Type: Journal Article
Venue: Journal of Geophysical Research
Pun, B. K. (2008), Development and initial application of the sesquiversion of MADRID, J. Geophys. Res., 113, D12212, doi:10.1029/2008JD009888.
Resource Link: http://www.agu.org/journals/ABS/2008/2008JD009888.shtml
Organic aerosol (OA) models are typically formulated on the basis of empirical parameters or from first principles based on molecular constituents and properties. The overarching theme of sesqui-MADRID is the representation of primary and secondary organic aerosol constituents using surrogate molecular structures. Empirical partitioning characteristics guide the selection of surrogate secondary OA (SOA). Semivolatile primary OA (POA) are represented by selective compounds of key source types. The assignment of surrogate structures allows the use of group contribution methods for the calculation of thermodynamic properties such as activity coefficients, vapor pressures, and enthalpies of vaporization. A new activity coefficient model represents the interactions among POA, SOA, and water. The inclusion of water increases the total OA concentration by increasing the absorbing medium. If activity coefficients are assumed to be unity, OA increases substantially. Treatment of nonideality is essential in OA modules. When thermodynamically favorable, two phases with different hydrophobicity are modeled. Phase separation of OA is more common when water is considered as part of the absorbing medium than when it is not. Phase separation results in a small increase in total OA, because hydrophobic and hydrophilic compounds can exist in two liquid phases rather than repel each other in a single phase. Allowing POA to evaporate reduces the fraction of POA relative to SOA. The decrease can lead to a less than proportional decrease (or an increase) in total OA due to changes in composition, activity coefficients, and partitioning of individual constituents. A three-dimensional demonstration of sesqui-MADRID for a typical summer day shows an overall increase in OA in the southeastern United States compared to an empirical approach. Sesqui-MADRID predicts higher contributions from volatile and hydrophilic constituents and similar concentrations for constituents that are hydrophobic and less volatile.