Abstract. Ozone holds a certain fascination in atmospheric science. It is ubiquitous in the atmosphere, central to tropospheric oxidation chemistry, yet harmful to human and ecosystem health as well as being an important greenhouse gas. It is not emitted into the atmosphere but is a byproduct of the very oxidation chemistry it largely initiates. Much effort is focused on the reduction of surface levels of ozone owing to its health and vegetation impacts, but recent efforts to achieve reductions in exposure at a country scale have proved difficult to achieve owing to increases in background ozone at the zonal hemispheric scale. There is also a growing realisation that the role of ozone as a short-lived climate pollutant could be important in integrated air quality climate change mitigation. This review examines current understanding of the processes regulating tropospheric ozone at global to local scales from both measurements and models. It takes the view that knowledge across the scales is important for dealing with air quality and climate change in a synergistic manner. The review shows that there remain a number of clear challenges for ozone such as explaining surface trends, incorporating new chemical understanding, ozone-climate coupling, and a better assessment of impacts. There is a clear and present need to treat ozone across the range of scales, a transboundary issue, but with an emphasis on the hemispheric scales. New observational opportunities are offered both by satellites and small sensors that bridge the scales.

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[1] Oceanic emissions of gaseous organic iodine-atom precursors have the potential to significantly affect atmospheric chemistry and climate, however there is currently considerable uncertainty associated with quantifying their sources. We present sea-air fluxes calculated from simultaneous air and seawater measurements of a comprehensive range of volatile organic iodine compounds (VOICs), including CH 3 I and the less commonly reported dihalomethanes CH 2 ICl, CH 2 IBr and CH 2 I 2 , made during two cruises in the Atlantic Ocean between 15-58°N. The combined dihalomethane flux provides a global iodine source (∼0.33 ± 0.19 Tg I y −1 ) comparable to that of CH 3 I, and a surface iodine atom source 3-4 times higher. However, a 1D atmospheric model reveals that, in the tropical east Atlantic Ocean in the vicinity of Cape Verde, even these combined VOIC fluxes are capable of supporting only ∼10-25% of the observed IO levels, and suggests that a substantial (340-640 nmol I m −2 d −1 ) additional photochemical source of iodine is required.

Abstract. Ozone holds a certain fascination in atmospheric science. It is ubiquitous in the atmosphere, central to tropospheric oxidation chemistry, yet harmful to human and ecosystem health as well as being an important greenhouse gas. It is not emitted into the atmosphere but is a by-product of the very oxidation chemistry it largely initiates. Much effort is focussed on the reduction of surface levels of ozone owing to its health impacts but recent efforts to achieve reductions in exposure at a country scale have proved difficult to achieve due to increases in background ozone at the zonal hemispheric scale. There is also a growing realisation that the role of ozone as a short-lived climate pollutant could be important in integrated air quality climate-change mitigation. This review examines current understanding of the processes regulating tropospheric ozone at global to local scales from both measurements and models. It takes the view that knowledge across the scales is important for dealing with air quality and climate change in a synergistic manner.

Abstract. Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO 3 in regionally polluted areas with large NO x levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NO x . Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO 3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO 3 . A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO 3 +isoprene reaction, though not directly measured, were estimated to account for 2-9% of total reactive nitrogen. The mass of isoprene oxidized by NO 3 was comparable to and correlated with the organic aerosol Correspondence to: S. S. Brown (steven.s.brown@noaa.gov) loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1-17%, and isoprene SOA mass derived from NO 3 was calculated to exceed that due to OH by approximately 50%.

[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. Cl atoms in the marine atmosphere may significantly impact the lifetimes of methane and other hydrocarbons. However, the existing estimates of Cl atom levels in marine air are based on indirect evidence. Here we present measurements of the Cl precursors HOCl and Cl2 in the marine boundary layer during June of 2009 at the Cape Verde Atmospheric Observatory in the eastern tropical Atlantic. These are the first measurements of tropospheric HOCl. HOCl and Cl2 levels were low in air with open ocean back trajectories, with maximum levels always below 60 and 10 ppt (pmol/mol), respectively. In air with trajectories originating over Europe, HOCl and Cl2 levels were higher, with HOCl maxima exceeding 100 ppt each day and Cl2 reaching up to 35 ppt. The increased Cl cycling associated with long distance pollutant transport over the oceans likely impacts a wide geographic area and represents a mechanism by which human activities have increased the reactivity of the marine atmosphere. Data-constrained model simulations indicate that Cl atoms account for approximately 15 % of methane destruction on days when aged polluted air arrives at the site. A photochemical model does not adequately simulate the observed abundances of HOCl and Cl2, raising the possibility of an unknown HOCl source.

Abstract. A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved if an additional sea-air flux of inorganic iodine (e.g., I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.