Presentation on theme: "Microscale structure of air ion spatial and temporal distribution: some problems Hyytiälä 20060315"— Presentation transcript:
Microscale structure of air ion spatial and temporal distribution: some problems Hyytiälä 20060315 Hannes.Tammet@ut.ee
CONTENT Plain electrode effect Distribution of ionization rate Distribution of ion concentration Fluctuations of ion concentration Measurements in Hyytiälä 2005 Vertical profile of mobility? What should be measured? How to start design of an instrument? A simple example: the integral counter
h h E EE EoEo Air-Earth vertical current j = E does not depend on height. Conductivity near ground + is about a half of free air conductivity + + . Conclusion: E o 2E . Free air electric field is about 100 V/m. It follows electric field on the ground surface about 200 V/m. The difference is created by space charge carried by cluster ions and aerosol particles. = o E 100 V/m corresponds to 5.5×10 5 e/cm 2. This is about 1800 e/cm 3 distributed to 3 m. A cluster ion in 150 V/m field travels during 2 minutes of its life about 3 m.
The role of aerosol particles in electrode effect: see a paper by Tapio J. Tuomi Tuomi, T.J. (1980) Atmospheric electrode effect: approximate theory and wintertime observations. PAGEOPH, 119, 31-45. Advanced theory including turbulent diffusion: see papers by John C. Willett, e.g. Willett, J.C. (1983) The turbulent electrode effect as influenced by interfacial ion transfer. JGR, 88, 8453-8469.
Distribution of ionization rate Ionizing agents: cosmic rays gamma radiation from ground alpha radiation from air alpha radiation from surfaces beta radiation from surfaces
Distribution of ion concentration Concentration of cluster ions depends on the ionization rate and on the ion sinks. The sinks are: cluster ions of opposite polarity ( 10 – 20%), aerosol particles ( 50 – 90%), surfaces like conifer needles ( 0 – 40%). The distribution of ion concentration in still air is shaped mostly by the profile of the ionization rate.
Fluctuations of ion concentration The ions from local sources are distributed by wind like smoke from fire. Measurements (July-August 1963) with an aspiration counter providing = 0.4 s (flow rate 1.5 m 3 s –1 ). Parameters of fluctuations: These parameters depend on the time step t i+1 – t i
What should be measured? Cluster ions at height of 14 m are partially originated from forest and partially from upper air. We need additionally measurements at height of 67 m and at different heights inside of forest. Multipoint measurements with universal mobility spectrometers are expensive and troublesome. When analyzing the measurements only few integral parameters of mobility distribution are used like the cluster ion concentration and average mobility, concentration of ion of sizes 1.5…3 nm and 3…7.5 nm. Measuring of 4 parameters is expected to be cheaper than measurement of 16 mobility fractions.
How to start design of an instrument? V1V1 V2V2 V=0 y2y2y1y1 sheat air (?) Parameters of analyzer x 1, x 2, … Signals y 1, y 2, … Quantities to measure z 1, z 2, … When x is fixed, the regressions z i = f i (y 1, y 2,…) can be evaluated using existing data set of mobility distribution measurements. When real measurements and regression estimates are known, a summary error of estimates can be evaluated. Next the components of x can be modified and determined the effect of modification on . Finally the minimum problem of on the space of technically possible values of x can be posed and solved. Manufacturing expenses can be included as a factor of objective function.
Simple example: the integral counter N cluster = y n / ( e) Z cluster = const / V x ? = f (y 3 – y 2 ) y ynyn VxVx V1V1 V2V2 V3V3 y1y1 y2y2 y3y3 V