Figure 2. TDLAS NO₂ spectrum, 1593.1 cm^-1 to 1593.5 cm^-1 acquired at Harvard Forest 19 October 1999. 30 1-second spectra taken over the course of an hour have been co-averaged and ratioed against an averaged background spectrum. The fit corresponds to 514 ppt NO₂. The H₂O feature on the left side of the spectrum indicates a small difference between ambient water vapor during the background and signal spectra accumulation intervals. The difference is easily taken into account by including H₂O in the fit.