Abstract
Abstract. Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodynamic equilibrium model are used to study the partitioning of semivolatile inorganic species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. For particle sizes up to 1μm diameter, semi-volatile partitioning requires 15–30 min to equilibrate; longer time is typically required during the night and early morning hours. Aerosol and gas-phase speciation always exhibits substantial temporal variability, so that aerosol composition measurements (bulk or size-resolved) obtained over large integration periods are not reflective of its true state. When the aerosol sulfate-to-nitrate molar ratio is less than unity, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equivalent sodium" (rather than explicitly) in the thermodynamic equilibrium calculations introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramatically. This suggests that explicitly considering crustals in the thermodynamic calculations is required to accurately predict the partitioning and phase state of aerosols.
Highlights
Atmospheric particulate matter plays a central role in atmospheric phenomena like visibility reduction, public health, formation of acid rain and climate change
We evaluate the ability of ISORROPIA-II to reproduce the observed partitioning of ammonia, nitrate and chloride, which will test the expectation that equilibrium partitioning of semivolatile aerosol species is attained somewhere between 6 and 30 min
This study focuses on thermodynamical modeling of gasaerosol partitioning sampled during the Megacity Initiative: Local and Global Research Observations (MILAGRO) 2006 campaign in Mexico City
Summary
Atmospheric particulate matter plays a central role in atmospheric phenomena like visibility reduction, public health, formation of acid rain and climate change. An important question is whether equilibrium models (all of which embody simplified representations of aerosol composition) can adequately predict the equilibrium partitioning of semivolatile inorganic species This is often assessed by comparing model predictions against measurements, assuming thermodynamic equilibrium applies between the aerosol and gas phases. We use ISORROPIA-II, which treats the thermodynamics of the K+-Ca2+-Mg2+-NH+4 Na+-SO24−-HSO−4 -NO−3 -Cl−-H2O aerosol system, to a) concurrently test the model prediction skill and thermodynamic equilibrium assumption for the Mexico City aerosol during the MILAGRO 2006 campaign, b) gain insight on the preferred phase behavior of the aerosol (i.e. deliquescent or metastable) and equilibration timescale, and, c) assess the importance of neglecting crustal species (or treating them as equivalent sodium) in thermodynamic calculations. The MILAGRO 2006 dataset analyzed here is ideal for the objectives of this study, because of significant concentrations of all the inorganic species mentioned above
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