TROPOSPHERIC FORMALDEHYDE CONCENTRATION AT THE MAUNA-LOA-OBSERVATORY DURING THE MAUNA-LOA-OBSERVATORY PHOTOCHEMISTRY EXPERIMENT 2

The concentration of formaldehyde at Mauna Loa Observatory, Hawaii, wa s determined during four Mauna Loa Observatory Photochemistry Experime nt 2 (MLOPEX 2) measurement intensives between September 1991 and Augu st 1992. The observed diurnal variations, 200-900 parts per trillion b y volume (pptv) during daytime and 60-200 pptv during nighttime, resul ted mainly from the local air circulation pattern whereby island modif ied marine boundary layer air prevailed during the day and free tropos pheric air dominated during the night. A seasonal variation was also o bserved; the median/mean values of all data points are: 149/196, 129/1 49, 143/178, and 181/211 pptv for autumn, winter, spring, and summer i ntensives, respectively. During nighttime downslope flow periods which brought in free tropospheric air to the measurement site, the formald ehyde concentrations (median/mean) were 122/123, 110/112, 120/125, and 140/137 pptv for autumn, winter, spring, and summer, respectively. Th is seasonal dependence may be attributable to changes in solar insolat ion and NO concentrations. A simple box model calculation constrained by the experimentally determined concentrations of CH3OOH yielded a fo rmaldehyde concentration (without/with heterogeneous removal) for free tropospheric air, at 7 degrees C, of 155/140, 125/115, 210/195, and 2 20/205 pptv for autumn, winter, spring and summer, respectively. The c alculated values are in good agreement with the measured concentration s for winter (within 27/15%, without/with heterogeneous removal) and f all (within 14/5%), but are significantly higher for spring (75/63%) a nd summer (57/46%).