3.1. Isoprene Oxidation
 Isoprene is the most important precursor for both dicarbonyls, contributing 47% of glyoxal and 79% of methylglyoxal globally. Our detailed isoprene oxidation mechanism in GEOS-Chem is adapted from MCMv3.1 and is illustrated in Figure 1. Oxidation by NO3 produces both dicarbonyls with high yields but accounts for only a small fraction of isoprene loss. Oxidation by OH yields glyoxal and methylglyoxal as second-generation products and also as third-generation products via the intermediates glycolaldehyde (HOCH2CHO) and hydroxyacetone (HOCH2C(O)CH3). In the presence of NO, glyoxal and methylglyoxal are produced in a matter of hours following isoprene oxidation. Under low NOx conditions, the dicarbonyl production can be delayed for days due to formation of ROOH reservoirs. The global mean molar yields of glyoxal from isoprene are 4.6% as a second generation product and 1.6% as a third generation product. The mean molar yields of methylglyoxal from isoprene are 13% as a second generation product and 11% as a third generation product. Global biogenic isoprene emission estimated by MEGAN [Guenther et al., 2006] is 410 Tg a−1, producing 21 Tg a−1 glyoxal and 110 Tg a−1 methylglyoxal.
Figure 1. Glyoxal and methylglyoxal production from the oxidation of isoprene by OH and NO3. The molar yields, shown in percentages, are calculated using the chemical mechanism in GEOS-Chem based on MCMv3.1 [Saunders et al., 2003; Bloss et al., 2005]. The diagram assumes that organic peroxy radicals (RO2) react with NO only; as discussed in the text, reaction with HO2 to form peroxides does not change the ultimate product yields. The branching ratios for carbonyl decomposition are computed assuming [O3] = 40 ppb, [OH] = 4 × 106 molecules cm−3, and July mean surface photolysis rates at 45°N latitude at noon.
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 We elaborate further on the dicarbonyl production from glycolaldehyde and hydroxyacetone, as these compounds are produced from the oxidation of a number of VOCs discussed below and are also directly emitted from biomass burning (Table 1). Both are removed by photolysis and oxidation by OH, the latter process producing glyoxal and methylglyoxal [Orlando et al., 1999; Bacher et al., 2001; Magneron et al., 2005; JPL, 2006]:
A recent laboratory study found that the molar yield of methylglyoxal from hydroxyacetone oxidation in reaction (2) is temperature-dependent and decreases from 82% at 298K to 49% at 236K [Butkovskaya et al., 2006]. Here we assume this yield to be unity following MCMv3.1. Rate constants for OH oxidation are 1.1 × 10−11 cm3 molecule−1 s−1 for glycolaldehyde [Bacher et al., 2001] and 3 × 10−12 cm3 molecule−1 s−1 for hydroxyacetone [Orlando et al., 1999]. Glycolaldehyde is also oxidized by NO3 with a rate constant of 1.44 × 10−12 exp(−1862/T) [MCMv3.1], not producing glyoxal. Photolysis frequencies for glycoladehyde and hydroxyacetone are calculated using absorption cross sections and quantum yield data from JPL . Glycolaldehyde is sufficiently water-soluble to be removed by dry and wet deposition, with an effective Henry's Law constant H* = 4.1 × 104 exp [4.6 × 103 (1/T – 1/298)] M atm−1 accounting for hydrolysis [Betterton and Hoffmann, 1988]. On a global scale, the molar yield of glyoxal from glycolaldehyde is 9.9% and the molar yield of methylglyoxal from hydroxyacetone is 75%.
3.2. Other Sources
 Acetone is the second largest source of methylglyoxal and has a long atmospheric lifetime (22 d against OH oxidation and photolysis in GEOS-Chem), enabling SOA production in the free troposphere. Primary sources of acetone are mainly biogenic, including terrestrial vegetation and dissolved organic matter in the ocean [Jacob et al., 2002]. In addition, acetone is produced in the atmosphere by OH oxidation of monoterpenes, methylbutenol, and isoalkanes; these secondary sources will be discussed below. Acetone oxidation by OH in MCMv3.1 produces methylglyoxal and hydroxyacetone when NOx is low, as a result of the RO2 radicals reacting with HO2 or with other RO2 radicals. Annual methylglyoxal production from primary acetone is 10 Tg a−1, contributing 7% of the global total methylglyoxal source.
 Several monoterpenes including α-pinene, Δ3-carene, geraniol, and citral produce glyoxal and methylglyoxal when oxidized by ozone [Yu et al., 1998; Fick et al., 2003, 2004; Nunes et al., 2005]. Measured molar yields for α-pinene ozonolysis range from 4% to 9% for glyoxal and 1% to 11% for methylglyoxal and appear to be sensitive to temperature and relative humidity [Fick et al., 2003, 2004]. We assume an instantaneous and constant molar yield of 5% for both dicarbonyls from all monoterpenes when oxidized by O3. In addition, monoterpene oxidation by both OH and O3 produces 6.9 Tg a−1 acetone [Jacob et al., 2002], which further oxidizes to produce methylglyoxal. Global emission of monoterpenes is 160 Tg a−1 in the GEIA inventory [Guenther et al., 1995]. The mean overall molar yield of glyoxal from oxidation of monoterpenes is 2.8%, producing 1.8 Tg a−1 glyoxal. The corresponding mean molar yield of methylglyoxal is 4.2%, producing 3.5 Tg a−1 methylglyoxal.
 Methylbutenol (MBO), emitted by North American pine trees, is oxidized by OH with a global mean lifetime of 7.1 h to produce glycolaldehyde and from there glyoxal. The reported molar yields of glycolaldehyde from MBO under high-NOx conditions range from 50% to 78% [Atkinson and Arey, 2003; Carrasco et al., 2007]. We assume a constant molar yield of 63% following MCMv3.1. In addition, MBO oxidation produces 2.5 Tg a−1 acetone [Jacob et al., 2002], which can then produce methylglyoxal as described above. Global MBO emission is 9.6 Tg a−1 in the GEIA inventory, all from North America [Guenther et al., 1995], producing 0.35 Tg a−1 glyoxal and 0.5 Tg a−1 methylglyoxal.
 C3–C5 isoalkanes (propane, isobutane, isopentane) are mostly anthropogenic and are oxidized in the atmosphere by OH to produce methylglyoxal by way of acetone [Jacob et al., 2002]. Propane produces acetone with 75% yield. Global propane emission is 16 Tg a−1, which produces 2.7 Tg a−1 methylglyoxal. Higher isoalkanes contribute an additional 1.0 Tg a−1 methylglyoxal.
 Ethylene and higher alkenes are emitted by vegetation and human activities. Ethylene (C2H4) is mainly oxidized by OH with a mean lifetime of 1.7 d and produces glycolaldehyde, precursor to glyoxal. GEOS-Chem includes detailed ethylene photochemistry based on MCMv3.1 [Fu et al., 2007]. The yield of glycolaldehyde is determined by the branching of HOC2H4O• radical decomposition, which depends on temperature and pressure [IUPAC, 2006]. As a result, the yield of glycolaldehyde from ethylene ranges from approximately 30% at the surface to near 100% in the upper troposphere. The global average molar yield of glyoxal from ethylene is 5.7%, producing 2.5 Tg a−1 glyoxal.
 The only higher (>C2) alkene that produces significant amounts of dicarbonyls according to MCMv3.1 is propene, which is oxidized by OH to form β-hydroxyalkyl peroxy radicals. Under low NOx conditions, these radicals can react with HO2 or other RO2 radicals to form hydroxyacetone. The oxidation of isoalkenes by OH produces acetone but is a negligibly small source [Jacob et al., 2002]. GEOS-Chem represents all higher alkenes as one single lumped species with the reactivity of propene. We scale the hydroxyacetone yield from propene in our chemical mechanism by assuming the molar ratio of propene to higher alkenes to be 1.5, based on the emission ratios from EDGARv2.0 and Goldstein et al. . The resulting global average yield of methylglyoxal from higher alkenes is 7.7%, producing methylglyoxal at a rate of 4.1 Tg a−1.
 Acetylene (C2H2) is the second largest source of glyoxal and the most important anthropogenic precursor [Xiao et al., 2007]. It is emitted by combustion and has a global mean lifetime of 18 d against oxidation by OH. The measured molar yield of glyoxal from acetylene is 70% ±30% [Bohn and Zetzsch, 1998]; we assume a yield of 63% following MCMv3.1. The resulting glyoxal production is 8.9 Tg a−1, which is 20% of the total glyoxal source. Because of its long lifetime, acetylene provides a free tropospheric source of glyoxal and hence of SOA.
 Aromatics are mainly emitted by combustion and solvent use. Chamber studies show that they produce glyoxal and methylglyoxal during the first stage of OH oxidation as a result of ring-cleavage [Volkamer et al., 2001]. We averaged the measured dicarbonyl molar yields reported in literature: 0.25 ± 0.086 glyoxal for benzene, 0.16 ± 0.10 glyoxal and 0.12 ± 0.05 methylglyoxal for toluene, and 0.16 ± 0.12 glyoxal and 0.23 ± 0.11 methylglyoxal for xylene [Tuazon et al., 1986; Atkinson, 1990; Yu et al., 1997; Bethel et al., 2000; Volkamer et al., 2001; Atkinson and Avery, 2003; Zhao et al., 2005; Berndt and Böge, 2006]. Benzene and toluene, with lifetimes exceeding days, enhance the dicarbonyl concentrations in the outflow of anthropogenic emission regions.
 Direct biofuel and biomass burning emissions of glyoxal, methylglyoxal, and their intermediate precursors glycolaldehyde and hydroxyacetone have been reported in several studies [McDonald et al., 2000; Hays et al., 2002; Christian et al., 2003; Greenberg et al., 2006]. We use emission ratios relative to CO from the literature (Table 1). The corresponding dicarbonyl sources from the direct emissions of glycolaldehyde and hydroxyacetone are relatively small, but the direct emissions of glyoxal and methylglyoxal represent a significant contribution to their global budgets (Table 1).
 In addition to gas phase production, hydrates of glyoxal and methylglyoxal may also be produced in the aqueous phase. Model studies by Lim et al.  and Warneck  suggest that aqueous-phase oxidation of glycolaldehyde hydrates and other hydroxy and hydroperoxy aldehydes can be major sources of glyoxal hydrate and methylglyoxal hydrate in cloud water. However, since we assume that the uptake of dicarbonyls in the aqueous phase is irreversible, this aqueous production does not contribute to the gas-phase dicarbonyl budgets and we neglect it here. It could however serve as an additional SOA formation pathway [Lim et al., 2005].
 Table 1 summarizes our global budgets for glyoxal and methylglyoxal. The global source of glyoxal is 45 Tg a−1, including 55% from biogenic precursors, 20% from open biomass burning, 17% from biofuel use, and 8% from other anthropogenic emissions. The global source of methylglyoxal is 140 Tg a−1, including 87% from biogenic precursors, 5% from open biomass burning, 3% from biofuel use, and 5% from other anthropogenic emissions. In terms of their potential as SOA sources in the free troposphere, 23% of glyoxal and 15% of methylglyoxal are produced by precursors with lifetimes longer than two days, and these precursors are principally anthropogenic for glyoxal and biogenic for methylglyoxal. There is significant uncertainty attached to all the terms in Table 1, as is apparent from the discussion above. The most important uncertainty is likely the global emission of isoprene, as current estimates range from 280 to 850 Tg a−1 [Wiedinmyer et al., 2004], resulting in a global source uncertainty of roughly 50% for both dicarbonyls. On the basis of a larger global isoprene emission of 500 Tg a−1, Myriokefalitakis et al.  estimated 56 Tg a−1 production of glyoxal, in proportional agreement with our analysis.