Atmospheric Chemistry: Composition, Reactions, and Environmental Impact
Steady state balance between protons and electrons. Rocks = eruptive volatile atmosphere + sea water sediments. Determined by atmospheric composition, seawater pH, redox potential of 0.75. Non-photosynthetic microorganisms act as catalysts. pE = E / 2.303RT / F in 25 ºC.
pE0 = E0 / 2.303RT / F in 25 ºC. F = Faraday constant. pE = -log (ae-). It accounts for the effects of different electrode potential activities. Fe3+ + e– ⇌ Fe2+, E0 = +0.77 volts, pE0 = 13.2. The Nernst equation is: E = E0 + 2.303RT/nF log [Fe3+] / [Fe2+], pE = pE0 + 1/n log [Fe3+] / [Fe2+].
Trace Metal Concentration
a1 corresponds to the pH at which the aquo-complex is partially protonated. If pH a1, the metal is coordinated with water molecules. If pH > pKa1, OH– replaces water molecules in the coordination sites. Z2/r indicates the possibility of forming ionic links. Na, K have a low Z2/r and form permanent complexes with water. A high value of Z2/r (Al(III)) is deprotonated more easily. Metals with a high oxidation state (Cr(VI), Mo(VI)) are oxoanions. Type A (hard) cations have a noble gas electron configuration and interact electrostatically with ligands (fluoride and oxygen donor ligands such as Al3+, Ca2+). Type B or soft cations, with 10-12 electrons in the last layer, preferably interact covalently with ligands containing S or N (e.g., Cd2+, Ag+, Hg2+). Alkali and alkaline-earth ions are considered Type A and present as free aqueous ions with a very low tendency to form complexes. Elements with higher oxidation states (As(V), Cr(VI)) are present as hydrolyzed species. Ligands: Ammonia, nitrogen compounds decay; Phosphates – detergents, fertilizers; Cyanide – gold extraction process; EDTA – industrial laundry, photography, textiles, paper industry, detergents; NTA – phosphate substitute in detergents.
Photochemistry
Energy of molecules derived from different movements: Translational, Rotational, Vibrational, Electronic. Contribution is through heat. The higher the temperature, the greater the input. Each molecule requires approximately ½ kT. The most energetic radiation is capable of ionizing and dissociating molecules in the upper atmosphere, explaining the temperature increase in the thermosphere and ionosphere formation. The fraction of molecules that absorb photons in a specific proceeding, Φ = Nreacting molecules / Nmolecules that absorb photons, depends on the photochemical process.
Resulting Secondary Photochemical Processes
Of the species produced in the primary processes. Depends on the species produced in primary processes: ozone hole, tropospheric oxidation processes resulting from the formation of OH· radicals.
Chemistry of the Troposphere
ppm, ppb, ppt – number of gas molecules in a million, billion, trillion molecules of air. 1 ppbv = 1×10-6 moles or molecules per mole of air molecules. Mixing ratio: air molecules / cm3 at standard P and T. Density: Composite n/v = p/RT, 1 atm / 0.082 x 298 = 0.0409 moles/L. n/v = 0.0409 moles/L x 10-3 L/cm3 x 6.02×1023 molecules/mol = 2.46×1019 molecules/cm3. Mass/volume (g/m3). The chemistry of the troposphere is dominated by the chemistry of OH· radicals: hydroxyl (OH·), hydroperoxyl (HO2·), alkylperoxyl (RO2·), atomic oxygen (O(3P) ground state, O(1D) excited state), singlet state molecular oxygen (O2(1Δg)), and nitrate (NO3). Reactions are characterized by H abstraction from hydrocarbons, resulting in the formation of alkyl radicals and alkylperoxyl radicals. Important reactions in low-temperature combustion (autoignition and others). Oxidized organic compounds contribute to tropospheric ozone formation. Highly reactive species react with NO, NO2, HO2, and other peroxyl radicals. Concentration varies during the day, depending on daily photochemical activity: 150 ppt (day), 10 ppt (night). Alkoxyl radicals promote the formation of products from atmospheric oxidation. Reactions of alkenes with ozone, initiated by radicals, are important in the production of OH radicals in urban and rural atmospheres. These reactions are slower than the addition reactions of OH, contributing to the oxidative degradation of alkenes.
Tropospheric Ozone
Injections of air from the stratosphere. Destroyed at sea level. 0.1 to 0.01 ppm, maximum in spring/summer depending on latitude. Lifetime: 2 to 3 months. Tropospheric generation: 400 nm photolysis of NO2. Consumed by photolysis, NO2, HO2, and hydrocarbons (HC). The impact on emission levels is complex, depending on NOx levels.
Oxidation of Methane (CH4)
A cycle common to other organic compounds. Long photochemical lifetime. Concentration ~1.8 ppmv. Reactions form the base chemistry of the troposphere. Natural Oil: Terpenes (ppm). Degrade via reaction with ozone and OH. Petroleum Combustion/Incineration/Evaporation of Solvents, Oil. NMHC: Long lifetime (300 ppb, medium 1 week, 1 ppb reagents, 12 hours 10 ppb). React with HO·, forming NO2 stored as PANs. Sulfur: SO2, DMS, H2S from fossil fuels, biomass burning, oceans, soil, and volcanoes. Volcanic eruptions and anthropogenic sources contribute 0.2 ppb (lifetime 3 days). Anomalously, H2S is oxidized to sulfuric acid (H2SO4), which joins particulate matter.
Stratospheric Ozone
The tropopause, a region limiting the troposphere and stratosphere, is between 9-18 km. An “inversion” impedes the vertical movement of gases, keeping them perfectly stratified. Under normal conditions, an equilibrium system exists where ozone molecules are formed and destroyed at equal rates, maintaining a constant concentration. Ozone is much rarer than normal oxygen in the upper atmosphere: for every 10 million air molecules, about 2 million are normal oxygen and only 3 are ozone. Rupture of O2 spreads from 100-150 km to 50 km, where it reacts with O2. The maximum radiation zone for ozone formation is at the equator.
Ozone Variation Factors
> V undreds stratospheric, solar cycle although not very large proportions, chlorinated compounds from forest fires and degradation of marine life, volcanism, sulfate aerosols quantities of chlorine, explosion.
chemistry of the troposphere is one of the most important trace gases in the atmosphere, is a greenhouse gas, is an oxidant and precursor of tropospheric oxidants such as OH, NO3, Toxic for organisms, material damages, sources: diffusion from the stratosphere and of training as a secondary pollutant in the degradation of VOCs, ozone increased from 10-15 ppb to 30-40ppb current large-scale effect, decreases crop production / global rate of ozone formation is trposférico: BO3 = FO3-DO3 , ozone formation is the result of photolysis of NO2 to form peroxy radical / ozone destruction is through the intercorversión of HO2 OH / OH reaction initiates a large number of atmospheric components, detergent of the atmosphere, concentrations near surface ~ 106 – 107cm-3, very fast and efficiently recycled / end of the cycle HNO3 and H2O2 photolysis or react with OH reversing the steps, but reactions are slow (lifetimes of several days), Both are very soluble (less H2O2 ) m Laundry by precipitation, dry deposition, in upper troposphere, there is no dry deposition and wet removal is limited by / k limit cycle reactions methyl hydroperoxide (CH3OOH) can photolysis with OH with a lifetime of ~ 2 days, radical Regenerate the system, important source of radicals in the upper tropical troposphere, Moderately soluble and can be removed by dry or wet, Decreases radical /
greenhouse Social concern at the increase in atmospheric temperature-greenhouse effect
Heat storage air-water and CO 2-absorb infrared energy-Increased concentration of greenhouse gases = greater global warming
Natural greenhouse effect, relaying from the land of l> 4000 nm: Absorption by atmospheric gases
:8000-12000 nm emición to outside atmosphere by window: Increased from 33 ° C average temperature on earth 288 ° K