Abstract
Abstract. We present the development of ANISORROPIA, the discrete adjoint of the ISORROPIA thermodynamic equilibrium model that treats the Na+-SO42−- HSO4−-NH4+ -NO3−-Cl−-H2O aerosol system, and we demonstrate its sensitivity analysis capabilities. ANISORROPIA calculates sensitivities of an inorganic species in aerosol or gas phase with respect to the total concentrations of each species present with less than a two-fold increase in computational time over the concentration calculations. Due to the highly nonlinear and discontinuous solution surface of ISORROPIA, evaluation of the adjoint required a new, complex-variable version of the model, which determines first-order sensitivities with machine precision and avoids cancellation errors arising from finite difference calculations. The adjoint is verified over an atmospherically relevant range of concentrations, temperature, and relative humidity. We apply ANISORROPIA to recent field campaign results from Atlanta, GA, USA, and Mexico City, Mexico, to characterize the inorganic aerosol sensitivities of these distinct urban air masses. The variability in the relationship between fine mode inorganic aerosol mass and precursor concentrations shown has important implications for air quality and climate.
Highlights
Atmospheric chemical transport models (CTMs) are used for many purposes including assessment of radiative climate forcing of aerosol (Forster et al, 2007) and evaluation of the effectiveness of emissions control strategies designed to reduce exposure of humans to particulates (US EPA, 2004)
The partitioned concentrations given by ANISORROPIA were evaluated against ISORROPIA v.2.1 for an atmospherically relevant range of relative humidity (RH) (5–95 %), temperature (268–308 K), and total concentrations to ensure that the forward calculations are not altered by the modifications of the code required to calculate the adjoint derivatives
Having ensured that the sensitivities would be evaluated for nearly identical simulated equilibrium concentrations, the accuracy of the adjoint sensitivities was assessed by comparison to sensitivities calculated using the complex variable method
Summary
Atmospheric chemical transport models (CTMs) are used for many purposes including assessment of radiative climate forcing of aerosol (Forster et al, 2007) and evaluation of the effectiveness of emissions control strategies designed to reduce exposure of humans to particulates (US EPA, 2004). The accuracy of these estimates is limited by model representations of chemical and physical processes as well as model parameters (e.g., emissions rates). Adjoint-based sensitivity analysis enhances the ability to assess the relative influence of aerosol precursor emissions on air quality metrics and aerosol radiative forcing of climate as well as providing a means of refining emissions estimates with observations in an inverse modeling framework
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