Compensating for the Effects of Stray Light in Single-Monochromator Brewer Spectrophotometer Ozone Retrieval

Figures & data

Fig. 1 Slit functions measured with an HeCd-laser for the single Brewer #037 and a double Brewer #107 at Izaña 2011. For Brewer #107 there are some spikes in the wing caused by internal reflections. On average, for Brewer #037 the relative intensity outside the triangular core is about two orders of magnitude higher than the relative intensity for Brewer #107.

Table 1. Slit function fits for Brewers #037 and #107. Fits have been divided into three parts: a triangular core with FWHM 0.6 nm, an exponentially decaying wing, and a flat outer region.

Brewer	Difference to laser peak, δ(nm)	Slit function
#037	|δ| ≤ 0.6 nm	S1(δ) = 1 − 1.6642|δ|
	|δ| ≤ 15 nm and |δ| > 0.6 nm	S2(δ) = exp(−3.72 × 10^−6|δ|^3 + 8.88 × 10^−4|δ|^2 − 9.34 × 10^−2|δ| − 6.01) + 2.12 × 10^−5
	|δ| > 15 nm	S3 = S2(15 nm)
#107	|δ| ≤ 0.6 nm	S1(δ) = 1−1.6665|δ|
	|δ| ≤ 2.5 nm and |δ| > 0.6 nm	S2(δ) = exp(−0.36|δ| − 7.22) + 2.26 × 10^−7
	|δ| > 2.5 nm	S3 = S2 (2.5 nm)

Fig. 2 Brewer #037 direct response constructed by modelling the Brewer DS measurements.

Fig. 3 Calibration plots for the modelled ideal Brewer and Brewer #037.

Fig. 4 Ratios, a_n, of modelled count rates of ozone retrieval wavelengths between Brewers with an ideal slit function (triangle, FWHM 0.6 nm) and with the Brewer #037 slit.

Fig. 5 Additive correction β calculated from every simulated DS measurement using Eq. (7). To be used as a correction with real data, a representative fit has been made as a function of the single Brewer ozone slant column.

Fig. 6 Brewer #037 ETC and absorption coefficients fitted for original and stray-light corrected data to match Brewer #157 at low air mass values.

Fig. 7 Relative difference between Brewer #037 total ozone columns and Brewer #185/#157 total ozone columns before and after stray-light correction.

Table 2. Statistics of relative differences between the single Brewer #037 and double Brewer #157/#185 ((MS11₀₃₇ − MS11_157/185)/MS11_157/185) before and after the stray-light corrections have been applied. Statistics have been divided into nine areas (columns) according to the ozone slant column of the double Brewers.

Ozone slant column (DU)	300–500	500–700	700–900	900–1100	1100–1300	1300–1500	1500–1700	1700–1900	1900–2300
Original data, mean difference (%)	0.00	0.00	0.00	−0.18	−0.53	−1.00	−1.72	−2.93	−4.74
Original data, st. deviation (%)	0.50	0.32	0.33	0.32	0.35	0.44	0.49	0.73	1.36
Corrected data, mean difference (%)	0.01	−0.02	0.05	0.07	0.03	−0.08	−0.29	−0.74	−1.29
Corrected data, st. deviation (%)	0.50	0.33	0.34	0.31	0.34	0.43	0.51	0.77	1.37