Measurements and modelling of I₂, IO, OIO, BrO and NO₃ in the mid-latitude marine boundary layer

Abstract. Time series observations of molecular iodine (I₂), iodine oxides (IO, OIO), bromine oxide (BrO), and the nitrate radical (NO₃) 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 B³Π(0⁺_u)-X¹Σ⁺_g electronic transition between 535 and 575 nm. The I₂ mixing ratio was found to vary from below the detection limit (~5 ppt) up to a nighttime maximum of 93 ppt. Along with I₂, observations of IO, OIO and NO₃ 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 I₂ and NO₃ 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 NO₃ as a function of solar zenith angle (SZA) during sunrise, from which vertical profiles of NO₃ through the troposphere were obtained. On several occasions a positive gradient of NO₃ was observed over the first 2 km, possibly due to dimethyl sulphide (DMS) removing NO₃ at the ocean surface.