Atmospheric Chemistry and Physics, vol. 6, issue 6 (2006) pp. 1513-1528
Time series observations of molecular iodine (I-2), 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 west coast of Ireland.
I2 was detected using the B-3 Pi(0(u)(+))- X(1)Sigma(g)(+) electronic
transition between 535 and 575 nm. The I-2 mixing ratio was found
to vary from below the detection limit ( similar to 5 ppt) up to
a night-time 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 I-2 and NO3
is the likely nighttime source of these radicals. The BrO mixing
ratio varied from below the detection limit at night ( similar to
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.
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