A calibration procedure which accounts for non-linearity in single-monochromator Brewer ozone spectrophotometer measurements
Authors: Vaziri Zanjani Zahra , Moeini Omid , McElroy Tom , Barton David , Savastiouk Vladimir 
Published: January 2019

Abstract. It is now known that single-monochromator Brewer spectrophotometer
ozone and sulfur dioxide measurements suffer from non-linearity at large
ozone slant column amounts due to the presence of instrumental stray light
caused by scattering within the optics of the instrument. Because of the
large gradient in the ozone absorption spectrum in the near-ultraviolet, the
atmospheric spectra measured by the instrument possess a very large gradient
in intensity in the 300 to 325 nm wavelength region. This results in a
significant sensitivity to stray light when there is more than 1000 Dobson
units (DU) of ozone in the light path. As the light path (air mass) through
ozone increases, the stray-light effect on the measurements also increases.
The measurements can be of the order of 10 %, low for an ozone column of
600 DU and an air mass factor of 3 (1800 DU slant column amount), which is an
example of conditions that produce large slant column amounts. Primary calibrations for the Brewer instrument are carried out at Mauna Loa
Observatory in Hawaii and Izana Observatory in Tenerife. They are done
using the Langley plot method to extrapolate a set of measurements made
under a constant ozone vertical column to an extraterrestrial calibration
constant. Since the effects of a small non-linearity at moderate ozone
paths may still be important, a better calibration procedure should account
for the non-linearity of the instrument response. Studies involving the
scanning of a laser source have been used to characterize the stray-light
response of the Brewer (Fioletov et al., 2000), but until recently
these data have not been used to elucidate the relationship between the
stray-light response and the ozone measurement non-linearity. In a study done by Karppinen et al. (2015), a method for correcting stray
light has been presented that uses an additive correction, which is
determined via instrument slit characterization and a radiative transfer
model simulation and is then applied to the single Brewer data
(Karppinen et al., 2015).
The European Brewer Network is also applying stray-light corrections, which
includes an iterative process that results in correcting the single Brewer
data to agree with double Brewer data (Rimmer et
al., 2018; Redondas et al., 2018). The
first model requires measurements of the slit function and the latter method
relies on a calibrated instrument, such as a double Brewer, to characterize
the instrument and to determine a correction for stray light. This paper presents a simple and practical method to correct for the
effects of stray light, which includes a mathematical model of the instrument
response and a non-linear retrieval approach that calculates the best values
for the model parameters. The model can then be used in reverse to provide
more accurate ozone values up to a defined maximum ozone slant path. The
parameterization used was validated using an instrument physical model
simulation. This model can be applied independently to any Brewer instrument
and correct for the effects of stray light.