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.
Highlights
Poor air quality related to anthropogenic activity has been estimated to contribute up to nearly 7 million premature deaths globally on an annual basis (World Health Organization, 2014)
We found light-emitting diodes (LEDs) with bandwidths of a few tens of nanometers to be preferred over laser diodes
Light intensity in the ozone photolysis cell is continuously measured by a photodiode and output in the data stream, and this light intensity value can be used to infer that adequate ozone is present to quantitatively consume Nitric oxide (NO)
Summary
Poor air quality related to anthropogenic activity has been estimated to contribute up to nearly 7 million premature deaths globally on an annual basis (World Health Organization, 2014). To obtain the low absorbance precisions of 3 × 10−6 stated earlier, it is important to measure the reference light intensity (Io) every 5 to 10 s due to small intensity fluctuations in typical light sources This requires total cell flush times of 2.5 to 5 s (to measure both I and Io), which is incompatible with White cells unless excessively large (and impractical) flow rates are used (> 10 L min−1). Because those measurements are made close together in time, variations in the lamp intensity between measurements is small, resulting in higher precision relative to a White cell or Herriott cell of the same path length Using this approach, measurements of ambient concentrations of NO2, SO2, and black carbon by direct absorbance in the gas phase become feasible and economical. The folded tubular photometer design will be discussed as it pertains to direct absorbance measurements of other atmospheric species such as SO2 and BC
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
