<title>Nitric acid and nitrogen dioxide flux measurements: a new application of tunable diode laser absorption spectroscopy</title>

A dual tunable diode laser absorption spectrometer (TDLAS) for continuous field measurement of nitric acid and nitrogen dioxide eddy covariance fluxes is described and preliminary field results are presented. The dual TDLAS simultaneously measures nitric acid (HNO₃) and nitrogen dioxide (NO₂) by direct absorption spectroscopy over a long path enclosed in an astigmatic Herriott multipass cell. The technique provides sufficient precision and time response (200 ppt RMS in 1 second) needed to record ambient variations and deposition rates by the eddy-covariance method. Real-time fitting of the integrated spectra over multiple absorption features makes the system appropriate for continuous field measurements while retaining the highly selective quality of direct absorption measurements and minimizing potential interferences. This method also produces an absolute, spectroscopic determination of concentration within the multipass cell, eliminating the need for calibrated gas mixtures in the field.