ENVIRONMENTAL CHEMISTRY (Air II) Chem. 3030
Stratospheric chemistry and the ozone layer; principles of photochemistry, light absorption by molecules, noncatalytic and catalytic process of ozone distraction, free radicals, Cl and Br as X catalysts, the ozone hole and its consequences, chlorofluorocarbons (CFCs). Ground-level (tropospheric) air chemistry; ground-level ozone and photochemical smog, oxidation of methane, hydrocarbons and atmospheric SO2, acid rain, ecological effects of outdoor air pollutants, indoor air pollution: formaldehyde, NO2, CO, tobacco smoke, asbestos, radioactivity from radon gas. The greenhouse effect and global warming; energy absorption, the major and minor greenhouse gases: CO2, water vapour, methane, N2O, CFCs. Environmental consequences of energy use: CO2 emissions, solar energy, conventional and alternative fuels, nuclear energy. The chemistry of natural waters; acid-base chemistry, CO2/carbonate system, ion concentations, alkalinity, seawater, redox chemistry in natural waters, oxygen demand, the pE scale, sulphur and nitrogen compounds, ion complexes, stratification, precipitation. Soil chemistry; soil components, weathering process, aerobic, anaerobic soils, water-sediment-soil system.
Photochemical smog oxidation reactions are initiated by the hydroxyl radical (produced in large part due to the presence of NO from combustion emissions). Hydrocarbons and other volatile organic compounds are the oxidisable substrates. The products of the smog-producing reactions include partially oxidised hydrocarbons, CO, aldehydes and ketones, residual nitrogen oxides and ozone
Reducing photochemical smog and ground-level ozone: VOCs (C=C) NOx
ACID RAIN (atmospheric precipitation of substantial acid, more acidic than natural) H2SO4 and HNO3 from SO2 and NOx(primary pollutants) • Sources: • volcanos, e.g. Mount Pinatubo in the Phillipines is known to have contributed a lot of sulphate aerosols to the Arctic • crude oil – petroleum industry • Claus reaction – removal of sulphides: 2H2S + SO2→ 3S + 2H2O • petrochemical processes (CH3SH, (CH3)2S, CH3SSCH3) • TRS – total reduced sulphur • smelting metal ores with sulphides
The sulphuric acid in precipitation originates from a number of chemical precursors. Reduced sulphur compounds can be oxidised to SO2 with hydroxyl radical as the primary oxidising agent. The SO2 then dissolves in water droplets and is further oxidised to sulphuric acid via several homogeneous and heterogeneous processes
Environmental consequences of photochemical smog and acid rain • wet deposition • dry deposition • acidification depends strongly on the soil composition • limestone neutralises acid, granite or quartz are strongly affected • acidified lakes • growth of water animals and plants • DOC decreased – higher penetration of UV light to the lower water levels • elevated conc. of dissolved Al and Fe • deterioration of soils – washing out plant nutrients • effect on trees • agriculture crops • ozone reacts with ethylene forming free radicals – resulting in slowed photosynthesis
Air particulates Solid or liquid particles suspended in air. They are not visible to the naked eye, but form a haze that reduces visibility. Size: 2nm – 100 µm Coarse particles > 2.5 µm < fine particles Soot or inorganic soot or sulphate, nitrate aerosols Basic due to a soil content acidic due to unneutralised acids PM index – amount of particulate matter per volume (µg/m3) PM10 = 10 µm PM10 = all fine particles <10 µm – inhalable PM2.5– respirable PM0.05 - ultrafine
Distribution of numbers of aerosol particles vs. size in a typical urban environment
Aerosol particles cover a size range from 1 nm to 100 μm in diameter, but it is particles in the range 0.01 – 10 μm that are most stable in suspension. Particles smaller than 0.01 μm in diameter tend to coagulate into larger units, while those larger than 10 μm readily settle out. The aerosol fraction that consists of very fine particles is of concern to human health because of its association with respiratory problems.
Chemistry of the troposphere – free radicals Prediction of fate of gases emitted into the air
Prediction of fate of airborne free radicals
Oxidation of methane in the troposphere (the rep.of other alkenes and VOCs with single bonds) CH4 – no water soluble, does not absorb sunlight, no multiple bonds – abstraction of H by OH* CH3*– addition of O2 to produce peroxy radical Peroxy radical oxidize by transfer of O atom O2 abstracts H to produce formaldehyde Formaldehyde decompose photochemically and H adds to O2 O2 addition to double bondH-C=O OH* addition to triple bond C≡O O2 abstracts the H to produce CO2 and OOH* radical CH4 + 5O2 + NO*→(UV-A) CO2 + H2O + NO2* + 4HOO*
Indoor air pollution • Formaldehyde, H2C=O – stable intermediate in the oxidation of methane and VOCs • sources: cigarette smoke, synthetic materials, glues, dyes, resins • CARCINOGEN • NO2 and CO • Sources: combustion processes • NO2 dissolves in biological tissues, OXIDANT • CO bonds to haemoglobin and stops oxygen bonding • Tobacco smoke • Very complex toxic action – solids (tar), liquids and VOCs • Asbestos • CARCINOGEN • Radon –Radioactivity
THE GREEN HOUSE EFFECT AND GLOBAL WARMING
Average air temp. increases as a result of the build-up of carbon dioxide and other “greenhouse” gases in the atmosphere. The mechanism of greenhouse effect: Incoming energy to the Earth is mostly visible light from Sun Outgoing energy is in a form of infrared radiation (heat)
Balance of energy: From the total light coming to Earth – 50% absorbed by surface - 20% absorbed by gases (O3, O2, CO2, H2O and water droplets) - 30% reflected back into space (by clouds, snow, ice, sand) Some gases in air can temporarily absorb IR light of specific wavelengths and later re-emit in all directions (randomly); some are redirected to the earth’s surface – GREENHOUSE EFFECT Normally gases in the atmosphere operate as a ‘blanket’ for the Earth keeping the same level of coming in and going out radiation. Increasing the conc. of the gases in air which absorb the IR radiation will increase re-direction of IR and keep more heat on the Earth - ENHANCED GREENHOUSE EFFECT (distinguish from naturally operated for millennia)
CO2 O=C=O 4.26 µm – antisymmetric stretch vibration 15.0 µm – bond-angle bending vibration The CO2 molecules absorb now about half of the outgoing thermal IR radiation. H2O H-O-H 6.3 µm –bending vibration from 18.0 µm– rotation without vibration Emission of CO2 – 0.4% annually Higher temp. causes increase of water vapour → more absorption of IR radiation = double effect
Other IR absorbing gases: CH4 N2O O3 CFCs Theoretical intensity of thermal IR light that would be leaving the Earth without absorption by greenhouse gases 8-13 µm - window Experimentally measured intensity of thermal IR light leaving the Earth
GREENHOUSE GASES ATMOSPHERIC RESIDENCE TIME, Tavg Tavg = C / R C – total atmospheric amount R – average rate of input or output per unit time
Incoming sunlight Reflection Airborne particle Absorption Effects of aerosols All solids and liquids (atmospheric particles) have the ability to reflect light. The common aerosols are: clouds; ammonium sulphate and other sulphate-based solid aerosols; biomass aerosols such as black carbon or soot. Sulphate aerosols can be from natural oceanic sulphide (e.g. dimethyl sulphide) and anthropogenic sources of SO2. While sulphate aerosols tend to backscatter incoming light, soot adds to positive radiative forcing.
The amount of sunlight reflected into space by anthropogenic aerosols (Watts per m2 of the Earth surface)
Effects of global warming • If CO2 is not reduced,according to UN Environmental Plan and computer modelling, by the year 2040 the average global temp. will be 1 degree higher than at present. This will lead to: • Rainfall increase • Sea level rise by about 18 cm (due to the melting of ice)- less of land • temp. and moisture changes – some ecosystems destabilised • Antarctic and Greenland ice sheets will melt • monsoon rains more heavy • some areas fully dry • and……more unknown consequences!
Country-level climate change impacts for 2080 based on a) Max-Plank model, b) Hadley model, c) Canadian model (Fischer at al. 2001)
Water and CO2 absorb large amounts of IR radiation and have the potential to contribute to a warmer climate than would otherwise exist on the Earth. Several other trace gases – methane, ozone, NO and CFCs – also absorb IR radiation, mostly in the “window” region. All these are called greenhouse gases. Both natural and anthropogenic aerosols contribute an additional warming effect.