Formaldehyde methods comparison in the remote lower troposphere during the Mauna Loa photochemistry experiment 2

Five methods for the measurement of CH2O vapor were compared under remote tropospheric conditions. The techniques included an aqueous-scrubber enzyme fluorescence method (URIcoil), TDLAS (UNI), 2,4-dinitrophenylhydrazine (DNPH) impregnated-cartridges (BNL/WSUcart), an aqueous-scrubber DNPH mettled (BNL/WSUcoil), and an unpublished aqueous-scrubber immobilized-enzyme fluorescence method (NCARbound). This was part of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) which was performed in four similar to 30-day intensives. In MLOPEX 2a (fall 1991), the URIcoil technique indicated higher concentrations relative to the BNL/WSUcoil which were higher than UNI. The limited number of NCARbound measurements for MLOPEX 2a were greater than the other measurements and least reliable. During MLOPEX 2b (winter 1992), URIcoil, UNI, BNL/WSUcoil, and NCAR-bound gave comparable measurements of CH2O with the latter again being consistently higher than the first three methods and its performance and reliability was improved over MLOPEX 2a. URIcoil and BNL/WSUcoil were comparable in MLOPEX 2c (spring 1992) and 2d (summer 1992), whereas, BNL/WSUcart was consistently higher than both for these intensives. The major source of difference between techniques was ascribed to variations in the analytical-procedural field blanks. Calibration differences were the most likely cause of URIcoil being higher than BNL/WSUcoil and BNL/WSUcoil being higher than UNI during MLOPEX 2a. Lower free troposphere median concentrations were between 100 and 150 ppt for all seasons and without a seasonal trend. These values were a factor of 2 to 3 lower than model estimates for this study site and were in keeping with earlier work.