Collision‐induced absorption by N₂ near 2.16 µm: Calculations, model, and consequences for atmospheric remote sensing

Abstract

Classical molecular dynamics simulations (CMDS) are used for calculations of the collision‐induced absorption (CIA) by pure N2 in the (2.1–2.2 µm) region of the first overtone band. They lead to reasonable (±15%) agreement with the only two laboratory measurements available, at 97 K and room temperature. Based on these experiment/theory comparisons, empirical corrections are made to the CMDS‐calculated CIA of pure N2 in the 200–300 K temperature range. In addition, the contribution of N2‐O2 collisions is, in the absence of any laboratory measurement, calculated and a simple semiempirical model (the first of its kind) is built in order to predict the CIA of N2 under Earth atmosphere conditions. This is successfully validated by comparisons with ground‐based atmospheric transmission spectra in the 2.1–2.2 µm region.

Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST

“…For these emission spectra, we include line lists for the following molecules: CO 2 , H 2 O, CH 4 , CO, NH 3 , H 2 S, C 2 H 2 , C 2 H 4 , C 2 H 6 , HCN, O 2 , O 3 , NO, NO 2 , and SO 2 . We include collision-induced opacities (CIA) from N 2 -N 2 , O 2 -O 2 , CO 2 -CO 2 , and N 2 -CH 4 (Gruszka & Borysow 1997;Baranov et al 2004;Lee et al 2016;Bezard et al 1990;Wordsworth et al 2010;Borysow & Tang 1993;Lafferty et al 1996;Hartmann et al 2017), as well as hydrogen and helium CIA opacities (Richard et al 2012).…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…CMDS also enabled the modeling of CIA in the region of the first overtone of N 2 with the analysis (Fig. 17) of the spectral contributions of the single and double (or simultaneous) transitions [417]. The comparisons with measurements made in [417] also show that, while the CMDS slightly overestimate absorption at room temperature, they strongly underestimate (~40 %) it around 90 K. Note that CMDS were very recently made for the fundamental band of N 2 colliding with H 2 O [348].…”

“…17) of the spectral contributions of the single and double (or simultaneous) transitions [417]. The comparisons with measurements made in [417] also show that, while the CMDS slightly overestimate absorption at room temperature, they strongly underestimate (~40 %) it around 90 K. Note that CMDS were very recently made for the fundamental band of N 2 colliding with H 2 O [348]. Agreement within the uncertainties of measurements [418] was obtained and the calculations confirm the considerable enhancement of the peak value and width of the CIA spectrum for N 2 -H 2 O collisions with respect to those for pure N 2 .…”

“…The red and magenta lines are the calculated full CIA before and after correction by a factor of 0.9. After [417].…”

Indirect Influence of Humidity on Atmospheric Spectra Near 4 μm

“…For a synthetic view of the results obtained for different conditions of atmospheric humidity and observations (zenith angle), each measured spectrum was fit by adjusting the amounts of all species of significant absorption, including N 2 , as previously done in the region of the N 2 overtone band (Hartmann et al, 2017). The difference between the N 2 vmr x N2 retrieved this way and the nominal value ( R x N2 = 78.09% of the total dry air content) provides a measurement of the quality of the modeling of the N 2 CIA.…”