SOA from Atmospheric Oxidation of Monoterpenes

Biogenic volatile organic compound (BVOC) emissions are an order of magnitude greater than anthropogenic VOC emissions on a global scale (1,2). Monoterpenes (C10H16) comprise a significant portion of BVOC emissions (2, 3), and it is important to understand the atmospheric fates of monoterpenes and their oxidation products. The most commonly occurring monoterpenes (4) are shown in Figure 1. The emission patterns of the various monoterpenes strongly depend on the type of vegetation and on the environmental conditions, however D-limonene makes up the majority of monoterpene emissions over orange groves, while a-pinene and p-pinene dominate over most other kinds of forests, especially those composed of oaks and conifers (3, 5). The involvement of monoterpenes in the formation of tropospheric aerosol was recognized in 1960 by Went (6), who observed that a blue haze formed over pine needles in the presence of ozone. Since then, a great deal has been learned about the reactions between naturally emitted monoterpenes and atmospheric oxidants. It is now well established that oxidation of isoprene and monoterpenes is a significant source of SOA, contributing some 60 Tg C/year to the global SOA budget (7-9).

Figure 2 is a highly simplified diagram of the processes involved in the atmospheric processing of monoterpenes. Gas-phase monoterpenes readily react with the major atmospheric oxidants such as ozone (03), hydroxyl radical (OH), and nitrate radical (N03). During the day, their concentrations are controlled by OH and 03, and at night they are controlled by N03, with monoterpene lifetimes on the order of a few hours in both cases. Regardless of the initial oxidant, gasphase oxidation of monoterpenes results in a wide variety of polyfiinctional carboxylic acids, ketones, aldehydes, peroxides, and alcohols (10-19). Many of these species have sufficiently low vapor pressure to partition into pre-existing particulate matter. In addition, monoterpenes can partition into aqueous particles or cloud droplets by wet deposition and undergo oxidation via aqueous chemistry (20-23), with droplets subsequently drying out into organic particles.

The chemical composition of monoterpene SOA is generally quite complex, with a number of products still remaining uncharacterized. Pinonaldehyde, pinonic acid, nor-pinonic acid, and pinic acid have been identified as major condensed phase products of the laboratory ozonolysis of a-pinene (24-26). Nopinone was identified as the major particle-phase product of P-pinene ozonolysis (11, 24); norpinic and pinic acids were also produced (11). Limononaldehdye, keto-limonene, keto-limononaldehyde, limononic acid, and keto-limononic acid accounted for -60% of the observed aerosol mass in ozonolysis of D-limonene (27). The latter three products have also been identified as major constituents of SOA resulting from the photooxidation of D-limonene in the presence of NOx (28). In the OH-initiated oxidation of D-limonene, both keto-limonene and limononaldehyde were identified as major low volatility products (17). Oxidation of a-pinene and D-limonene by N03 yielded pinonaldehyde and limononic acid, respectively, and unidentified nitrates (29).

a-pinene p-pinene A3-carene d-limonene camphene myrcene a-terpinene

Figure 1. Structures of the most commonly occurring monoterpenes.

P-phellandrene sabinene p-cymene ocimene a-thujene terpinolene y-terpinene

Figure 1. Structures of the most commonly occurring monoterpenes.

Pinene Oxidation

Figure 2. Simplified diagram of atmospheric processing of monoterpenes and their oxidation products.

Partitioning into aerosol particles

Figure 2. Simplified diagram of atmospheric processing of monoterpenes and their oxidation products.

In addition to the "usual suspects" listed above, several research groups have identified organic peroxides (30-32) and polyfunctional species (12, 14, 15, 33) amongst the products of monoterpene oxidation. There is also increasing evidence that oligomeric species play a key role in the monoterpene SOA formation and subsequent aging (14, 34-43). For example, hundreds of individual compounds in the m/z range of 200-700 were observed by highresolution mass spectrometry amongst products of ozonolysis of a-pinene (40) and D-limonene (38). Such oligomeric species were attributed to various condensation reactions involving aldehydes, hydroperoxides, carbonyl oxides, and other reactive groups (20, 30, 31,41, 44).

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