Folded tubular photometer for atmospheric measurements of NO₂ and NO

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 NO_<i>x</i> 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