ARTICLE pubs.acs.org/JPCA
H2O Broadening of a CO2 Line and Its Nearest Neighbors Near 6360 cm1 C. J. Wallace, C. Jeon, C. N. Anderson, and D. K. Havey* Department of Chemistry and Biochemistry, James Madison University, MSC 4501, Harrisonburg, Virginia 22807, United States ABSTRACT: Remote sensing of CO2 requires high-fidelity reference data of spectral line parameters to be successful. The 6360 cm1 region is commonly used by satellites, field campaigns, and point-source gas sensors because it contains well-characterized and relatively isolated transitions of appropriate line strengths for atmospheric applications. However, the presence of gases other than CO2, N2, and O2 can be a source of uncertainty for atmospheric measurements. Near 6360 cm1, there are numerous H2O and HDO transitions. Water makes up approximately 14% of Earth’s lower atmosphere and can interfere with remote sensing measurements by (1) appearing as a direct spectral interference or (2) acting as a foreign broadener for CO2 lines. The primary goal of this work was to quantify H2O broadening of CO2 through precision spectroscopy measurements on the R16e transition at 6359.967 cm1 and its two nearest neighbors. A secondary goal was to assess the accuracy of H2O reference line parameters in the HITRAN 2008 database for spectrally removing typical levels of moisture from air samples containing approximately 400 ppm of CO2.
1. INTRODUCTION In atmospheric chemistry and climate change, greenhouse gases, most notably carbon dioxide, are critical species. A key issue for the next few decades will be to accurately measure the amount of atmospheric CO2 produced at the local, national, and international scale. However, this is a difficult technical problem because useful measurements involve quantifying small changes on top of a relatively large atmospheric background of approximately 400 ppm and have stringent uncertainty requirements. Remote sensing missions typically use optical spectroscopy to quantify column-averaged concentrations of small atmospheric molecules such as CO2. However, the target precision requirements for these missions can be demanding.1 Thus, high-fidelity reference data on spectral line parameters are imperative for accurate retrieval algorithms that convert measured optical absorption into gas concentration. NASA’s Orbiting Carbon Observatory (OCO) and JAXA’s GOSAT are two remote sensing missions that aim to measure CO2 concentrations with
