Investigation of isotopic fractionation of water vapor in greater Los Angeles basin using HDO retrievals from a mountaintop remote sensing observatory

Remote sensing measurements of water vapor isotopologues, such as HDO, represent an important source of information that can be applied to better understand modern climate dynamics and hydrological processes. We present new retrievals of HDO from the California Laboratory for Atmospheric Remote Sensing (CLARS), which houses a mountaintop Fourier transform spectrometer (FTS) operating in the near-infrared range that has made daily observations of the Los Angeles basin since 2011. This dataset provides both high spatial and temporal resolution with which to examine HDO/H₂O isotopic ratios for a coastal area influenced by a large urban population. Retrievals are performed by applying a modified GFIT algorithm, similar to that used by the Total Carbon Column Observing Network (TCCON), to solar spectra reflected from 33 surface locations distributed throughout the LA basin. Retrievals using a set of nine spectral windows with central wavenumbers ranging from 4054 cm^-1 to 6458 cm^-1 are compared, and optimal windows for minimizing spectral residuals and uncertainties in HDO concentrations are identified. Retrievals of HDO and H₂O are used to compute δD, a quantity defined as the fractional deviation of the HDO/H₂O ratio from the standard reference ratio which is useful in examining water vapor fractionation. Derived δD values are compared to observations from nearby TCCON tower sites. Finally, a preliminary examination of the temporal variability of δD on diurnal, monthly, and annual time scales is conducted for various targets in the LA basin. Observations from CLARS-FTS offer a unique opportunity to investigate isotopic fractionation of water vapor over a region using greater temporal resolution compared to satellite datasets, which often only provide daily δD measurements for a specific area. Furthermore, the set of HDO retrievals and δD observations presented here represent a new source of information for future studies and models which could greatly enhance understanding of hydrological processes in the greater Los Angeles area.