EPA Re-Thinks OTM-37 for Condensable Particulate

    Aug 25, 2019

    I recently wrote an article on the long and painful history of condensable particulate measurement techniques for the Summer 2019 edition of the WPCA News, a publication of the Worldwide Pollution Control Association. The focus was on the “dilution method” currently referred to as OTM-37. I’ll hit a few of the highlights here but I recommend downloading the newsletter for the complete scoop.

    Condensable Particulate Matter (CPM) has had as long a regulatory history as its more density-endowed sibling Filterable Particulate Matter (FPM). In our recent podcast interview with Walt Smith, he reminisced about the good old days when EPA first proposed methods to measure particulate from a stack (Method 5) in 1977. In the proposed Method 5 rule, both FPM and CPM were included. In the final rule, CPM measurement was removed due to the many uncertainties involved with collecting the samples and interpreting the results. Have no fear though because CPM would have its day 15 years later with the promulgation of Method 202.

    EPA’s specific definition of CPM is…

    “Material that is vapor phase at stack conditions, but condenses and/or reacts upon cooling and dilution in the ambient air to form solid or liquid PM immediately after discharge from the stack.  Note that all condensable PM is assumed to be in the PM2.5size fraction.”40 CFR 51

    So gaseous compounds in the stack that turn into solid particles when they cool in the ambient air. Keep that in mind… ambient air.

    Method 202 was problematic from the get-go. Hot, wet stack gas was bubbled through big ice- cold test tubes filled with water… not exactly a good surrogate for ambient air. The Method was plagued with too many options and, most importantly, suffered from extremely high bias issues from false particulate created during the sampling process. Apparently, if you bubble reactive gases in a test tube… you get reactions. Who knew?

    To solve this problem, EPA drastically modified Method 202 to minimize contact between the water and the stack gas. In 2011 the “new” Method 202 was born. But the baby was still colicky. While the method modifications reduced the bias, they did not eliminate it – particularly where both SO2 and ammonia were present in the gas stream. Say… I don’t know… maybe downstream of an SCR.

    So remember the definition of CPM? Condenses or reacts in ambient air? Wouldn’t it be great if there were a way to simulate what actually happens to stack gas as it exits the stack and mixes and cools in the ambient air? Well friends, it turns out EPA has been working for years on just such a method. That’s OTM-37. It introduces cool, clean dilution air to the stack gas and provides mixing just like nature intended.

    Since this method mimics what actually happens to a gas plume in the ambient air, EPA has called this method the “gold standard” of particulate measurement.

    Everything seemed to be chugging along for Pinocchio to turn into a real boy… I mean for OTM-37 to be a real test method. Until one fateful day…

    Here’s where you download the WPCA News. Enjoy!