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Revisiting integrating nephelometer measurements.
- Source :
-
Atmospheric Environment . Feb2024, Vol. 319, pN.PAG-N.PAG. 1p. - Publication Year :
- 2024
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Abstract
- Integrating nephelometers are designed to measure the scattering coefficient, b sp , of an aerosol introduced into its sampling chamber. They have been and are used extensively in laboratory studies and short-term intensive aerosol characterization studies to validate and improve optical aerosol models. They are used to measure long-term temporal and spatial trends of the particle scattering coefficient, and to measure trends in the scattering enhancement factor, f(RH). The National Park Service (NPS) has operated Optec NGN nephelometers with an open sampling chamber, referred to as NGN ambient , designed to measure total (coarse plus fine) particle scattering at near ambient relative humidity (RH). However, the NGN is no longer manufactured and will be replaced with Ambilabs 2WIN nephelometers. An intercomparison study was initiated to compare the NGN ambient , an enclosed heated NGN, two 2WIN nephelometers, one held at RH ≤ 40% and another modified to sample at near ambient RH, and a TSI (Shoreview, MN, USA). The objective of the study was to evaluate the suitability of the 2WIN to replace the NGN ambient and more generally to assess the suitability of the 2WIN instrument to be used to track long term trends in bsp and f(RH) values. The two 2WIN's, TSI and the heated NGN have enclosed sampling chambers with PM2.5 inlets. Measurements of particle physical and chemical properties and sampling chamber RH allowed for an assessment of the inter-comparability of relationships between optical models and nephelometer measurements. Heating of the sampling chamber to reduce RH to less than 40% did not significantly volatilize particle nitrate but heating to reduce the RH to near 20% did. It was also determined that measurement of the scattering enhancement function, f(RH) , was underestimated in this study, by about 20% for RH levels of 70% or greater, when the reference RH nephelometer was held at RH ≤ 40% compared to when the RH ≤ 10%. The study highlighted the importance of careful calibration of the instrument and careful documentation of the sampling chamber environment in which the measurement is made. Knowing precisely the RH in the sensor volume is critical to the interpretation of measured b sp levels. Monitoring of long-term temporal and spatial trends should be done either with the sampling chamber at or near ambient RH to track changes in ambient b sp or at RH ≤ 10% to understand changes in particulate mass concentrations contributing to b sp. Understanding changes in f(RH) over time and space should be done with the reference b sp at RH ≤ 10%. The lower RH levels should be achieved by some method other than heating the chamber to avoid volatilization of the aerosol species such as ammonium nitrate. Key Points. • When atmospheric particle size and physical and chemical characteristics are known it is possible to estimate particulate scattering as measured by a nephelometer if operation of the nephelometer does not inadvertently modify the aerosol, for example heating the aerosol such that volatile species are lost. • Even though nephelometers are easily and accurately calibrated the measurement of particulate scattering is very sensitive to temperature and RH in the sampling chamber. Therefore, to track long-term spatial and temporal trends in aerosol scattering it is critical to control the humidity in the sampling chamber, preferably at relative humidity levels near 20% or less and by some method other than heating the aerosol. • All commercially available nephelometers modify the aerosol because of inadvertent heating and modifications must be made to the nephelometers to operate at near-ambient conditions. • Slight heating of the aerosol such that relative humidity did not exceed 40% in this study apparently did not volatilize ammonium nitrate particles. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 13522310
- Volume :
- 319
- Database :
- Academic Search Index
- Journal :
- Atmospheric Environment
- Publication Type :
- Academic Journal
- Accession number :
- 174639447
- Full Text :
- https://doi.org/10.1016/j.atmosenv.2023.120237