SEASONAL MEASUREMENTS OF NONMETHANE HYDROCARBONS AND CARBON-MONOXIDE AT THE MAUNA-LOA-OBSERVATORY DURING THE MAUNA-LOA-OBSERVATORY PHOTOCHEMISTRY EXPERIMENT 2

Measurements of carbon monoxide and nonmethane hydrocarbons were made in situ at the Mauna Loa Observatory from September 1991 through Augus t 1992. A distinct seasonal cycle in concentrations was observed for C O and most NMHCs, with higher average concentrations from January thro ugh April and much lower average concentrations from mid-May through O ctober. The sources of CO and NMHCs appeared to vary with season and a ppeared more homogeneous in winter (urban/industrial) and spring (unid entified). The ocean around Hawaii did not pen appear to be a major so urce of CO and most NMHCs, but was the major source of ethane and prop ene. Ethene and propene were always present in free tropospheric air s ampled, indicating daily exchange between the free troposphere and the marine boundary layer. Air arriving at MLO during free tropospheric f low periods had seasonally similar photochemical age (approximately 20 days), with no significant fresh source inputs of NMHCs (except ethen e and propene) within at least 10 days. The influence of atmospheric d ilution processes on atmospheric concentrations was weakest in the spr ing period, when the influence of chemical loss processes was greatest . Aircraft measurements made during the spring agreed in trends and ab solute concentrations with ground-based measurement made at MLO. The t emporal trends in concentrations of NMHCs within altitude ranges were similar. These trends varied with altitude and characterized a layered atmosphere with differing chemical composition. Acetone and methyl et hyl ketone were detected in most samples. N-alkyl aldehydes were also detected but may be artifacts formed from organic aerosols collected i n the sampling process.