Recommendations for Nationwide Approval of Nafion™ Dryers Upstream of UV-Absorption Ozone Analyzers
Citation:
Halliday, H., C. Johnson, R. Long, R. Vanderpool, AND A. Whitehill. Recommendations for Nationwide Approval of Nafion™ Dryers Upstream of UV-Absorption Ozone Analyzers. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-20/390, 2020.
Impact/Purpose:
UV Photometric-based ambient O3 analyzers can experience significant water vapor interferences resulting in increased measurement noise, biased concentration measurements, and slow instrument response times. The magnitude and duration of these biases is particularly noticeable during sampling events where the O3 analyzer experiences rapid changes in the sampled air’s moisture content. EPA implemented the use of a Perma Pure Nafion™ tube dryer upstream of their Thermo Environmental Model 49i O3 analyzers at 30 CASTNET NAAQS monitoring sites as part of a limited evaluation and has found this approach minimizes spectroscopic water vapor interferences to an acceptable degree. Based on the results of this investigation, the EPA ORD has determined that the use of Nafion™ dryers upstream of UV Photometric-based O3 analyzers on a nationwide basis is a valid means to improve the data quality of O3 compliance measurements.
Description:
Ultraviolet (UV) Photometry-based ambient Federal Equivalent Method (FEM) ozone (O3) analyzers can experience significant water vapor interferences resulting in increased measurement noise, biased concentration measurements, and slow instrument response times. The magnitude and duration of these biases is particularly noticeable during sampling events where the O3 analyzer experiences rapid changes in the sampled air’s moisture content. The U.S. Environmental Protection Agency (EPA) implemented the use of a Perma Pure (Lakewood, NJ) Nafion™ tube dryerupstream of their ThermoEnvironmental (Franklin, MA) Model 49i O3 analyzers at 30 Clean Air Status of Trends Network (CASTNET) National Ambient Air Quality Standard (NAAQS) monitoring sites as part of a limited evaluation and has found that this approach minimizes spectroscopic water vapor interferences to an acceptable degree. This document summarizes the pertinent qualitative and quantitative information to foster further discussion so that a consensus decision can be made whether to approve the routine use Nafion™ dryers on a nationwide basis. The operating principle of UV Photometer-based O3 analyzers is presented along with a discussion of known measurement interferences, with a particular emphasis on atmospheric water vapor and approaches used to minimize the magnitude of this water vapor interference. Multiple studies are discussed which demonstrates that no inadvertent loss of O3 occurs in the Nafion™ tube dryer. The design and operational specifications of CASTNET’s Nafion™ dryer system are presented along with test results of the system’s use since the Office of Air and Radiation’s (OAR) approval to CASTNET evaluation. Based on the results of this investigation, the EPA Office of Research and Development (ORD) has determined that the use of Nafion™ dryers upstream of UV Photometric-based O3 analyzers on a nationwide basis is a valid means to improve the data quality of O3 compliance measurements.