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Measurements and modelling of I-2, IO, OIO, BrO and NO3 in the mid-latitude marine boundary layer

by A Saiz-Lopez, J A Shillito, H Coe, J M C Plane
ATMOSPHERIC CHEMISTRY AND PHYSICS ()

Abstract

Time series observations of molecular iodine (I-2), iodine oxides\n(IO, OIO), bromine oxide (BrO), and the nitrate radical (NO3) in\nthe mid-latitude coastal marine boundary layer (MBL) are reported.\nMeasurements were made using a new long-path DOAS instrument during\na summertime campaign at Mace Head on the west coast of Ireland.\nI2 was detected using the B-3 Pi(0(u)(+))- X(1)Sigma(g)(+) electronic\ntransition between 535 and 575 nm. The I-2 mixing ratio was found\nto vary from below the detection limit ( similar to 5 ppt) up to\na night-time maximum of 93 ppt. Along with I2, observations of IO,\nOIO and NO3 were also made during the night. Surprisingly, IO and\nOIO were detected at mixing ratios up to 2.5 and 10.8 ppt, respectively.\nA model is employed to show that the reaction between I-2 and NO3\nis the likely nighttime source of these radicals. The BrO mixing\nratio varied from below the detection limit at night ( similar to\n1 ppt) to a maximum of 6 ppt in the first hours after sunrise. A\nbromine chemistry model is used to simulate the diurnal behaviour\nof the BrO radical, demonstrating the importance of halogen recycling\nthrough sea-salt aerosol. In the same campaign a zenith sky DOAS\nwas employed to determine the column density variation of NO3 as\na function of solar zenith angle (SZA) during sunrise, from which\nvertical profiles of NO3 through the troposphere were obtained. On\nseveral occasions a positive gradient of NO3 was observed over the\nfirst 2 km, possibly due to dimethyl sulphide ( DMS) removing NO3\nat the ocean surface.

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