The sunrise column abundance of NO3 has been measured at Fritz Peak, Colorado (40 ¿N, 105 ¿W), using visible absorption spectroscopy with scattered sky light as the light source. Measurements were made in the ''off-axis'' configuration, viewing the sky near the horizon. This is a convenient alternative to direct Moon measurements, and the tropospheric component of the total NO3 column can be measured with comparable signal-to-noise levels to the direct Moon data. The average tropospheric NO3 vertical column measured during August--October 1993 was about 4¿1013 molecule cm-2. Assuming a layer 1 km thick in the lowest part of the troposphere, this corresponds to 4¿108 molecule cm-3, or about 20 pptv. Under these conditions, the nighttime oxidation of isoprene by NO3 is comparable to the daytime oxidation by OH. ¿ American Geophysical Union 1996