Time series observations of molecular iodine (I 2 ), iodine oxides (IO, OIO), bromine oxide (BrO), and the nitrate radical (NO 3 ) in the mid-latitude coastal marine boundary layer (MBL) are reported. Measurements were made using a new long-path DOAS instrument during a summertime campaign at Mace Head on the west coast of Ireland. I 2 was detected using the B 3 (0 + u )−X 1 + g electronic transition between 535 and 575 nm. The I 2 mixing ratio was found to vary from below the detection limit

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... 5 ppt) up to a nighttime maximum of 93 ppt. Along with I 2 , observations of IO, OIO and NO 3 were also made during the night. Surprisingly, IO and OIO were detected at mixing ratios up to 2.5 and 10.8 ppt, respectively. A model is employed to show that the reaction between I 2 and NO 3 is the likely nighttime source of these radicals. The BrO mixing ratio varied from below the detection limit at night (∼1 ppt) to a maximum of 6 ppt in the first hours after sunrise. A bromine chemistry model is used to simulate the diurnal behaviour of the BrO radical, demonstrating the importance of halogen recycling through sea-salt aerosol. In the same campaign a zenith sky DOAS was employed to determine the column density variation of NO 3 as a function of solar zenith angle (SZA) during sunrise, from which vertical profiles of NO 3 through the troposphere were obtained. On several occasions a positive gradient of NO 3 was observed over the first 2 km, possibly due to dimethyl sulphide (DMS) removing NO 3 at the ocean surface.