water (spring-tide), the plant is exposed to ambient air and may release gaseous iodine in some way to the atmosphere. The results of our re-analysis of spectra from the PARFORCE campaign in 1998 support this theory. Hence, we feel that we can provide an explanation for the different I 2 levels in Brittany and Mace Head.

Abstract. The speciation of iodine in atmospheric aerosol is currently poorly understood. Models predict negligible iodide concentrations but accumulation of iodate in aerosol, both of which is not confirmed by recent measurements. We present an updated aqueous phase iodine chemistry scheme for use in atmospheric chemistry models and discuss sensitivity studies with the marine boundary layer model MIS-TRA. These studies show that iodate can be reduced in acidic aerosol by inorganic reactions, i.e., iodate does not necessarily accumulate in particles. Furthermore, the transformation of particulate iodide to volatile iodine species likely has been overestimated in previous model studies due to negligence of collision-induced upper limits for the reaction rates. However, inorganic reaction cycles still do not seem to be sufficient to reproduce the observed range of iodide -iodate speciation in atmospheric aerosol. Therefore, we also investigate the effects of the recently suggested reaction of HOI with dissolved organic matter to produce iodide. If this reaction is fast enough to compete with the inorganic mechanism, it would not only directly lead to enhanced iodide concentrations but, indirectly via speed-up of the inorganic iodate reduction cycles, also to a decrease in iodate concentrations. Hence, according to our model studies, organic iodine chemistry, combined with inorganic reaction cycles, is able to reproduce observations. The presented chemistry cycles are highly dependent on pH and thus offer an explanation for the large observed variability of the iodide -iodate speciation in atmospheric aerosol.