Folded tubular photometer for atmospheric measurements of NO<sub>2</sub> and NO

Abstract

Abstract. We describe and characterize a modular folded tubular photometer for making direct measurements of the concentrations of nitrogen dioxide (NO2) and specify how this method could be extended to measure other pollutants such as sulfur dioxide (SO2), ozone (O3), and black carbon particulate matter. Direct absorbance measurements using this photometer can be made across the spectral range from the ultraviolet (UV) to the near infrared. The absorbance cell makes use of modular components (tubular detection cells and mirror cubes) that allow construction of path lengths of up to 2 m or more while maintaining low cell volumes. The long path lengths and low cell volumes enable sensitive detection of ambient air pollutants down to low part-per-billion levels for gas species and aerosol extinctions down to 1 Mm−1, corresponding to ∼ 0.1 µg m−3 for black carbon particulates. Pressure equalization throughout the stages of the absorbance measurement is shown to be critical to accurate measurements of analyte concentrations. The present paper describes the application of this photometer to direct measurements of nitrogen dioxide (NO2) and the incorporation of design features that also enable measurement of nitric oxide (NO) in the same instrument. Excellent agreement for ambient measurements along an urban roadside was found for both NO2 and NO measured by the folded tubular photometer compared to existing standard techniques. Compared to commonly used methods for measurements of NOx species, the advantages of this approach include (1) an absolute quantification for NO2 based on the Beer–Lambert law, thereby greatly reducing the frequency at which calibrations are required; (2) the direct measurement of NO2 concentration without prior conversion to NO as is required for the commonly used chemiluminescence method; (3) the use of modular components that allow construction of absorbance detection cells of varying lengths for extending the dynamic range of concentrations that can be measured; (4) a more economical instrument than other currently available direct measurement techniques for NO2; and (5) the potential for simultaneous detection of additional species such as SO2, O3, and black carbon in the same instrument. In contrast to other commercially available direct NO2 measurements, such as cavity-attenuated phase-shift spectroscopy (CAPS), the folded tubular photometer also measures NO simultaneously in the same apparatus by quantitatively converting NO to NO2 with ozone, which is then detected by direct absorbance.

Fast particulate nitrate formation via N<sub>2</sub>O<sub>5</sub> uptake aloft in winter in Beijing

“…The model 405 nm instrument measures NO 2 directly based on the absorbance at 405 nm, and NO is measured by adding excess O 3 (conversion efficiency ∼ 100 %). The limit of detection of both NO and NO 2 is 1 part per billion volume (ppbv), with an accuracy of 2 ppbv or 2 % of the reading, and the time resolution is 10 s (Birks et al, 2018). The model 106-L instrument measures O 3 based on the absorbance at 254 nm, with a precision of 1 ppbv, or 2 % of the reading, and a limit of detection of 3 ppbv.…”

Portable Calibrator for NO Based on Photolysis of N<sub>2</sub>O and a Combined NO<sub>2</sub>/NO/O<sub>3</sub> Source for Field Calibrations of Air Pollution Monitors

“…However, the stand-alone ozone calibrator (Model 306) operates using rather different flow rates (and therefore, residence times) than the ozone photolysis cell in the NO2/NO/O3 calibrator described here. Birks et al (2018a) found that 500 chemical loss of ozone due to OH and HO2 radicals (generated either by water photolysis at 184.9 nm or by ozone photolysis and subsequent reaction of O( 1 D) with H2O) was a negligible effect on the ozone output in the stand-alone ozone calibrator (the Model 306). The only effect of water vapor was the small dilution of the O2 precursor by water vapor in the photolysis cell that results in a small of reduction of the ozone generated (up to ~2 %).…”

“…Therefore, for NO2-only analyzers the GPT calibration method requires either (1) a second 105 instrument that can measure the loss of NO or (2) a NIST-traceable ozone source, such that the loss of ozone can be correlated with the formation of NO2. Note that the standard GPT calibration procedures can still be applied to methods that directly measure NO2 and then indirectly measure NO (the opposite of the chemiluminescence technique) -such as in the long-path folded tubular photometer (FTP) developed in our group that measures direct NO2 absorbance at 405 nm (Birks et al, 2018a). 110…”

Portable calibrator for NO based on the photolysis of N<sub>2</sub>O and a combined NO<sub>2</sub>∕NO∕O<sub>3</sub> source for field calibrations of air pollution monitors

“…The fre-quency of calibration depends upon the species being measured and the instrumental approach. A detailed discussion of ozone detection methods and calibration protocols is given in a previous publication describing a portable ozone calibrator (Birks et al, 2018b) and will not be repeated here. In the past, NO x calibration methods were developed primarily for use with analyzers based on the chemiluminescence (CL) reaction of NO with an excess of ozone, which is the most widely used method for quantifying NO and, following its conversion to NO, NO 2 (Fontijn et al, 1970;Ridley and Howlett, 1974;Kley and McFarland, 1980;Steffenson and Stedman, 1974;Demerjian, 2000).…”

“…Therefore, for NO 2 -only analyzers the GPT calibration method requires either (1) a second instrument that can measure the loss of NO or (2) a NIST-traceable ozone source such that the loss of ozone can be correlated with the formation of NO 2 . Note that the standard GPT calibration procedures can still be applied to methods that directly measure NO 2 and then indirectly measure NO (the opposite of the chemiluminescence technique) -such as in the long-path folded tubular photometer (FTP) developed in our group that measures direct NO 2 absorbance at 405 nm (Birks et al, 2018a).…”