Abstract
Abstract. The OH oxidation of α-pinene under both low- and high-NOx environments was studied in the Caltech atmospheric chambers. Ozone was kept low to ensure OH was the oxidant. The initial α-pinene concentration was 20–50 ppb to ensure that the dominant peroxy radical pathway under low-NOx conditions is reaction with HO2, produced from reaction of OH with H2O2, and under high-NOx conditions, reactions with NO. Here we present the gas-phase results observed. Under low-NOx conditions the main first generation oxidation products are a number of α-pinene hydroxy hydroperoxides and pinonaldehyde, accounting for over 40% of the yield. In all, 65–75% of the carbon can be accounted for in the gas phase; this excludes first-generation products that enter the particle phase. We suggest that pinonaldehyde forms from RO2 + HO2 through an alkoxy radical channel that regenerates OH, a mechanism typically associated with acyl peroxy radicals, not alkyl peroxy radicals. The OH oxidation and photolysis of α-pinene hydroxy hydroperoxides leads to further production of pinonaldehyde, resulting in total pinonaldehyde yield from low-NOx OH oxidation of ~33%. The low-NOx OH oxidation of pinonaldehyde produces a number of carboxylic acids and peroxyacids known to be important secondary organic aerosol components. Under high-NOx conditions, pinonaldehyde was also found to be the major first-generation OH oxidation product. The high-NOx OH oxidation of pinonaldehyde did not produce carboxylic acids and peroxyacids. A number of organonitrates and peroxyacyl nitrates are observed and identified from α-pinene and pinonaldehyde.
Highlights
The emissions of biogenic volatile organic compounds (BVOCs) far outnumber those of anthropogenic VOCs (Guenther et al, 1995; Steinbrecher et al, 2009; Monks et al, 2009)
We isolate the peroxy radical reaction pathways to investigate the photochemistry of α-pinene. We have studied these reactions under low-NOx conditions similar to those found in the atmosphere, where RO2 + HO2 is the dominant peroxy radical reaction, and other reactions are suppressed (RO2 + RO2 and reactions with O3)
Under low-NOx conditions, photolysis of hydrogen peroxide (H2O2) was the OH source, while for the highNOx experiments the photolysis of nitrous acid (HONO) or methyl nitrite (CH3ONO) produced OH
Summary
The emissions of biogenic volatile organic compounds (BVOCs) far outnumber those of anthropogenic VOCs (Guenther et al, 1995; Steinbrecher et al, 2009; Monks et al, 2009). Important BVOCs include isoprene, (flux of ∼500 Tg C yr−1) and the monoterpenes (∼127 Tg C yr−1) of which α-pinene accounts for ∼50 Tg C yr−1 (Guenther et al, 1995; Chung and Seinfeld, 2002). Because they are unsaturated, these compounds are highly reactive towards OH, O3, and NO3 and play an important role in tropospheric chemistry.
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