Global modeling of secondary organic aerosol formation from aromatic hydrocarbons: high- vs low-yield pathways

Abstract

Abstract. Formation of SOA from the aromatic species toluene, xylene, and, for the first time, benzene, is added to a global chemical transport model. A simple mechanism is presented that accounts for competition between low and high-yield pathways of SOA formation, wherein secondary gas-phase products react further with either nitrogen oxide (NO) or hydroperoxy radical (HO2) to yield semi- or non-volatile products, respectively. Aromatic species yield more SOA when they react with OH in regions where the [NO]/[HO2] ratios are lower. The SOA yield thus depends upon the distribution of aromatic emissions, with biomass burning emissions being in areas with lower [NO]/[HO2] ratios, and the reactivity of the aromatic with respect to OH, as a lower initial reactivity allows transport away from industrial source regions, where [NO]/[HO2] ratios are higher, to more remote regions, where this ratio is lower and, hence, the ultimate yield of SOA is higher. As a result, benzene is estimated to be the most important aromatic species with regards to formation of SOA, with a total production nearly equal that of toluene and xylene combined. In total, while only 39% percent of the aromatic species react via the low-NOx pathway, 72% of the aromatic SOA is formed via this mechanism. Predicted SOA concentrations from aromatics in the Eastern United States and Eastern Europe are actually largest during the summer, when the [NO]/[HO2] ratio is lower. Global production of SOA from aromatic sources is estimated at 3.5 Tg/yr, resulting in a global burden of 0.08 Tg, twice as large as previous estimates. The contribution of these largely anthropogenic sources to global SOA is still small relative to biogenic sources, which are estimated to comprise 90% of the global SOA burden, about half of which comes from isoprene. Compared to recent observations, it would appear there are additional pathways beyond those accounted for here for production of anthropogenic SOA. However, owing to differences in spatial distributions of sources and seasons of peak production, there are still regions in which aromatic SOA produced via the mechanisms identified here are predicted to contribute substantially to, and even dominate, the local SOA concentrations, such as outflow regions from North America and South East Asia during the wintertime, though total SOA concentrations there are small (~0.1 μg/m³).

Predicted change in global secondary organic aerosol concentrations in response to future climate, emissions, and land use change

“…Global production is dominated by isoprene (19 Tg C a −1 ), with over half of annual isoprene production taking place in the Southern Hemisphere due primarily to the forested regions of the Amazon and southern Africa. Over 75% of the anthropogenic production (global total 1.4 Tg C a −1 ) takes place in the Northern Hemisphere, which is hemispherically aseasonal, but varies locally [ Henze et al , 2008]. Emissions of monoterpenes simulated here are roughly half of previous estimates (section 2.3), resulting in a small contribution (global production 3.7 Tg C a −1 ) to total simulated SOA.…”

“…Emissions of monoterpenes simulated here are roughly half of previous estimates (section 2.3), resulting in a small contribution (global production 3.7 Tg C a −1 ) to total simulated SOA. SOA production in present‐day totals 24.3 Tg C a −1 , with a global annual mean burden of 0.59 Tg C. Henze et al [2008] report SOA production from aromatic, monoterpene, and isoprene of 3.5 Tg a −1 , 8.2 Tg a −1 and 13.2 Tg a −1 . Upon accounting for differences in precursor emissions employed in these simulations, we find that while SOA production and the resulting burden from aromatic sources simulated here agrees well with the estimates of Henze et al [2008], production from biogenic sources appears to be more than a factor of two more efficient in our simulation.…”

“…SOA production in present‐day totals 24.3 Tg C a −1 , with a global annual mean burden of 0.59 Tg C. Henze et al [2008] report SOA production from aromatic, monoterpene, and isoprene of 3.5 Tg a −1 , 8.2 Tg a −1 and 13.2 Tg a −1 . Upon accounting for differences in precursor emissions employed in these simulations, we find that while SOA production and the resulting burden from aromatic sources simulated here agrees well with the estimates of Henze et al [2008], production from biogenic sources appears to be more than a factor of two more efficient in our simulation. We attribute this to enhanced POA emissions, the tropical tropospheric cold bias of CCSM3 [ Collins et al , 2006b] and differences in convection.…”

“…Anthropogenic SOA from the oxidation of aromatics (benzene, toluene and xylene) by OH is included on the basis of recent results of Ng et al [2007a]. These yields are sensitive to NO x concentrations and we follow the treatment of Henze et al [2008] to simulate the formation of SOA from the reaction of aromatic oxidation products with peroxy radicals (HO 2 ) or nitric oxide (NO). In this formulation, aromatic oxidation products in low NOx conditions are essentially nonvolatile.…”

“…Nitrogen oxide concentrations have recently been recognized as an important control on SOA formation efficiency [ Kroll et al , 2005; Presto et al , 2005; Song et al , 2005]. The implementation of aromatic SOA formation followed here [ Henze et al , 2008] explicitly assesses the competition between low and high NOx pathways. We examine how the NO x /VOC ratio, and therefore the SOA production efficiency, is predicted to change in the future in section 4.…”