Detection of NO3 and N2O5 by thermal dissociation with LIF detection of NO2 and NO3

The nitrate radical, NO3, is believed to be an important sink of anthropogenic alkenes and many unsaturated biogenic volatile organic compounds. The chemistry of NO3 and its reservoir N2O5 are also effective NOx sinks due to these same oxidation reactions which produce organic nitrates and HNO3 and to heterogeneous chemistry on aerosol surfaces producing HNO3. Measurements of NO3 and N2O5 by 10 different instruments were obtained simultaneously during the 2007 NO3/N2O5 intercomparison at the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber in Juelich, Germany. Here we compare our measurements of NO3 and N2O5 by two independent Thermal Dissociation - Laser Induced Fluorescence (TD-LIF) techniques that employ detection of NO3 and NO2. In one instrument NO3 is detected by electronic excitation at 662nm with an inexpensive, low-power (30mW) diode laser followed by collection of red shifted fluorescence (700-750 nm). N2O5 is detected in this instrument by thermal dissociation to NO3 and NO2 in a second heated inlet at 200°C followed by LIF detection of NO3. N2O5 is calculated by subtracting the two signals. In this instrument detection limits of 74 pptv and 38 pptv have been achieved (signal/noise = 2) for NO3 and the sum of NO3 + N2O5 respectively with 5 minutes of signal averaging. In a second instrument NO2 is excited with a diode laser at 408nm and fluorescence longer than 650nm is collected. N2O5 is detected in this instrument by thermal dissociation to NO2 using a heated inlet held at 180C. NO3 has also been observed to be detected in this instrument using a heated inlet at 600°C, with decreased efficiency relative to the NO2 fragment from N2O5. In this paper, details of the instrument design and calibration, discussion of the inlet transmission efficiency and the mechanism for NO3 detection in the NO2 LIF system are discussed.