Spatial and temporal variability of inorganic chlorine in Northwestern Europe

Abstract:

Chlorine is well known to be a strong oxidant in the atmosphere;
chlorine reactivity impacts the formation of tropospheric ozone, the
oxidation of methane and non-methane hydrocarbons, and the cycling of
nitrogen, sulphur and mercury. An accurate assessment of the role
played by chlorine in tropospheric chemical processes is complicated
by the scarce knowledge of its sources, sinks and distribution.

We report observations of inorganic chlorine species (Cl₂, ClNO₂,
particulate chloride) taken over the period 2014-2015 at three
different locations in Britain: an urban site a hundred kilometers
from the ocean (Leicester), a coastal site mostly affected by ship
traffic (Penlee Point, Cornwall) and a coastal site experiencing
either clean air from the North Sea or polluted air from inland
(Weybourne, Norfolk).

This dataset provides a first look into the geographical distribution
and seasonal variability of chlorine in Northwestern Europe: the
results suggest that, during the night, ClNO₂ is ubiquitous with
concentrations in the range of hundreds to thousands of pptV at all
locations, whereas Cl₂ can be observed only at coastal sites, with
concentrations of a few tens of pptV. The implications of the
widespread presence of these forms of inorganic chlorine for ozone
production and, in general, for the oxidative processes in the lower
atmosphere are discussed with the help of a wide range of supporting
measurements.