Scheme of the equilibrium Environmental Compartments Model

Life-cycle of gaseous contaminants

Processes leading to atmospheric deposition

Dry and wet atmospheric deposition Wet deposition Rainout, washout and deposition of aerosols are one- way transport processes that transfer chemicals from the air to water and soils. Most efficient for soluble gases and for aerosols with diameters > 1 µm Dry deposition Airborne particles and dry gases

Processes affecting the compositio of rain droplets

Deposition routes of some acid- forming gases and ammonia

Solubility of gases in liquids Gas solubility is an equilibrium process controlled by the gas (partial) pressure. At constant temperature, the number of molecules transferred from the gas phase to the liquid phase (solution) is the same as the number of molecules transferred the opposite way. Increasing pressure leads to higher solubility:

Henry’s Law Distribution of a gas between gas phase (air) and liquid phase (water) is described by the air-water distribution coefficient K aw = C(air)/C(aq) where C(air) is the concentration of a chemical in the air and C(aq) its concentration in water. C(air) can be calculated from the equation of state C(air) = n i /V = p i /RT n i is the number of moles of the chemical in volume V of air and p i is partial pressure of the chemical: p i = x i p where p is the (total atmospheric) pressure and x i molar fraction of the chemical in the air. If this chemical is below its critical point and aqueous solution is saturated, i.e. C(aq) = C S, partial pressure is equal to the saturated vapor pressure – p S More commonly applied is the Henry’s law constant H H = p S / C S = ( p i /C(aq) ) = K aw RT

Solubility of gases in liquids – example Calculate the solubility of oxygen in water at 25°C (H = Pa m 3 mol -1 ) O 2 in air= 20,95 % (molar percentage) P O2 = 0,2095 p atm C[O 2 aq ] = p O2 / H Normal pressure = 10 5 Pa = 1 bar = 1 atm = 760 mm Hg (torr) (conversion to atm is not exact) p O2 = 0, = ·10 4 Pa C[O 2 aq ] = p O2 /H = 2,7 10 –4 mol/l

Pressure in a closed system containing only pure liquid and gas phases of a given compound is called its (saturated) vapor pressure. Vapor pressure depends only on temperature. Many empirical correlation equations are used for this relation, most often the Antoine equation where A, B, C are constants derived from experimental data and valid only in some limited temperature interval. This equation is usually applied for vapor pressures from 1 to about 200 kPa. Vapor pressure

Measurements of vapor pressure – static method H 2 O (l) H 2 O (g) startequilibrium

Phase diagram of water

Factors affecting gas solubility Pressure According to Henry’s law, solubility of a gas is proportional to its partial pressure in the air. Because atmospheric pressure is more or less constant, it depends just on the concentration of this gas in the air Temperature Connected to gas expansivity, i.e. the equation of state: higher temperature leads to more expanded gases and thus lower concentration. Even more important is that Henry’s law constant of gases is growing with temperature. Both effects lead to lower gas solubility Salts Gases are „salted-out“ from the solution: higher salinity leads to lower gas solubility Chemical reactions in water If the gas is reacting with water, its solubility is increased

Oxygen in the air Oxygen equilibrium is attained on one side by de- oxygenation (aerobic processes of biochemical degradation of organic compounds) and on the other side by re-aeration (dissolution of oxygen from the air, if water is less than saturated). Equilibrium amount of dissolved oxygen depends on temperature. Equilibrium concentration of aqueous oxygen vs. temperature (mg/l) t(°C) O 2 (mg/l) Concentrations below 4 mg/l are lethal to fish and other water organisms

Non-equilibrium dynamics of gas dissolution: the oxygen curve

Re-aeration processes The rate of oxygen dissolution depends on the water surface quality: Still water dissolves 1.4 mg O 2 per m 2 / day Stirred water dissolves 5.5 mg O 2 per m 2 / day Turbulent water dissolves 50 mg O 2 per m 2 / day The rate of re-aeration exponentially depends on the oxygen deficit – the above numbers refer to stationary state. Oxygen deficit can be increased by sudden increase of contamination and/or by increased temperature.