Composition and structure of the atmosphere

Transcription

1 Composition and structure of the atmosphere

2 Atmosphere Earth s atmosphere is a thin layer of gas held around the surface by gravity. 90% of the atmosphere s mass is within 15 km of the earth s surface Earth's radius is about 6400 km The atmosphere is like a layer of paint on a basketball

3 Atmosphere The atmosphere protects life on Earth by: absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), And reducing temperature extremes between day and night (the diurnal temperature variation). The Mass of the atmosphere: It is about kg, three quarters of which is within about 11 km of the surface.

4 Atmosphere The Earth's atmosphere is characterized by variations of temperature and pressure with height. In fact, the variation of the average temperature profile with altitude is the basis for distinguishing the layers of the atmosphere.

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6 Atmospheric Structure Troposphere. The lowest layer of the atmosphere, extending from the Earth's surface up to the tropopause, which is at km altitude depending on latitude and time of year (Higher near equator and lower towards poles); characterized by decreasing temperature with height; rapid vertical mixing (baring BL). Stratosphere. Extends from the tropopause to the stratopause (From ~ 45 to 55 km altitude); temperature increases with altitude. Mesosphere. Extends from the stratopause to the mesopause (From ~ 80 to 90 km altitude); temperature decreases with altitude to the mesopause, which is the coldest point in the atmosphere.

7 Composition of the Atmosphere

8 Atmosphere The very cold temperature of the tropopause layer at the top of the troposphere serves as a barrier that causes water vapor to condense to ice so that it cannot reach altitudes at which it would photodissociate through the action of intense highenergy ultraviolet radiation. If this happened, the hydrogen produced would escape the earth s atmosphere and be lost. The atmospheric layer directly above the troposphere is the stratosphere, in which the temperature rises to a maximum of about -2 C with increasing altitude.

9 Atmosphere The heating effect is caused by the absorption of ultraviolet radiation energy by ozone. Ozone serves as a natural atmospheric filter to prevent this light from reaching the surface, thereby protecting Earth's life from damages. The upper regions of the mesosphere and higher define a region, called the exosphere, from which molecules and ions can completely escape the atmosphere. Extending to the far outer reaches of the atmosphere is the thermosphere, in which the highly rarified gas reaches temperatures as high as 1200 C by the absorption of very energetic radiation of wavelengths less than approximately 200 nm by gas species in this region.

10 Atmosphere In Thermosphere temperatures increase again with altitude due to absorption of strong UV solar radiation by N2 and O2. The troposphere and stratosphere account together for 99.9% of total atmospheric mass and are the domains of main interest from an environmental perspective. The fraction of total atmospheric weight located above altitude z is P(z)/P(0). At 80 km altitude the atmospheric pressure is down to 0.01 hpa, meaning that % of the atmosphere is below that altitude.

11 It has become common use in atmospheric chemistry to describe volume mixing ratios by the following units: Volume mixing ratio= 1mL/1m 3 =1 ml/10 6 ml = 10-6 E.g., 1 ml SO2 in 10 6 mlof Air Unit Conversion parts per million (ppm) 10-6 μmol mol-1 parts per billion (ppb) 10-9 nmol mol-1 parts per trillion (ppt) pmol mol-1 These quantities are sometimes distinguished by an added v (for volume) and m (for mass), that is, Ppmv parts per million by volume Ppmm parts per million by mass Another common unit used to measure the concentration is the unit of mass per unit volume. The interconversion of these units is simple as exemplified later.

12 Ideal Gas Equation The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases. The ideal gas law is often written as: P V = nrt where the letters denote pressure, volume, amount, the ideal gas constant, and temperature of the gas, respectively. The value of R can be expressed in a number of units, the most useful in environmental chemistry relate to litres and atmospheres. Therefore: It is helpful to estimate the volume of gas at any temperature and Pressure, The most useful value to remember is at STP

13 what is the concentration of 1ppm by volume of Ozone expressed in g/m 3 at 25 C and 750 mm Hg pressure? Solution: We know at STP 1 mol of ozone occupies L Using Ideal gas equation Where P1 = 760 mm Hg, T1 = 273 K and P2 = 750 mm Hg, T2= 298 K We get: V2= L Mol wt of Ozone = 48 Now In these conditions L contains 48 g of Ozone So 1 L (1000 ml) will contain g of Ozone I ml will contain g of Ozone This 1 ml of Ozone in 1 m-3 of air can be expressed as

14 Atmospheric Scales Of Motion Four rough categories of atmospheric scales of motion: 1. Microscale. Phenomena occurring on scales of the order of m, such as the meandering and dispersion of a chimney plume 2. Mesoscale. Phenomena occurring on scales of tens to hundreds of kilometers, such as land-sea breezes, and fronts. Fig: Spatial and temporal scales of variability for atmospheric constituents. 3. Synoptic Scale. Motions of whole weather systems, on scales of hundreds to thousands of kilometers. 4. Global Scale. Phenomena occurring on scales exceeding km.

15 Spatial scales characteristic of various atmospheric chemical phenomena are given in the Table There is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosolclimate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, (4) tropospheric oxidative capacity and stratospheric ozone depletion.

16 Fate of air pollutants and role of atmosphere

17 Hazardous air pollutants Photolysis Chemical reaction with OH radical, nitrate radical and O3. Reaction with OH radical and O3 is predominant. Photolysis is chemical fragmentation or rearrangement of a chemical upon the adsorption of light of appropriate wavelength. Photolysis is only important during daytime only for those chemicals that strongly absorb light. Formaldehyde and acetaldehyde form as a result. Major removal mechanism is OH abstraction of addition. Product of reaction would be CO2 and CO. Atmospheric lifetime generally less than 1 day.

18 Nitrogen Dioxide

19 Nitrogen Dioxide

20 Photochemical oxidants Unlike other pollutants the photochemical oxidants results entirely from atmospheric reaction. Thus they are called secondary pollutants. O 3 is primary photochemical oxidant. O 3 formation is basically attributed to the nitrogen dioxide photolytic cycle. Hydrocarbon modifies this cycle by reacting with atomic oxygen to form free radicals (highly reactive organic species)

21 Sulfur oxides Sulfur oxides may be both primary and secondary pollutants. Power plants, industries, volcanoes and ocean emit SO 2, SO 3, and SO 4 2- directly as primary pollutants. Biological decay process and some industrial sources emit H2S, which is oxidized to for SO Pg (Natural sources) and 45 Tg (anthropogenic sources)

22 Problem An coal power plant burning coal at the rate of 1.00 kg/s. If the analysis of coal reveals a sulfur content of 3% what is the annual rate of emission of SO 2. sulfur content of the ash 5% of input sulfur

23 Fate of SO2 in the atmosphere

24 IIT Delhi

25 Particulate Matter Particles come in different shapes and sizes Particle sizes Ultra-fine particles (<0.1 µm) Fine particles (0.1 to 2.5 µm) Coarse particles (2.5 to 10 µm) PM10 Section 7 Chemical Aspects of Air Pollution Crustal material 29 Carbon chain agglomerates AREP GAW, WMO Report

26 Particulate Matter Composition Geological Material suspended dust consists mainly of oxides of Al, Si, Ca, Ti, Fe, and other metal oxides Ammonium ammonium bisulfate, sulfate, and nitrate are most common Sulfate results from conversion of SO 2 gas to sulfate-containing particles Nitrate results from a reversible gas/particle equilibrium between ammonia (NH 3 ), nitric acid (HNO 3 ), and particulate ammonium nitrate NaCl salt is found in PM near sea coasts and after de-icing materials are applied Organic Carbon (OC) consists of hundreds of separate compounds containing mainly carbon, hydrogen, and oxygen Elemental Carbon (EC) composed of carbon without much hydrocarbon or oxygen. EC is black, often called soot. Liquid Water soluble nitrates, sulfates, ammonium, sodium, other inorganic ions, and some organic material absorb water vapor from the atmosphere 31 Chow and Watson(1997)

27 Particulate Matter Chemistry Sources Chemical Processes Mechanic al Sea salt Dust Combusti on Motor vehicles Industrial Fires Other gaseous Biogenic Anthropoge nic Emissions Particles NaCl Crust al Particles Soot Metals OC Gases NO x SO 2 VOCs NH 3 Gases VOCs NH 3 NO Meteorological Processes x Winds Temperature Solar radiation Vertical mixing PM Formation gases condense onto particles cloud/fog processes condensation and coagulation photochemical production cloud/fog processes Clouds, fog Temperature Relative humidity Solar 32radiation PM Transport/Loss transpo rt sedimentation (dry deposition) wet deposition Winds Precipitation Temperature Relative humidity Winds Sample Collection Measurement Issues Inlet cut points Vaporization of nitrate H 2 O, VOCs Adsorption of VOCs Absorption of H 2 O AREP GAW, WMO Report

28 Comparison of CPCB (India) and WHO Standards for Particulate Air Pollution R. Gopalaswami et al. A Study on Effects of Weather, Vehicular Traffic and Other Sources of Particulate Air Pollution on the City of Delhi for the Year Journal of Environment Pollution and Human Health, 2016, Vol. 4, No. 2, doi: /jephh Component of Particulate Air Pollution 24-Hr Annual CPCB (India) WHO CPCB WHO Particulate matter (PM 10 ), µg/m µg/m3 AQI 174 Unhealthy 50 µg/m 3 AQI 137 Unhealthy to Sensitive Groups 60 µg/m 3 AQI = 153, Unhealthy 20 µg/m 3 AQI =68 Moderate Particulate matter (PM 2.5 ), µg/m 3 60 µg/m3 AQI 153 Unhealthy 25 µg/m 3 AQI =78, moderate AQI =112, Unhealthy to sensitive groups 10 µg/m 3 AQI = 42, Good

29 Table. Monthly Average Particulate Air Pollution (PM 2.5) over a 3-year period in Delhi and Beijing Summer Season Winter and Early Summer Monsoon (Rainy) Season in India Winter Month Site Not Specified Measured Values of Particulate Air Pollution (PM 2.5) In Micrograms/ cubic-metre Site Not Specified Site at Chanak yap uri Delhi Beijing Delhi Beijing Delhi Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec R. Gopalaswami et al. A Study on Effects of Weather, Vehicular Traffic and Other Sources of Particulate Air Pollution on the City of Delhi for the Year Journal of Environment Pollution and Human Health, 2016, Vol. 4, No. 2, doi: /jephh

30 Venkataraman et al. Atmos. Chem. Phys., 18, ,

31 Ozone Presence of O3 in the upper atmosphere km and up provides a barrier to UV radiation. Small amount that do seep through provide you with your summer tan. To much UV will cause skin cancer. O2 also serves as barrier to UV radiation, it absorbs only over a narrow band centred at a wave length of 0.2 m.

32 Ozone destruction Photoreaction of O3 and O3 destruction by chlorofluorocarbon

33 Ozone destruction

34 Helsinki Declaration Eight countries met in Helsinki, Finland in the spring of 1989 and Helsinki declaration took place as follows. All joint the 1985 Vienna Convention for the protection of O3 layerand the follow up Montreal protocol. Phase out production and consumption of O3 depleting CFCs no later than Phase out production and consumption as soon as feasible of halons and such chemicals as carbon tetrachloride and methyl chloroform that also contribute to O3 depletion. Commit themselves to accelerated development of environmentally acceptable alternative chemical and technology. Make relevant scientific information, research results and training available to developing countries.

35 Alternative to CFCs Hydrofluorocarbons (HFCs) Hydrochlorofluorocarbons (HCFCs) In contrast to CFCs HFCs and HCFCs contain one or more C-H bonds. This makes them susceptible to attack by OH radicals in the lower atmosphere. HFCs do not contain chlorine they do not have the O3 depletion potential. HCFCs contains chlorine, this chlorine is not transported to the stratosphere because of OH scavenging in the troposphere is relatively efficient.