Biometeorology, ESPM 129, Water, humidity and trace gases 1Lecture 14, Water, Humidity, Pressure and Trace Gases, Part 1 September 28, 2012 Instructor: Dennis Baldocchi Professor of Biometeorology Ecosystem Science Division Department of Environmental Science, Policy and Management 345 Hilgard Hall University of California, Berkeley Berkeley, CA 94720 Topics to be covered: 1. Physical and chemical properties of water 2. Chemical Potential of Water 3. Gas Laws absolute humidity relative humidity virtual temperature Saturation Vapor Pressure Clausius-Clapyeron Equation 4. Rainfall/Drought. L14.1 Introduction Water, in liquid and vapor forms, plays many roles in plant biology. With respect to plant physiology, its foremost function is in hydrating tissue, aiding cell expansion and growth and being a medium for the transfer of nutrients from the soil, through the roots and to the leaves and fruits. How well a leaf is hydrated affects stomatal mechanics directly and indirectly. The turgidity of guard cells is what causes stomata to open. Indirect stomatal responses are triggered by hormonal signals, ABA, which are released when the soil dries, atmospheric humidity deficits and transpiration rates. In turn, how open or closed the stomata are affect transpiration, leaf temperature, the uptake of pollutants and the soil water budget. In regards to biochemistry, water is an active participant in photosynthetic reactions.Biometeorology, ESPM 129, Water, humidity and trace gases 2 In the atmosphere, water exists as a vapor, liquid (rain, dew, fog, clouds) and solid (snow, frost). Atmospheric humidity sets the driving potential for canopy transpiration and soil evaporation. At saturation, suspended droplets form, causing fog and clouds. Clouds are highly reflective and diminish the radiation balance at the earth surface. Aerosols are more included to form when humidity is high. Water plays a role in the formation of OH, the hydroxyl radical, the atmosphere's vacuum cleaner, and hence the atmospheric composition of the atmosphere. Water vapor is one of the most effective greenhouse gases, too. Biometeorologists, plant pathologists and entomologists are interested in humidity because it facilitates the hatching of microbial spores and insect and spider eggs. Atmospheric humidity is governed by biospheric sources and sinks and advection. Sources of humidity include evaporation, transpiration, and sublimation. Losses of atmospheric humidity occur by precipitation, snow, dew and frost. Advection occurs when humid or dry air masses move over a region or by the entrainment of dry air from the troposphere into the planetary boundary layer. Ultimately, water is continuously SoilRootleafatmosphereRboundary LayerRstomataXylemRxylemRroot/soilBiometeorology, ESPM 129, Water, humidity and trace gases 3cycled on earth, between fresh and salt water reservoirs. Precipitation over land either evaporates, runs-off into lakes and rivers or is stored in the soil, glaciers and groundwater. Water that condenses into clouds can eventually return to the land when the clouds rain. The study of this cycling is formally within the domain of hydrology, but biometeorologist work actively in this area through their research on evaporation. L14.2. Physical Properties of Water Water is the main constituent of most plant cells. It can constitute up to 95% of lettuce and less than 5% of dry seeds. Water is of further importance for it is the medium by which the diffusion of solutes in cells and the soil take place. Water, H2O, has a molecular weight of 18 g mole-1. Its melting point is 0 oC. Its boiling point is 100 oC. It is densest at 4 oC. It is incompressible. Its latent heat of vaporization is 2442 J g-1 (at 25 C). On a mass basis, water has one of the highest rates of evaporation of any liquid. The specific heat of water is 1 calorie per gram at 14.5 C or using the preferred S.I. units 4.184 J g-1. Table 1 Summary of the 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 The arrangement of the water molecule is unique, compared to other low weight compounds (e.g. CO2, CH4). The atoms H-O-H are not arranged in a straight line.(180 degrees). Instead, they form an angle of 105 degrees, forming a polar molecule.Biometeorology, ESPM 129, Water, humidity and trace gases 4 Figure 1 source www.princeton.edu/~chm112/project/ water%20molecule.gif The hydrogen atoms approximate naked protons on the surface of the oxygen atom. The net charge is zero, but the arrangement of atoms form a slightly positive charge on one side of the molecule and a negative one on the other side. This arrangement allows hydrogen bonding. It enables water to remain liquid at room temperature and minimizes vaporization. This is an exception for low weighted molecules. Consequently, water is a very effective solvent, due to its small molecular size, its polar characteristic and its high dialectric (80) constant. The high dielectric constant of water is a property that is exploited when measuring soil moisture with a time domain reflectometer. Table 2 Temperature dependent properties of water (Campbell, Norman, 1998) 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 L16.3 Chemical Potential of Water From everyday experience we know that water moves from regions of high to low elevations, as observed by watching a water fall, or the pure to salty regions as fresh dilutes sea water as it crosses a hydraulic barrier. The concept of chemical potential quantifies the driving force for movement of water between two locations. The concept can also be used to determine whether a substance will undergo a chemical reaction. The chemical potential has units of energy (J mol-1). The chemical potential of a chemical species is a function of its concentration, the pressure of the system, electrical