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

This is the authors' abstract. We don't add key points for this paper.

Measurements of nitryl chloride (ClNO 2 ) and its precursors (O 3 , NO 2 , particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO 2 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 ClNO 2 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, ClNO 2 levels were controlled by the concentrations of O 3 and NO 2 , rather than by the uptake and reaction of N 2 O 5 with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO 2 can be explained in terms of O 3 ‐limited and NO 2 ‐limited regimes affecting the formation of the N 2 O 5 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 ClNO 2 , mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O 3 . A higher‐resolution (0.25° × 0.3125°) version of GEOS‐Chem showed better agreement with the observations, although it still overestimated ClNO 2 concentrations during summer.