Tunable diode laser absorption measurements of H₂O₂ and HCHO during the Mauna Loa Observatory Photochemistry Experiment

The tunable diode laser absorption spectrometric technique was employed to measure H 2 O 2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) fall and winter intensives. Comparisons were made between the measurements made with the Tunable Diode Laser Absorption Spectrometer (TDLAS) technique and several other methods. The half‐hourly average mixing ratios varied between 1300 parts per trillion by volume (pptv) and the instrument detection limit of 50 – 100 pptv for H 2 O 2 and between 400 pptv and the instrument detection limit ∼25 pptv for HCHO. Daily average mixing ratios for H 2 O 2 were 550 ± 79 pptv (fall) and 540 ± 80 pptv (winter), and for HCHO were 130 ± 21 pptv (fall) and 200 ± 35 (winter). HCHO showed strong diurnal behaviour with maxima in the 1200 – 1700 time period whereas H 2 O 2 showed essentially no diurnal variation. Clean tropospheric air sampled during downslope conditions between 2000 and 0600 had HCHO mixing ratios averaging 80 pptv in the fall and 160 pptv in the winter. These values are considerably lower than model predictions.