ATMOSPHERIC CHEMISTRY OF ISOPRENE AND OF ITS CARBONYL PRODUCTS

The carbonyl products of isoprene, methacrolein (MTA), methyl vinyl ketone (MVK), hydroxyacetaldehyde, and hydroxyacetone have been identified and their concentrations measured in experiments involving sunlight irradiations of 1 ppm organic and 200 ppb NO in purified air. The MTA/MVK yield ratio was 1.4 for isoprene. The hydroxycarbonyl/methylglyoxal yield ratio was 4.3 for MTA and was 1.9 for MVK. The peroxyacyl nitrates PAN [RC(O)OONO2, R = CH3-] and MPAN [R = CH2=C-(CH3)-] have been measured. MPAN/PAN concentration ratios were 0.65 +/- 0.04 for isoprene and 2.3 +/- 0.1 for MTA. A search for hydroxy-PAN (R = CH2OH-) as a reaction product was not conclusive. The nitrogen-containing products NO2, PAN, and MPAN accounted for 78-90% of the reacted NO. The reaction of ozone with isoprene, MTA, and MVK has been studied with excess cyclohexane added as a scavenger for OH which is formed in the ozone-organic reaction. The ozone-organic reaction rate constants were 1.02 +/- 0.05, 4.72 +/- 0.09, and 8.95 +/- 0.25 x 10(-18) cm3 molecule-1 s-1 for MTA (18 +/- 2-degrees-C), MVK (18 +/- 2-degrees-C), and isoprene (20 +/- 2-degrees-C), respectively. Carbonyl products accounted for 70-92% of the reacted organic. The MTA/MVK yield ratio from isoprene was 2.6. The results are discussed in terms of OH-organic and ozone-organic atmospheric oxidation pathways.