Stray and sky-scattered light in a Brewer spectrophotometer: Error assessment and correction

An investigation into the effects of internal stray light and sky-scattered light in the Mk II Brewer spectrophotometer was conducted using model simulated Brewer measurements based on laserscans of the instrument function, including the stray light wings. Their impact on both column ozone and SO₂ was determined. Stray and skylight were found to cause small underestimates of the ozone column for high sun (small air masses) and increase with lower sun angles (larger air masses) and larger ozone columns. A simple three-parameter correction factor was developed that, when applied to Brewer measurements made at Toronto, discrepancies with simultaneous double-Brewer measurements were greatly reduced. Errors in SO₂ behaved in a similar manner and often led to negative column values.