Evaluation of Aeris MIRA, Picarro CRDS G2307, and DNPH-based sampling for long-term formaldehyde monitoring efforts

Current formaldehyde measurement networks rely on the TO-11A offline chemical derivatization technique, which can be resource intensive and limited in temporal resolution. In this work, we evaluate the field performance of three new commercial instruments for continuous in-situ formaldehyde monitoring: the Picarro cavity ringdown spectroscopy (CRDS) G2307 gas concentration analyzer and Aeris Technologies’ mid-infrared absorption (MIRA) Pico and Ultra gas analyzers. All instruments require regular drift correction, with baseline drifts over a 1-week period of ambient sampling of 1 ppb, 4 ppb, and 20 ppb for the G2307, Ultra, and Pico, respectively. Baseline drifts are easily corrected with frequent instrument zeroing using DNPH scrubbers, while Drierite, molecular sieves, and heated hopcalite fail to remove all incoming HCHO. Drift-corrected 3σ limits of detection (LOD) determined from regular instrument zeroing were relatively comparable at 0.055 ppb (Picarro G2307), 0.065 ppb (Aeris Ultra), and 0.08 ppb (Aeris Pico) for a 20 min integration time. We find that after correcting for a 30-40% bias in the Pico measurements, all instruments agree within 5% and are well correlated with each other (all R 2≥0.70). Picarro G2307 HCHO observations are more than 50% higher than co-located TO-11A HCHO measurements (R2 = 0.92, slope = 1.47, int = 1 ppb HCHO), which is in contrast to previous comparisons where measurements were biased low by 1–2 ppb. We attribute this discrepancy to the previous versions of the spectral fitting algorithm as well as the zeroing method. The temperature stabilization upgrade of the Ultra offers improved baseline stability over the previously described Pico version, reducing the maximum drift rate by a factor of 13 and improves precision of a 10 min average by 13 ppt. Using a 6-month deployment period, we demonstrate that all instruments provide a reliable measurement of ambient HCHO concentrations in an urban environment and, when compared with previous observations, find that midday summertime HCHO concentrations have reduced by approximately 50% in the last two decades.