Ground level ozone measurements, sometimes referred to as in situ measurements are those measurements that occur close to the ground, i.e. within the boundary layer which is approximately 1 kilometre in height. Ongoing network measurements from the Canadian Air and Precipitation Monitoring Network (CAPMoN) and studies such as those from Alert, NU and Mount Sutton, QC are also included within the published data sets. In CAPMoN, measurements are made every minute and averaged at five minute intervals. Every 30 days a remote calibration is performed using an on-site secondary reference traceable to a primary standard (a Canadian standard referenced to the National Institute of Standards and Technology (NIST), at Gaithersburg, Maryland, USA) and all measurements are corrected back to the primary standard.