The boundary layer concentration of ozone was measured with a Dasibi ozone instrument that monitors surface ozone with an ultraviolet absorption measurement. Ambient air is pumped through a cell and the intensity of ultraviolet light shining through this cell from a mercury lamp is measured. Periodically the airflow is diverted to a chemical filter that removes ozone from the air and the lamp's intensity measured as a control = regular calibration. Corrections are required to allow for the ambient pressure and the instrument temperature, so these parameters are logged by a computer along with the intensity. The data are processed directly by instrument and recorded as ozone concentration measurements.