A calibration procedure which accounts for non-linearity in single-monochromator Brewer ozone spectrophotometer measurements
2018
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 lightcaused 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 lightwhen 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-lighteffect 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 massfactor 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-lightresponse of the Brewer (Fioletov et al., 2000), but until recently these data have not been used to elucidate the relationship between the
stray-lightresponse 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-lig
ht corrections, which includes an iterative process that results in correcting the single Brewer data to agree with double Brewer data (Rimmer et al., 2018;
Redondaset 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 lig
ht. This paper presents a simple and practical method to correct for the effects of stray lig
ht, which includes a mathematical model of the instrument response and a non-linear retrieval approach that calculates the
best valuesfor 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 lig
ht.
Keywords:
-
Correction
-
Source
-
Cite
-
Save
14
References
2
Citations
NaN
KQI