Water Vapor Near‐UV Absorption: Laboratory Spectrum, Field Evidence, and Atmospheric Impacts. Issue 24 (18th December 2019)

• Record Type: Journal Article
• Authors: Pei, Linsen; Min, Qilong; Du, Yuyi; Wang, Zhechen; Yin, Bangsheng; Yang, Kai; Disterhoft, Patrick; Pongetti, Thomas; Zhu, Lei
• Is Part Of: Journal of geophysical research. Volume 124:Issue 24(2019)
• Page Start: 14310
• Page End: 14324
• Publication Date: 2019-12-18
• Subjects: water vapor -- near‐UV absorption -- absorption cross‐section -- radiation modeling -- ozone retrievals -- field UV residual spectra; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD030724
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Absorption of solar radiation by water vapor in the near‐UV region is a poorly understood but important issue in atmospheric science. To better understand water vapor near‐UV absorption, we constructed a cavity ring‐down spectrometer with bandwidth of 5 cm −1 (~0.05 nm) and obtained water vapor absorption cross sections at 1‐nm increments in the 290‐ to 350‐nm region. Water vapor displays structured absorption over this range with maximum and minimum cross sections of 8.4 × 10 −25 and 1.6 × 10 −25 cm 2 /molecule. Major water vapor absorption bands were observed at 293–295, 307–313, 319, 321–322, and 325 nm, with cross‐section values higher than 4.0 × 10 −25 cm 2 /molecule. To obtain further insight into major water vapor absorption bands, we measured water vapor absorption cross sections at 0.05‐nm intervals in the 292‐ to 296‐nm, 306‐ to 314‐nm, and 317‐ to 326‐nm region. Field UV residual spectra not only exhibited increased attenuation at higher atmospheric water vapor loadings but also showed structures suggested by the laboratory water vapor absorption spectrum. Spaceborne UV radiance spectra have spectral structures resembling the differential cross‐section spectrum constructed from the laboratory wavelength‐dependent water vapor absorption cross sections presented here. Incorporating water vapor absorption cross‐section data into a radiative transfer model yielded an estimated energy budget of 0.26 W/m 2 for the standard U.S. atmosphere and 0.76 W/m 2 for the tropics. This shows that water vapor near‐UV absorption is an important contributor for climate simulation and ozone retrievals. Plain Language Summary: Water vapor near‐UV absorption is an overlooked subject of core importance to atmospheric science. We constructed a cavity ring‐down spectrometer with a bandwidth comparable to those of field UV spectrometers and determined water vapor absorption cross sections at 1‐nm intervals in the 290‐ to 350‐nm region. We also measured water vapor absorption cross sections at 0.05‐nm intervals surrounding major absorption bands. We provide field evidence to support laboratory water vapor near‐UV absorption measurements and present comparisons of the estimated optical depth spectra of ozone with those of water vapor for the standard U.S. and tropical atmospheres. Our findings suggest that water vapor near‐UV absorption will significantly affect ozone retrieval from UV measurements, particularly in the tropical region. Incorporating cross‐section data into a radiative transfer model, we estimated that the energy budget of water vapor near‐UV absorption was about 0.26 W/m 2 for the standard U.S. atmosphere and 0.76 W/m 2 for the tropics. Since it was not thought that water vapor could have near‐UV absorption, the effect of such absorption is not currently included in radiation and climate simulation models. Our work on water vapor near‐UV absorption is expected to change the paradigm in atmospheric measurements from UV remote sensing observations and how atmospheric radiation and climate are modeled. Key Points: Water vapor displays structured absorption bands over the 290‐ to 350‐nm range, with cross sections in the range of 10 −24 to 10 −25 cm 2 /molecule Field evidence for water vapor near‐UV absorption is presented Water vapor near‐UV absorption affects O3 retrieval and modeling of atmospheric radiation and climate