Cavity Ring-Down Spectroscopy of Ambient NO₂ with Quantification and Elimination of Interferences

Abstract

Ambientdetection of NO2 by cavity ring-down spectroscopy is examined in the wavelength region near 405.23 nm, and possible interferences by particulates, water vapor, and carbon dioxide are characterized. Particulates can be efficiently removed by the use of a 0.45 microm fluoropolymer filter. Water vapor has a response of 2.8 ppb (NO2 equivalent) for 1.0% water vapor (80% relative humidity at 10 degrees C) in air at 405.23 nm in a broad continuous absorption feature. Carbon dioxide has a response of 0.8 ppb (NO2 equivalent) for 1.0% CO2 attributable to Rayleigh scattering and would not contribute significant interference in ambient measurements due to the lower ambient CO2 levels. Water vapor interference and in general broad background in the absorption spectrum can be accounted for by removing NO2 selectively in the ambient air stream with an annular denuder coated with sodium hydroxide and methoxyphenol (guiacol). Subtraction of the resulting background signal provides NO2 measurements with a limit of detection of 150 ppt/10 s (SIN = 3). Reliable NO2 measurements could be obtained by this method without the need for frequent calibration with calibration gas. Ambient NO2 measurements are carried out to demonstrate this method.

Measurement of aerosol optical extinction using diode laser cavity ringdown spectroscopy

“…This approach is particularly useful for the ambient measurements, in which the gas-phase background can be monitored and removed in real time, providing enhanced sensitivity and time resolution. To demonstrate this approach, the diode laser was chosen at the near-UV wavelength 408.5 nm where the ambient NO 2 absorbs [21]. For multiple wavelength measurements, diode lasers can also be selected to avoid or minimize the gas phase background.…”

“…A 0.45 m Teflon particle filter was installed in front of the reference channel to remove the particles in the ambient air stream, and the background level (mainly from ambient NO 2 ) [21] was obtained from the reference cell. The 0.…”

“…The 0. 45 Teflon filter has been demonstrated to efficiently remove all the particles in ambient air sample [21]. Meanwhile, the sample channel measured the background level plus the optical extinction of the particles.…”

“…The CRDS technique is absolute, quantitative, and sensitive, and it is ideal for spectroscopic detection of atmospheric trace species. Many atmospheric species, such as NO 2 [21][22][23], NO 3 [24,25], SO 2 [26] and peroxy radicals [27], have been measured by CRDS.…”

Peroxy radical detection for airborne atmospheric measurements using absorption spectroscopy of NO<sub>2</sub>

“…It is a combination of a NO 2 CRDS detector (Hargrove et al, 2006) and a 5 m-long (Teflon) tubing system enabling the peroxy radical chemical conversion. Its eCL is calibrated using rather high amounts (compared to typical atmospheric abundances) of HO 2 between 0.5 and 3 ppbv generated by thermal decomposition of H 2 O 2 (see Sect.…”