EPA Method T0-11A EPA outlines practices for measuring formaldehyde using the 4,2-DNPH denuder method, subsequent derivatization, and analysis by HPLC. While the DNPH method has been a stalwart of air quality monitoring for decades, it provides only time-averaged exposure estimates (typically 8 or 24-hour), requires offline analysis at third-party labs, and continues to experience suspected bias effects associated with ozone and water vapor. Because values are reported significantly post-hoc, formaldehyde also cannot be integrated into air quality monitoring forecasts. Furthermore, time-averaged values limit the ability of regulators to correlate source regions with real time wind data.
Over the last decade, commercially available formaldehyde instruments have allowed for sub-ppb measurements of formaldehyde, but questions of zero drift and scaling relative to TO-11A have impeded the ability to provide EPA method equivalence. Picarro Inc released its G2307 formaldehyde instrument in 2017, delivering sub-ppb sensitivity. Initial testing within the regulatory community has indicated that the instrument is highly repeatable at span, but that various analytical techniques disagree significantly on scaling, making it challenging to assess the absolute accuracy of the Picarro instrument’s factory calibration. Users have also noted that the typical zero drift on order 1 ppb introduces uncertainty to intercomparisons.

We present here advances in providing a zero-reference system for assessing and ultimately correcting out in real time the zero drift of this analyzer. We also present advances in establishing a “golden instrument” approach for documenting repeatable accuracy between production units, and that allows users to return their instrument to the factory for annual validation or recalibration. Finally, we present real-world intercomparisons performed by members of the air quality community over the past two years showing promisingly high degrees of correlation between the EPA Method and the Picarro instrument, and suggesting some possible shortcomings in 4,2-DNPH cartridges at low formaldehyde concentrations.