Ronald C. Cohen, University of California, Berkeley

The UC Berkeley thermal-dissociation laser-induced fluorescence (TD- LIF) instrument detects NO2 directly and detects total peroxynitrates (SPNs = PAN + PPN +N2O5 + HO2NO2 + . . .), total alkyl- and other thermally stable organic nitrates (SANs), and HNO3 following thermal dissociation of these NOy species to NO2.

NO2 Detection

Briefly, the airborne TD-LIF instrument uses a compact, diode pumped, Q-switched (8 kHz, 30 nsec pulse length), frequency doubled (532nm), Nd3+: YAG laser to pump a tunable dye laser (100mW @ 585nm with a linewidth of 0.06 cm-1) [Thornton et al., 2000]. The custom-built, etalon-tuned dye laser is used to tune the laser to excite a narrow rovibronic feature unique to NO2. The light from the dye laser is focused sequentially into two 40 pass White cells. Red-shifted fluorescent photons at wavelengths longer than 700 nm are collected and imaged onto the photocathode of a cooled GaAs photomultiplier tube. Dichroic filters manufactured using fused silica substrates and without any absorbing colored glass are used to reject Rayleigh, Raman and other background scattered light. Single fluorescent photons are counted using time-gated photon counting. The laser is alternately tuned between a strong NO2 resonance and the weaker continuum absorption to test for interferences, assess the background scattering, and for use in an algorithm that holds the laser frequency locked on the resonance feature. This instrument also incorporates a supersonic expansion in the detection region to increase the population of NO2 in the resonant rotational state [Cleary et al., 2002]. The figure below illustrates the room temperature NO2 reference cell absorption (upper trace) features and the jet-expansion flourescence (lower trace) spectrum from scanning the laser wavelength. For sampling the stepper motor index is moved back and forth between 0 (on-line) and 200 (off-line) steps.

The gas sampled from the external probe is expanded through a 300 µm pinhole into a chamber pumped to 250 mTorr (at sea level). The rotational temperature in the expansion is estimated to be approximately 25 K, and results in a 30 fold signal enhancement. The primary instrument calibration is the response to additions of NIST traceable NO2 standards of 5 ppmv diluted with zero air. The calibration is repeated as often as necessary to capture alignment changes or potential interferences from the atmosphere. We also frequently measure the instrument background signal by over-pressuring the inlet with zero air. The detection sensitivity of this instrument is 0.8 ppt/min at S/N=2. The uncertainty in the instrument zero is less than l ppt.

SPN, SAN and HNO3 detection

Adding a thermal dissociation pre-reactor to the LIF detector enables the detection of SPNs, SANs and HNO3 [Day et al., 2002; Wooldridge et al., 2010]. These species thermally dissociate to yield NO2 and a companion radical: XNO2 + heat -> X + NO2
The sample is rapidly heated in a quartz tube near the sampling point, producing an enhancement in NO2 over the ambient background. After flowing through a short region that allows the sample to cool to near ambient temperature, the sample is transported in PFA Teflon tubing to the LIF detection system where NO2 is observed. At a residence time of 30-90ms and a pressure of 1 atmosphere, approximate temperatures for complete dissociation are: 200°C for SPNs; 400°C for SANs; and finally 650°C for HNO3.

Physically, the instrument occupies 2 bays of a NASA DC-8 high rack (approx. 115 x 65 x 140 cm tall, 300 kg). One bay contains the laser system and LIF detection cells and the other contains the computer, data acquisition, calibration, laser dye and cooling water, and pumping systems.

Website & References: http://www.cchem.berkeley.edu/rccgrp/
Cleary, P.A. et al., (2002) Applied Optics 41(33): 6950-6956. Link
Day, D.A. et al., (2002) Journal of Geophysical Research 107 (D6):10.1029/2001JD00077 9. Link
Thornton, J.A. et al., (2000) Analytical Chemistry 72 (3): 528-539. Link
Wooldridge, P.J. et al., (2010), Atmos. Meas. Tech., 3, 593-607. Link