10 1 2012 Lecture 14 Water Humidity Pressure and Trace Gases Part 1 Physical and chemical properties of water Gas Laws Chemical Potential of Water Rainfall Drought Biometeorology ESPM 129 Atmospheric Gas Composition constitue nt percent by volume percent by mass molecular weight g mole 1 N2 78 091 75 5 28 02 O2 20 95 23 1 32 00 Ar 0 930 1 3 39 94 CO2 0 036 0 05 44 01 H2O 18 Biometeorology ESPM 129 1 10 1 2012 Water Gas Liquid Solid Biometeorology ESPM 129 Soil Plant Atmosphere Water Continuum 50 MPa atmosphere Rboundary Layer Rstomata leaf 0 7 MPa Xylem Water Moves UPWARD because it flows DOWNHILL Energetically Rxylem 0 2 MPa Root Rroot soil Soil 0 05 MPa ESPM 129 Biometeorology 2 10 1 2012 Triple Point for Water http chemwiki ucdavis edu api deki files 7399 f copy png Biometeorology ESPM 129 Phase Changes of Water How can we Exploit the Triple Point Curve to prevent water from condensing on our air sampling tube http www laetusinpraesens org musings images climate files phase1 jpg Biometeorology ESPM 129 3 10 1 2012 Properties of Water Property Value molecular weight 18 g mole 1 melting point 273 15 K boiling point 373 15 K latent heat of vaporization 2 442 MJ kg 1 or 44 00 kJ mol 1 at 20 C latent heat of sublimation 2 826 MJ kg 1 or 51 00 kJ mol 1at 0 C dialectric constant 80 thermal conductivity 0 599 W m 1 at 20 C heat capacity of water 4182 J kg 1 at 20 C molecular diffusivity water in air 2 42 10 5 m2 s 1 at 20 C Density 1 000 kg m 3 at 4 C Biometeorology ESPM 129 Properties of Water f T temperature density latent heat of evaporation kinematic viscosity oC Mg m 3 kJ mol 1 mm2 s 1 0 0 99987 45 0 1 79 4 1 0000 44 8 1 57 10 0 99973 44 6 1 31 20 0 99823 44 1 1 01 30 0 99568 43 7 0 80 40 0 99225 42 8 0 66 Biometeorology ESPM 129 4 10 1 2012 Measures of Atmospheric Moisture Water Potential Pa Relative Humidity Dew Point Temperature Wet Bulb Temperature Vapor Pressure Pa Vapor Pressure Deficit Pa Density mole m 3 Mixing Ratio Biometeorology ESPM 129 Charles Law and Boyle s Law pressure volume and temperature are inter related PV nRT R is the universal gas constant 8 3144 J mol 1 K 1 T is absolute temperature K P is pressure Pa N m 2 V is volume n is the number of moles Biometeorology ESPM 129 5 10 1 2012 the molar volume of air volume occupied by one mole of air n 1 for standard pressure 0 1013 MPa 101 3 kPa 1013 mb and temperature 273 15 K V nRT P 22 41 liters mole 1 or 0 02241 m3 mol 1 Biometeorology ESPM 129 Gas Law used by Meteorologists P RT m P RT m m is the molecular weight of a gas g mol 1 is the mass density g m 3 is 1 Biometeorology ESPM 129 6 10 1 2012 Partial Pressure Law P pN 2 pO2 p Ar pCO2 pH2O R p T m a ma v mv c mc p RT Biometeorology ESPM 129 Air Density P e ma e mv RT RT T dry Saturated oC Kg m 3 kg m 3 0 1 292 1 289 5 1 269 1 265 10 1 246 1 240 15 1 225 1 217 20 1 204 1 194 25 1 183 1 169 30 1 164 1 145 40 1 128 1 096 Does a baseball fly Less far on a humid day Biometeorology ESPM 129 7 10 1 2012 Water Potential of atmospheric humidity is function of its mole fraction 400 R T e ln a p Vw P 450 MPa 500 550 600 650 700 R Universal gas constant T absolute temperature ea vapor pressure P pressure Vw molal volume of water 18 05 10 6 m3 mole 1 0 5 10 15 20 25 30 35 40 ea hPa Biometeorology ESPM 129 mass gas density grams of a gas molecule per unit volume c nc mc pc mc V RT nc is the number of moles V is volume R is universal gas constant mc is the molecular mass g mole 1 of the compound Biometeorology ESPM 129 8 10 1 2012 molar gas density moles of a gas molecule per unit volume c nc p c c V RT mc nc is the number of moles V is volume mc is the molecular mass g mole 1 of the compound Biometeorology ESPM 129 Mass fraction mass per unit mass of air Mc c c mc ma a ma is the molecular weight of dry air g mol 1 mc is molecular weight of trace gas a is the mass density of dry air g m 3 c is the mass density of trace gas air g m 3 Biometeorology ESPM 129 9 10 1 2012 Mole fraction the number of moles of a trace gas divided by the total number of moles present in the mixture Moist air nc ma c pc n mc p Cc Dry air Cc nc p pc c nd pd pa pH2O Biometeorology ESPM 129 Mixing Ratio of Water Vapor Saturated Atmosphere Saturation vapor pressure T dry o Kg m 3 g m 3 0 1 292 4 85 5 1 269 6 8 10 1 246 9 4 15 1 225 12 07 20 1 204 17 31 25 1 183 30 40 C Mixing Ratio mmol m 3 ppt 269 377 6 02 522 670 12 1 23 1 23 06 961 1281 1 164 30 4 1688 41 9 1 128 51 2 2844 72 9 8 60 15 8 31 3 Biometeorology ESPM 129 10 10 1 2012 Saturation Vapor Pressure When a pool of water is at constant temperature in a closed container some water molecules are leaving the liquid and others are condensing and returning to the liquid Molecules in the head space exert a partial pressure on the system The equilibrium vapor pressure that occurs is called the saturation vapor pressure It is a function of temperature and is independent of pressure Biometeorology ESPM 129 Saturation Vapor Pressure Temperature Function bTc es Tc a exp c Tc Over Water a 0 611 b 17 502 c 240 97 kPa Over Ice a 0 611 b 21 87 c 265 5 14 12 es T kPa 10 8 6 4 2 0 0 10 20 30 40 50 60 T oC Biometeorology ESPM 129 11 10 1 2012 Boiling Point Pressure equals Saturation Vapor Pressure Saturation Vapor Pressure Temperature 120 Saturation Vapor Pressure kPa Sea Level 100 80 Mountains 60 40 20 0 0 20 40 60 80 100 120 Temperature C Biometeorology ESPM 129 Relative Humidity hr the ratio between the actual ea and saturation vapor pressures es T It ranges between zero and 1 0 with one indicating saturation 1 0 …