Diurnal variation of midlatitudinal NO₃ column abundance over table mountain facility, California

Abstract. The column abundance of NO₃ was measured over Table Mountain Facility, CA (34.4° N, 117.7° W) from May 2003 through September 2004, using lunar occultation near full moon with a grating spectrometer. The NO₃ column retrieval was performed with the differential optical absorption spectroscopy (DOAS) technique using both the 623 and 662 nm NO₃ absorption bands. Other spectral features such as Fraunhofer lines and absorption from water vapor and oxygen were removed using solar spectra obtained at different airmass factors. We observed a seasonal variation, with nocturnally averaged NO₃ columns between 5−7 × 10¹³ molec cm⁻² during October through March, and 5−22 × 10¹³ molec cm⁻² during April through September. A subset of the data, with diurnal variability vastly different from the temporal profile obtained from one-dimensional stratospheric model calculations, clearly has boundary layer contributions; this was confirmed by simultaneous long-path DOAS measurements. However, even the NO₃ columns that did follow the modeled time evolution were often much larger than modeled stratospheric partial columns constrained by realistic temperatures and ozone concentrations. This discrepancy is attributed to substantial tropospheric NO₃ in the free troposphere, which may have the same time dependence as stratospheric NO₃.