. Acidity, defined as pH, is a central component of aqueous
chemistry. In the atmosphere, the acidity of condensed phases (aerosol
particles, cloud water, and fog droplets) governs the phase partitioning of
semivolatile gases such as HNO3 , NH3 , HCl, and organic acids and
bases as well as chemical reaction rates. It has implications for the
atmospheric lifetime of pollutants, deposition, and human health. Despite
its fundamental role in atmospheric processes, only recently has this field
seen a growth in the number of studies on particle acidity. Even with this
growth, many fine-particle pH estimates must be based on thermodynamic model
calculations since no operational techniques exist for direct measurements.
Current information indicates acidic fine particles are ubiquitous, but
observationally constrained pH estimates are limited in spatial and temporal
coverage. Clouds and fogs are also generally acidic, but to a lesser degree
than particles, and have a range of pH that is quite sensitive to
anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient
ammonia. Historical measurements indicate that cloud and fog droplet pH has
changed in recent decades in response to controls on anthropogenic
emissions, while the limited trend data for aerosol particles indicate
acidity may be relatively constant due to the semivolatile nature of the
key acids and bases and buffering in particles. This paper reviews and
synthesizes the current state of knowledge on the acidity of atmospheric
condensed phases, specifically particles and cloud droplets. It includes
recommendations for estimating acidity and pH, standard nomenclature, a
synthesis of current pH estimates based on observations, and new model
calculations on the local and global scale.

/pdf/the-acidity-of-atmospheric-particles-and-clouds-2ga6ctwd3t.pdf

The acidity of atmospheric particles and clouds

Modelling of long-range transport of sulphur over Europe: A two-year model run and some model experiments

Numerical models that combine weather forecasting and atmospheric chemistry are here referred to as chemical weather forecasting models. Eighteen operational chemical weather forecasting models on regional and continental scales in Europe are described and compared in this article. Topics discussed in this article include how weather forecasting and atmospheric chemistry models are integrated into chemical weather forecasting systems, how physical processes are incorporated into the models through parameterization schemes, how the model architecture affects the predicted variables, and how air chemistry and aerosol processes are formulated. In addition, we discuss sensitivity analysis and evaluation of the models, user operational requirements, such as model availability and documentation, and output availability and dissemination. In this manner, this article allows for the evaluation of the relative strengths and weaknesses of the various modelling systems and modelling approaches. Finally, this article highlights the most prominent gaps of knowledge for chemical weather forecasting models and suggests potential priorities for future research directions, for the following selected focus areas: emission inventories, the integration of numerical weather prediction and atmospheric chemical transport models, boundary conditions and nesting of models, data assimilation of the various chemical species, improved understanding and parameterization of physical processes, better evaluation of models against data and the construction of model ensembles. © 2012 Author(s).

/pdf/a-review-of-operational-regional-scale-chemical-weather-33qgnupa5x.pdf

A review of operational, regional-scale, chemical weather forecasting models in Europe

Review: Atmospheric deposition and plant assimilation of gases and particles

Modeling the processing of aerosol and trace gases in clouds and fogs.

The concentration of sulfate in precipitation is predicted to be directly proportional to the sulfate concentration of the air ingested into the cloud and inversely proportional to cloud water concentration. In addition, for light to moderate precipitation rates, the removal of airborne sulfate by precipitation is forecast to be strongly dependent upon the mechanism of precipitation formation. If the precipitation is in the form of snow or originated as snow in the upper portions of the clouds, then the surface sulfate concentration of precipitation is proportional to R−0.3, where R is the precipitation rate. Should the precipitation form without the benefit of an initial ice growth stage, then the sulfate concentrations in precipitation water can increase by a factor of 2 to 10 or more over the ice-dependent predictions. Limited surface observations give support to these predictions. The analyses leading to the prediction of washout ratios are also used to forecast the mean oxidation rate of SO2...

Parameterization of Sulfate Removal by Precipitation.

Two case studies are used to examine the relationship of the sulfate concentration in surface precipitation to the microphysical characteristics of the precipitating cloud systems. The data from one case study support the contention that existing sulfate aerosol was incorporated into cloud water by the nucleation process and accounted for nearly all of the observed cloud and precipitation water sulfate concentration. These activated sulfate particles comprised nearly 60% of the clear-air sulfate mass concentration. Once nucleated, the sulfate particles accumulated water through the condensation process and were subsequently deposited at the surface after accretion on large snowflakes. The presumption of aqueous phase sulfate oxidation of SO2 was not necessary to account for the observed sulfate concentrations. The data from the second case study are more limited and difficult to interpret. Nucleation and below cloud washout appeared to be the main contributors to the surface sulfate concentration...

B. C. Scott

Papers

Parameterization of Sulfate Removal by Precipitation.

On the Concentration of Sulfate in Precipitation