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.