Contribution of First- versus Second-Generation Products to Secondary Organic Aerosols Formed in the Oxidation of Biogenic Hydrocarbons

Abstract

Biogenic hydrocarbons emitted by vegetation are important contributors to secondary organic aerosol (SOA), but the aerosol formation mechanisms are incompletely understood. In this study, the formation of aerosols and gas-phase products from the ozonolysis and photooxidation of a series of biogenic hydrocarbons (isoprene, 8 monoterpenes, 4 sesquiterpenes, and 3 oxygenated terpenes) are examined. By comparing aerosol growth (measured by Differential Mobility Analyzers, DMAs) and gas-phase concentrations (monitored by a Proton Transfer Reaction Mass Spectrometer, PTR-MS), we study the general mechanisms of SOA formation. Aerosol growth data are presented in terms of a “growth curve”, a plot of aerosol mass formed versus the amount of hydrocarbon reacted. From the shapes of the growth curves, it is found that all the hydrocarbons studied can be classified into two groups based entirely on the number of double bonds of the hydrocarbon, regardless of the reaction systems (ozonolysis or photooxidation) and the types of hydrocarbons studied: compounds with only one double bond and compounds with more than one double bond. For compounds with only one double bond, the first oxidation step is rate-limiting, and aerosols are formed mainly from low volatility first-generation oxidation products; whereas for compounds with more than one double bond, the second oxidation step may also be rate-limiting and second-generation products contribute substantially to SOA growth. This behavior is characterized by a vertical section in the growth curve, in which continued aerosol growth is observed even after all the parent hydrocarbon is consumed.

Secondary organic aerosol from ozone-initiated reactions with terpene-rich household products

“…Ozone and OH react with these oxidation products to form second-generation products and so on. Subsequent oxidation products can be even less volatile than first-generation products leading to increased SOA mass (Leungsakul et al, 2005;Ng et al, 2006;Zhang et al, 2006). We postulate that in the low AER case, the secondary oxidation products formed from the ozone-limonene reaction have additional time to react, forming even lower volatility products, which tend to condense and increase the total mass of SOA formed.…”

“…Several studies have shown that second-generation products of the ozone-limonene reaction make significant contributions to total SOA (Leungsakul et al, 2005;Ng et al, 2006;Zhang et al, 2006). The increased time in the chamber may also allow for more oxidation of the slower-reacting compounds, especially in AFR.…”

Mass yields of secondary organic aerosols from the oxidation of α-pinene and real plant emissions

“…4. Previous studies have shown the presence of a = CH 2 bond, the position of C=C bonds in the molecular ring structure and the numbers of C=C bonds in these types of compounds can affect the volatility of condensable products and the final yields Ng et al, 2006Ng et al, , 2007Bonn and Moortgat, 2002;Griffin et al, 1999). The interactive effects of these complex mixtures of different compounds and chemical mechanisms during the reactions with OH and O 3 are likely reasons for this scatter.…”

“…Aerosol yields can be determined from progressive changes in VOC concentration and in aerosol mass concentration during a single experiment for the compound with one C=C bond (Ng et al, 2006). Figure 2 shows SOA mass yields as a function of absorptive organic mass concentration from α-pinene oxidation (R1024, ref.…”

Known and Unexplored Organic Constituents in the Earth's Atmosphere

“…A key to understanding atmospheric organic aerosols is that the major transformation processes leading to SOA production are typically not the initial oxidation steps of the parent compound but rather the second-and later-generation oxidation steps, as was recently shown for the dominant biogenic compound isoprene and for many terpenes (38). SOA formation may therefore occur relatively far from the primary emission source, even for compounds such as isoprene, whose atmospheric lifetime is ~1 h.…”