Measurements were conducted near Kalamazoo, MI (42.2784°N, 85.6105°W) as part of the 2018 SNow and Atmospheric Chemistry at Kalamazoo (SNACK) campaign.The study was conducted from January 20 to February 24, 2018, at Western Michigan University in Kalamazoo, MI, USA.ClNO2 and N2O5 were measured at 1.5 m above ground level with a chemical ionization mass spectrometer (CIMS, THS Instruments), using iodide I(H2O)- as the reagent ion.

Cl2 calibrations were performed every 2 h during the campaign by adding Cl2, from permeation sources in N2, to the air flow. Characterization of the permeation sources was completed via the absorption method described by Liao et al 2011 (doi:10.1029/2010JD014788).ClNO2 and N2O5 were calibrated offline relative Cl2 (McNamara et al 2019, doi:10.1021/acs.est.9b01797).Background measurements were performed every 15 min by passing the air flow through a glass wool and stainless steel scrubber heated to 120 degrees C (McNamara et al 2021, doi:10.1021/acsearthspacechem.0c00317).CIMS has 30 min time resolution. Mole ratios are given as parts per trillion.Gaps in the data represent periods where the instruments were not operational, or when the CIMS was being used for other experiments. Only ambient data is provided.