Nitryl Chloride (ClNO2) acts as a source of highly reactive chlorine atoms as well as an important NOx reservoir. Measurements of ClNO2 at an operational UK landfill site are reported here for the first time. A peak concentration of 4 ppb of ClNO2 was found with a peak mean night time maximum of 0.9 ppb. Using models based upon the photolysis of observed ClNO2 and atmospheric chlorine chemistry, chlorine atom concentrations reaching in excess of 1.20 x 105 molecules cm-3 in the early morning following sunrise are calculated. These concentrations are approximately 10 times higher than previously reported in the UK, suggesting a significant impact on the oxidising capacity around such sites. Given the ubiquity of landfill sites regionally and globally, and the large abundances of Cl atoms from the photolysis of ClNO2, chlorine chemistry has a significant impact on ozone formation and VOCs oxidation as shown by WRF-Chem modelling.