SOA Formation from Glyoxal in the Aerosol Aqueous Phase: A case study from Mexico City using an explicit laboratory-based model
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This work investigated the formation and evolution of organic aerosols (OA) arising from anthropogenic and biogenic sources in a framework that combined state-of-the-science process and regional modeling, and their evaluation against advanced and emerging field measurements. Although OA are the dominant constituents of submicron particles, our understanding of their atmospheric lifecycle is limited, and current models fail to describe the observed amounts and properties of chemically formed secondary organic aerosols (SOA), leaving large uncertainties on the effects of SOA on climate. Our work has provided novel modeling constraints on sources, formation, aging and removal of SOA by investigating in particular (i) the contribution of trash burning emissions to OA levels in a megacity, (ii) the contribution of glyoxal to SOA formation in aqueous particles in California during CARES/CalNex and over the continental U.S., (iii) SOA formation and regional growth over a pine forest in Colorado and its sensitivity to anthropogenic NOx levels during BEACHON, and the sensitivity of SOA to (iv) the sunlight exposure during its atmospheric lifetime, and to (v) changes in solubility and removal of organic vapors in the urban plume (MILAGRO, Mexico City), and over the continental U.S.. We have also developed a parameterization of water solubility for condensable organic gases produced from major anthropogenic and biogenic precursors based on explicit chemical modeling, and made it available to the wider community. This work used for the first time constraints from the explicit model GECKO-A to improve SOA representation in 3D regional models such as WRF-Chem.
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Abstract. A new mechanism to simulate the formation of secondary organic aerosols (SOA) from reactive primary hydrocarbons is presented, together with comparisons with experimental smog chamber results and ambient measurements found in the literature. The SOA formation mechanism is based on an approach using calculated vapor pressures and a selection of species that can partition to the aerosol phase from a gas phase photochemical mechanism. The mechanism has been validated against smog chamber measurements using α-pinene, xylene and toluene as SOA precursors, and has an average error of 17%. Qualitative comparisons with smog chamber measurements using isoprene were also performed. A comparison against SOA production in the TORCH 2003 experiment (atmospheric measurements) had an average error of only 12%. This contrasts with previous efforts, in which it was necessary to increase partition coefficients by a factor of 500 in order to match the observed values. Calculations for rural and urban-influenced regions in the eastern U.S. suggest that most of the SOA is biogenic in origin, mainly originated from isoprene. A 0-dimensional calculation based on the New England Air Quality Study also showed good agreement with measured SOA, with about 40% of the total SOA from anthropogenic precursors. This mechanism can be implemented in a general circulation model (GCM) to estimate global SOA formation under ambient NOx and HOx levels.
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Secondary organic aerosol 1. Atmospheric chemical mechanism for production of molecular constituents
This series of three papers addresses the representation of secondary organic aerosol (SOA) in atmospheric models. SOA forms when gas‐phase organic species undergo oxidation, leading to products of sufficiently low vapor pressure that can partition between the gas and aerosol phases. The present paper, part 1, is devoted to the development of a gas‐phase atmospheric chemical mechanism designed to represent ozone chemistry as well as formation of individual organic oxidation products that are capable of forming SOA. The ozone chemistry in the mechanism draws upon the recent work of Stockwell et al. [1997] and Jenkin et al. [1997] and SAPRC‐97 and SAPRC‐99 (available from W.P.L. Carter at http://helium.ucr.edu/~carter/ ). The mechanism is evaluated in the three‐dimensional California Institute of Technology (CIT) model [ Meng et al. , 1998 ] by simulating gas‐phase concentrations in the South Coast Air Basin (SoCAB) of California over the period 27–29 August 1987. Total predicted concentrations of gas‐phase SOA compounds are compared with levels of SOA that have been inferred on the basis of ambient organic aerosol measurements during this period. These predicted concentrations indicate that the total gas‐phase potential of SOA‐forming compounds can account for observed aerosol concentrations. Part 2 develops a thermodynamic gas–aerosol partitioning module, and part 3 presents a full three‐dimensional simulation of gas and aerosol levels in the SoCAB during a 1993 episode.
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An intensive smog chamber study has revealed that secondary organic aerosol (SOA) formation follows Raoult's Law type gas/aerosol absorption thermodynamics. SOA formation was shown to occur via the gas/aerosol partitioning of semi-volatile, oxidation products rather than through the condensation of saturated, non-volatile products. The major consequence of this finding is that SOA yields are not constant, but rather are a function of the organic aerosol mass concentration. The theory has been used to successfully describe the aerosol formation potential of seventeen individual aromatic species, eight biogenic compounds, two different simple hydrocarbon precursor mixtures, and twelve different blends of whole gasoline vapor, in hundreds of smog chamber experiments. These results have been included in a 3-dimensional size- and chemically-resolved atmospheric chemical-transport model and used to simulate SOA formation in the South Coast Air Basin. The inherent dependence of SOA concentrations on primary organic aerosol (POA) concentrations, places strict constraints on organic and elemental carbon aerosol emissions inventories.
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[1] Recent laboratory and field studies have indicated that glyoxal is a potentially large contributor to secondary organic aerosol mass. We present in situ glyoxal measurements acquired with a recently developed, high sensitivity spectroscopic instrument during the CalNex 2010 field campaign in Pasadena, California. We use three methods to quantify the production and loss of glyoxal in Los Angeles and its contribution to organic aerosol. First, we calculate the difference between steady state sources and sinks of glyoxal at the Pasadena site, assuming that the remainder is available for aerosol uptake. Second, we use the Master Chemical Mechanism to construct a two-dimensional model for gas-phase glyoxal chemistry in Los Angeles, assuming that the difference between the modeled and measured glyoxal concentration is available for aerosol uptake. Third, we examine the nighttime loss of glyoxal in the absence of its photochemical sources and sinks. Using these methods we constrain the glyoxal loss to aerosol to be 0–5 × 10−5 s−1 during clear days and (1 ± 0.3) × 10−5 s−1 at night. Between 07:00–15:00 local time, the diurnally averaged secondary organic aerosol mass increases from 3.2 μg m−3 to a maximum of 8.8 μg m−3. The constraints on the glyoxal budget from this analysis indicate that it contributes 0–0.2 μg m−3 or 0–4% of the secondary organic aerosol mass.
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