Numerical simulation of chemical reactions occurring in fog droplets

Introduction

Fog provides a significant medium for chemical reactions occurring in liquid phase. Fog droplets are relatively small (5 < r < 50 µm) hence the surface-to-volume ratio is large. Through the larger overall surface the absorption of inorganic and organic gases from different sources may be enhanced. Depending on the stability of fog, the chemical processes may have more time to take place in fog droplets. Also, ground based sources of solid aerosol particles and gases may be in direct connection with fog.

Inorganic and organic components may change the pH of fog droplets and significant amount of sulphate ion can be formed due to the oxidation (e.g. by hydrogen-peroxide and ozone) of dissolved sulphur-dioxide. At the same time there are some organic components, e.g. formaldehyde, which also react with the dissolved sulphur-dioxide but produces hydroxymethanesulfonic acid (HMSA), thus decreases the possibility of producing sulphate ion through oxidation. These competitive processes are important in understanding the formation of sulphate ion in solution. In addition, the liquid phase concentration of compounds and also the sulphate ion formation strongly depends on the size of the droplets. Physical and chemical processes in fog may have an impact on both the size distribution and solubility of solid aerosol particles.

 

Numerical model

A box model with detailed microphysics and chemistry scheme with moving bin boundaries was used to simulate the following processes in fog:

• (i) Formation of drops on hygroscopic aerosol particles (ammonium-sulphate). Fog is formed due to cooling rate -0.0001 K/s.
• (ii) Condensational growth of drops.
• (iii) Scavenging of aerosol particles by water drops due to Brownian motion and phoretic forces.
• (iv) Absorption and desorption of inorganic (CO₂, H₂O₂, O₃, NH₃, SO₂) and organic (HCHO, HCOOH, CH₃COOH) gases, dissociation, change of pH, sulphate formation (oxidation of S(IV) by hydrogen-peroxide and by ozone and reaction of formaldehyde with S(IV)).

 

Results

Significant amount of HMSA formed in drops due to the reaction of S(IV) with formaldehyde. Taking into account this reaction, the amount of S(VI) formed is decreased compared to the case when no formaldehyde was present. Formation of HMSA modifies the solubility of the solid aerosol residue after evaporation of drops.