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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 5 | Copyright
Atmos. Meas. Tech., 11, 2821-2835, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 May 2018

Research article | 14 May 2018

Folded tubular photometer for atmospheric measurements of NO2 and NO

John W. Birks1, Peter C. Andersen1, Craig J. Williford1, Andrew A. Turnipseed1, Stanley E. Strunk1, Christine A. Ennis1, and Erick Mattson2 John W. Birks et al.
  • 12B Technologies, Inc., 2100 Central Ave., Suite 105, Boulder, CO 80301, USA
  • 2Colorado Dept. of Public Health and Environment, Air Pollution Control Division/Technical Services Program, 4300 Cherry Creek Drive South, Denver, CO 80246, USA

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 2m 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 1Mm−1, corresponding to  ∼ 0.1µgm−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.

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Short summary
A modular long-path folded tubular photometer for making absorbance measurements of air pollutant concentrations is described. The present paper applies this photometer to direct measurements of nitrogen dioxide and, indirectly, to nitric oxide. Excellent agreement for both was observed for measurements along an urban roadside compared with existing standard techniques. Advantages and extension of this technique to other atmospheric pollutants, including particulates, are discussed.
A modular long-path folded tubular photometer for making absorbance measurements of air...