Seasonal and geographical variability of nitryl chloride and its precursors in Northern Europe.

Measurements of nitryl chloride (ClNO₂) and its precursors (O₃, NO₂, particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO₂ was observed at all sites and in every season, with the highest concentrations between 00:00 and 04:00 GMT. The median nocturnal concentration of ClNO₂ ranged between the detection limit (4.2 ppt) and 139 ppt. A clear seasonal cycle, with maxima in spring and winter, and significant differences between locations in the same season were observed. The main source of particulate chloride was sea salt aerosol (including at Leicester, ∼200 km from the coast). In general, ClNO₂ levels were controlled by the concentrations of O₃ and NO₂, rather than by the uptake and reaction of N₂O₅ with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO₂ can be explained in terms of O₃‐limited and NO₂‐limited regimes affecting the formation of the N₂O₅ precursor. A global version of the GEOS‐Chem model at medium resolution (2° × 2.5°) was not able to fully capture the observed seasonality of ClNO₂, mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O₃. A higher‐resolution (0.25° × 0.3125°) version of GEOS‐Chem showed better agreement with the observations, although it still overestimated ClNO₂ concentrations during summer. [ABSTRACT FROM AUTHOR]