Liquid water absorption and scattering effects in DOAS retrievals over oceans

Abstract

Abstract. Spectral effects of liquid water are present in absorption (differential optical absorption spectroscopy -DOAS) measurements above the ocean and, if insufficiently removed, may interfere with trace gas absorptions, leading to wrong results. Currently available literature cross sections of liquid water absorption are provided in coarser resolution than DOAS applications require, and vibrational Raman scattering (VRS) is mostly not considered, or is compensated for using simulated pseudo cross sections from radiative transfer modeling.During the ship-based TransBrom campaign across the western Pacific in October 2009, MAX-DOAS (Multi-AXis differential optical absorption spectroscopy) measurements of light penetrating very clear natural waters were performed, achieving average underwater light paths of up to 50 m. From these measurements, the retrieval of a correction spectrum (H 2 O corr ) is presented, compensating simultaneously for insufficiencies in the liquid water absorption cross section and broad-banded VRS structures. Small-banded structures caused by VRS were found to be very efficiently compensated for by the intensity offset correction included in the DOAS fit. No interference between the H 2 O corr spectrum and phytoplankton absorption was found.In the MAX-DOAS tropospheric NO 2 retrieval, this method was able to compensate entirely for all liquid water effects that decrease the fit quality, and performed better than using a liquid water cross section in combination with a simulated VRS spectrum. The decrease in the residual root mean square (rms) of the DOAS fit depends on the measurement's contamination with liquid water structures, and ranges from ≈ 30 % for measurements slightly towards the water surface to several percent in small angles above the horizon. Furthermore, the H 2 O corr spectrum was found to prevent misfits of NO 2 slant columns, especially for very low NO 2 scenarios, and thus increases the reliability of the fit. In test fits on OMI satellite data, the H 2 O corr spectrum was found selectively above ocean surfaces, where it decreases the rms by up to ≈ 11 %.

On the relative absorption strengths of water vapour in the blue wavelength range

“…The influence of vibrational Raman scattering (VRS) of N 2 and O 2 as described in Lampel et al (2015) on the obtained results listed in Table 5 was not found to be significant. As also described in Lampel et al (2015), no significant contribution to observed ODs by VRS of liquid water (Vountas et al, 2003;Peters et al, 2014) was found most likely due to turbid water and consequently short light paths under water. VRS of water vapour itself is expected to result in an intensity offset and in differential structures especially at 459 and 463 nm, as it red shifts the solar spectrum by ≈ 3654cm −1 (Penney and Lapp, 1976;Murphy, 1978;Rizi et al, 2004).…”

An improved retrieval of tropospheric NO<sub>2</sub> from space over polluted regions using an Earth radiance reference

“…While the reference sector binning method used in this work will ensure that the geometric path length is near-identical in both cases, differences in the light paths caused by factors such as cloud cover and aerosol loading will need to be explicitly accounted for. The impact of vibrational Raman scattering on the DOAS fit (VRS, Peters et al, 2014) will also need to be accounted for, as the reference spectra is primarily measured over the Pacific Ocean. Accounting for these effects may further improve the ERrs-DOAS performance.…”

Investigating differences in DOAS retrieval codes using MAD-CAT campaign data

“…inside the spectrometer) which produces an intensity offset, also called stray light. In addition, other effects such as changes in the dark current can lead to intensity offsets and the vibrational Raman scattering (VRS) is known to produce spectral effects that are very similar to intensity offsets (Peters et al, 2014;Lampel et al, 2015). In the DOAS fit, pseudo cross sections are usually included in order to compensate for intensity offsets.…”