scholarly journals New representation of water activity based on a single solute specific constant to parameterize the hygroscopic growth of aerosols in atmospheric models

2012 ◽  
Vol 12 (12) ◽  
pp. 5429-5446 ◽  
Author(s):  
S. Metzger ◽  
B. Steil ◽  
L. Xu ◽  
J. E. Penner ◽  
J. Lelieveld
Keyword(s):  
Relative Humidity ◽  
Water Activity ◽  
Atmospheric Chemistry ◽  
Reference Model ◽  
Climate Modeling ◽  
Cloud Condensation Nuclei ◽  
Hygroscopic Growth ◽  
Wide Range ◽  
Kohler Theory ◽  
Key Factor

Abstract. Water activity is a key factor in aerosol thermodynamics and hygroscopic growth. We introduce a new representation of water activity (aw), which is empirically related to the solute molality (μs) through a single solute specific constant, νi. Our approach is widely applicable, considers the Kelvin effect and covers ideal solutions at high relative humidity (RH), including cloud condensation nuclei (CCN) activation. It also encompasses concentrated solutions with high ionic strength at low RH such as the relative humidity of deliquescence (RHD). The constant νi can thus be used to parameterize the aerosol hygroscopic growth over a wide range of particle sizes, from nanometer nucleation mode to micrometer coarse mode particles. In contrast to other aw-representations, our νi factor corrects the solute molality both linearly and in exponent form x · ax. We present four representations of our basic aw-parameterization at different levels of complexity for different aw-ranges, e.g. up to 0.95, 0.98 or 1. νi is constant over the selected aw-range, and in its most comprehensive form, the parameterization describes the entire aw range (0–1). In this work we focus on single solute solutions. νi can be pre-determined with a root-finding method from our water activity representation using an aw−μs data pair, e.g. at solute saturation using RHD and solubility measurements. Our aw and supersaturation (Köhler-theory) results compare well with the thermodynamic reference model E-AIM for the key compounds NaCl and (NH4)2SO4 relevant for CCN modeling and calibration studies. Envisaged applications include regional and global atmospheric chemistry and climate modeling.

scholarly journals Description of EQSAM4: gas-liquid-solid partitioning model for global simulations

2011 ◽  
Vol 4 (4) ◽  
pp. 2791-2847 ◽  
Author(s):  
S. Metzger ◽  
B. Steil ◽  
L. Xu ◽  
J. E. Penner ◽  
J. Lelieveld
Keyword(s):  
Water Uptake ◽  
Atmospheric Aerosols ◽  
Atmospheric Chemistry ◽  
Large Scale ◽  
Climate Model ◽  
Reference Model ◽  
Climate Modeling ◽  
Box Model ◽  
Hygroscopic Growth ◽  
Aerosol Model

Abstract. We introduce version 4 of the EQuilibrium Simplified Aerosol Model (EQSAM4), which is part of our aerosol chemistry-microphysics module (GMXe) and chemistry-climate model (EMAC). We focus on the relative humidity of deliquescence (RHD) based water uptake of atmospheric aerosols, as this is important for atmospheric chemistry and climate modeling, e.g. to calculate the aerosol optical depth (AOD). Since the main EQSAM4 applications will involve large-scale, long-term and high-resolution atmospheric chemistry-climate modeling with EMAC, computational efficiency is an important requirement. EQSAM4 parameterizes the composition and water uptake of multicomponent atmospheric aerosols by considering the gas-liquid-solid partitioning of single and mixed solutes. EQSAM4 builds on analytical, and hence CPU efficient, aerosol hygroscopic growth parameterizations to compute the aerosol liquid water content (AWC). The parameterizations are described in the companion paper (Metzger et al., 2011) and only require a compound specific coefficient νi to derive the single solute molality and the AWC for the whole range of water activity (aw). νi is pre-calculated and applied during runtime by using internal look-up tables. Here, the EQSAM4 equilibrium model is described and compared to the more explicit thermodynamic model ISORROPIA II. Both models are imbedded in EMAC/GMXe. Box model inter-comparisons, including the reference model E-AIM, and global simulations with EMAC show that gas-particle partitioning, including semi-volatiles and water, is in good agreement. A more comprehensive box model inter-comparison of EQSAM4 with EQUISOLV II is subject of the revised publication of Xu et al. (2009), i.e. Xu et al. (2011).

scholarly journals Aerosol hygroscopic growth parameterization based on a solute specific coefficient

2011 ◽  
Vol 11 (9) ◽  
pp. 24813-24855 ◽  
Author(s):  
S. Metzger ◽  
B. Steil ◽  
L. Xu ◽  
J. E. Penner ◽  
J. Lelieveld
Keyword(s):  
Relative Humidity ◽  
Atmospheric Aerosols ◽  
Reference Model ◽  
Empirical Relation ◽  
Companion Paper ◽  
Hygroscopic Growth ◽  
Computationally Efficient ◽  
Aerosol Model ◽  
Wide Range ◽  
Main Component

Abstract. Water is a main component of atmospheric aerosols and its amount depends on the particle chemical composition. We introduce a new parameterization for the aerosol hygroscopic growth factor (HGF), based on an empirical relation between water activity (aw) and solute molality (μs) through a single solute specific coefficient νi. Three main advantages are: (1) wide applicability, (2) simplicity and (3) analytical nature. (1) Our approach considers the Kelvin effect and covers ideal solutions at large relative humidity (RH), including CCN activation, as well as concentrated solutions with high ionic strength at low RH such as the relative humidity of deliquescence (RHD). (2) A single νi coefficient suffices to parameterize the HGF for a wide range of particle sizes, from nanometer nucleation mode to micrometer coarse mode particles. (3) In contrast to previous methods, our analytical aw parameterization depends not only on a linear correction factor for the solute molality, instead νi also appears in the exponent in form x · ax. According to our findings, νi can be assumed constant for the entire aw range (0–1). Thus, the νi based method is computationally efficient. In this work we focus on single solute solutions, where νi is pre-determined with the bisection method from our analytical equations using RHD measurements and the saturation molality μssat. The computed aerosol HGF and supersaturation (Köhler-theory) compare well with the results of the thermodynamic reference model E-AIM for the key compounds NaCl and (NH4)2SO4 relevant for CCN modeling and calibration studies. The equations introduced here provide the basis of our revised gas-liquid-solid partitioning model, i.e. version 4 of the EQuilibrium Simplified Aerosol Model (EQSAM4), described in a companion paper.

scholarly journals Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol – Part 2: Theoretical approaches

2009 ◽  
Vol 9 (12) ◽  
pp. 3999-4009 ◽  
Author(s):  
M. D. Petters ◽  
H. Wex ◽  
C. M. Carrico ◽  
E. Hallbauer ◽  
A. Massling ◽  
...  
Keyword(s):  
Water Activity ◽  
Secondary Organic Aerosol ◽  
Cloud Condensation Nuclei ◽  
Organic Aerosol ◽  
Water Soluble ◽  
Water Mixture ◽  
Hygroscopic Growth ◽  
Ideal Behavior ◽  
Hygroscopic Properties ◽  
Theoretical Approaches

Abstract. We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of α-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.

scholarly journals Accurate Measurements of Aerosol Hygroscopic Growth over a Wide Range in Relative Humidity

2016 ◽  
Vol 120 (25) ◽  
pp. 4376-4388 ◽  
Author(s):  
Grazia Rovelli ◽  
Rachael E. H. Miles ◽  
Jonathan P. Reid ◽  
Simon L. Clegg
Keyword(s):  
Relative Humidity ◽  
Hygroscopic Growth ◽  
Wide Range

scholarly journals Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol – Part 2: Theoretical approaches

2008 ◽  
Vol 8 (6) ◽  
pp. 20839-20867 ◽  
Author(s):  
M. D. Petters ◽  
H. Wex ◽  
C. M. Carrico ◽  
E. Hallbauer ◽  
A. Massling ◽  
...  
Keyword(s):  
Water Activity ◽  
Secondary Organic Aerosol ◽  
Cloud Condensation Nuclei ◽  
Organic Aerosol ◽  
Water Soluble ◽  
Water Mixture ◽  
Hygroscopic Growth ◽  
Ideal Behavior ◽  
Hygroscopic Properties ◽  
Theoretical Approaches

Abstract. We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of alpha-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.

scholarly journals Relative-humidity-dependent organic aerosol thermodynamics via an efficient reduced-complexity model

2019 ◽  
Vol 19 (21) ◽  
pp. 13383-13407 ◽  
Author(s):  
Kyle Gorkowski ◽  
Thomas C. Preston ◽  
Andreas Zuend
Keyword(s):  
Relative Humidity ◽  
Molar Mass ◽  
Cloud Condensation Nuclei ◽  
Organic Aerosol ◽  
Basis Set ◽  
Aqueous Mixtures ◽  
Organic Mixture ◽  
Organic Mixtures ◽  
Wide Range ◽  
Reduced Complexity

Abstract. Water plays an essential role in aerosol chemistry, gas–particle partitioning, and particle viscosity, but it is typically omitted in thermodynamic models describing the mixing within organic aerosol phases and the partitioning of semivolatile organics. In this study, we introduce the Binary Activity Thermodynamics (BAT) model, a water-sensitive reduced-complexity model treating the nonideal mixing of water and organics. The BAT model can process different levels of physicochemical mixture information enabling its application in the thermodynamic aerosol treatment within chemical transport models, the evaluation of humidity effects in environmental chamber studies, and the analysis of field observations. It is capable of using organic structure information including O:C, H:C, molar mass, and vapor pressure, which can be derived from identified compounds or estimated from bulk aerosol properties. A key feature of the BAT model is predicting the extent of liquid–liquid phase separation occurring within aqueous mixtures containing hydrophobic organics. This is crucial to simulating the abrupt change in water uptake behavior of moderately hygroscopic organics at high relative humidity, which is essential for capturing the correct behavior of organic aerosols serving as cloud condensation nuclei. For gas–particle partitioning predictions, we complement a volatility basis set (VBS) approach with the BAT model to account for nonideality and liquid–liquid equilibrium effects. To improve the computational efficiency of this approach, we trained two neural networks; the first for the prediction of aerosol water content at given relative humidity, and the second for the partitioning of semivolatile components. The integrated VBS + BAT model is benchmarked against high-fidelity molecular-level gas–particle equilibrium calculations based on the AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficient) model. Organic aerosol systems derived from α-pinene or isoprene oxidation are used for comparison. Predicted organic mass concentrations agree within less than a 5 % error in the isoprene case, which is a significant improvement over a traditional VBS implementation. In the case of the α-pinene system, the error is less than 2 % up to a relative humidity of 94 %, with larger errors past that point. The goal of the BAT model is to represent the bulk O:C and molar mass dependencies of a wide range of water–organic mixtures to a reasonable degree of accuracy. In this context, we discuss that the reduced-complexity effort may be poor at representing a specific binary water–organic mixture perfectly. However, the averaging effects of our reduced-complexity model become more representative when the mixture diversity increases in terms of organic functionality and number of components.

scholarly journals Assessment of NAAPS-RA performance in Maritime Southeast Asia during CAMP<sup>2</sup>Ex

2021 ◽  
Author(s):  
Eva-Lou Edwards ◽  
Jeffrey S. Reid ◽  
Peng Xian ◽  
Sharon P. Burton ◽  
Anthony L. Cook ◽  
...  
Keyword(s):  
Southeast Asia ◽  
Cloud Condensation Nuclei ◽  
Particle Mass ◽  
High Spectral Resolution ◽  
Light Extinction ◽  
Aerosol Extinction ◽  
Hygroscopic Growth ◽  
Extinction Coefficients ◽  
Wide Range ◽  
Aerosol Properties

Abstract. Monitoring and modeling aerosol particle lifecycle in Southeast Asia (SEA) is challenged by high cloud cover, complex meteorology, and the wide range of aerosol species, sources, and transformations found throughout the region. Satellite observations are limited, and there are few in situ observations of aerosol extinction profiles, aerosol properties, and environmental conditions. Therefore, accurate aerosol model outputs are crucial for the region. This work evaluates the Navy Aerosol Analysis and Prediction System Reanalysis (NAAPS-RA) aerosol optical thickness (AOT) and light extinction products using airborne aerosol and meteorological measurements from the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) in SEA. Modeled AOTs and extinction coefficients were compared to those retrieved with a High Spectral Resolution Lidar (HSRL-2). Correlations were highest for AOT in the mixed layer (AOTML; R2 = 0.83, bias = 0.00, root mean square error [RMSE] = 0.03) compared to total AOT (R2 = 0.68, bias = 0.01, RMSE = 0.14), although the correlations between the observations and 1° × 1° degree NAAPS-RA outputs were weaker in regions with strong gradients in aerosol properties, such as near areas of active convection. Correlations between simulated and retrieved aerosol extinction coefficients were highest from 145–500 m (R2 = 0.75, bias = 0.01 km−1, RMSE = 0.08 km−1) and decreased with increasing altitude (R2 = 0.69 and 0.26, bias = 0.00 and 0.00 km−1, RMSE = 0.09 and 0.00 km−1 for 500–1500 m and > 1500 m, respectively), which was likely a result of the use of bulk cloud mixing parameterizations. We also investigated the role of possible relative humidity (RH) errors in extinction simulations. Despite negative biases in modeled RH (−4.9, −7.7, and −2.3 % for altitudes < 500 m, 500–1500 m, and > 1500 m, respectively), AOT and extinction agreement with the HSRL-2 did not change significantly at any altitude when RHs from dropsondes were substituted into the model. Improvements may have been stunted due to errors in how NAAPS-RA modeled physics of particle hygroscopic growth, dry particle mass concentrations, and/or dry mass extinction efficiencies, especially when combined with AOT corrections from data assimilation. Specifically, the model overestimated the hygroscopicity of (i) smoke particles from biomass burning in the Maritime Continent (MC), and (ii) anthropogenic emissions transported from East Asia. This work provides insight into how certain environmental and microphysical properties influence AOT and extinction simulations, which can then be interpreted in the context of modeling global concentrations of particle mass and cloud condensation nuclei (CCN).

scholarly journals RH-dependent organic aerosol thermodynamics via an efficient reduced-complexity model

2019 ◽  
Author(s):  
Kyle Gorkowski ◽  
Thomas C. Preston ◽  
Andreas Zuend
Keyword(s):  
Relative Humidity ◽  
Molar Mass ◽  
Cloud Condensation Nuclei ◽  
Organic Aerosol ◽  
Basis Set ◽  
Organic Mixture ◽  
Alpha Pinene ◽  
Ideal Mixing ◽  
Wide Range ◽  
Reduced Complexity

Abstract. Water plays an essential role in aerosol chemistry, gas-particle partitioning, and particle viscosity, but it is typically omitted in thermodynamic models describing the mixing within organic aerosol phases and the partitioning of semivolatile organics. In this study, we introduce the Binary Activity Thermodynamics (BAT) model, a water-sensitive, reduced-complexity model treating the non-ideal mixing of water and organics. The BAT model can process different levels of physicochemical mixture information enabling its application in the thermodynamic aerosol treatment within chemical transport models, the evaluation of humidity effects in environmental chamber studies, and the analysis of field observations. It is capable of using organic structure information including O:C, H:C, molar mass, and vapor pressure, which can be derived from identified compounds or estimated from bulk aerosol properties. A key feature of the BAT model is predicting the extent of liquid-liquid phase separation occurring within aqueous mixtures containing hydrophobic organics. This is crucial to simulating the abrupt change in water uptake behavior of moderately hygroscopic organics at high relative humidity, which is essential for capturing the correct behavior of organic aerosols serving as cloud condensation nuclei. For gas-particle partitioning predictions, we complement a Volatility Basis Set (VBS) approach with the BAT model to account for non-ideality and liquid-liquid equilibrium effects. To improve the computational efficiency of this approach, we trained two neural networks; the first for the prediction of aerosol water content at given relative humidity, and the second for the partitioning of semivolatile components. The integrated VBS + BAT model is benchmarked against high-fidelity molecular-level gas-particle equilibrium calculations based on the AIOMFAC model. Organic aerosol systems derived from alpha-pinene or isoprene oxidation are used for comparison. Predicted organic mass concentrations agree within less than a 5 % error in the isoprene case, which is a significant improvement over a traditional VBS implementation. In the case of the alpha-pinene system, the error is less than 2 % up to a relative humidity of 94 %, with larger errors past that point. The goal of the BAT model is to represent the bulk O:C and molar mass dependencies of a wide range of water-organic mixtures to a reasonable degree of accuracy. In this context, we discuss that the reduced-complexity effort may be poor at representing a specific binary water-organic mixture perfectly. However, the averaging effects of our reduced-complexity model become more representative when the mixture diversity increases in terms of organic functionality and number of components.

scholarly journals Influence of common assumptions regarding aerosol composition and mixing state on predicted CCN concentration

2017 ◽  
Author(s):  
Manasi Mahish ◽  
Anne Jefferson ◽  
Don Collins
Keyword(s):  
Growth Factor ◽  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
Hygroscopic Growth ◽  
Aerosol Composition ◽  
Size Dependent ◽  
Critical Supersaturation ◽  
Kohler Theory ◽  
The Impact ◽  
Baseline Approach

Abstract. A 4-year record of aerosol size and hygroscopic growth factor distributions measured at the Department of Energy’s SGP ARM site in Oklahoma, U.S. were used to estimate supersaturation (S)-dependent cloud condensation nuclei concentrations (NCCN). Baseline or reference NCCN(S) spectra were estimated by using the data to create a matrix of size- and hygroscopicity-dependent number concentration (N) and then integrating for S > critical supersaturation (Sc) calculated for the same size and hygroscopicity pairs using κ-Köhler Theory. The accuracy of those estimates was assessed through comparison with the directly measured NCCN at the same site. Subsequently, NCCN was calculated using the same dataset but with an array of simplified treatments in which the aerosol was assumed to be either an internal or an external mixture and the hygroscopicity either assumed or based on averages derived from the growth factor distributions. The CCN spectra calculated using the simplified treatments were compared with those from the baseline approach to evaluate the impact of commonly used approximations. Among the simplified approaches, assuming the aerosol is an internal mixture with size-dependent hygroscopicity parameter (κ) resulted in estimates closest to those from the baseline approach over the range in S considered.

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