Deposition is the phenomenon by which the atmosphere cleans itself. It consists of the interchange of pollutant flows which occur at the earth's surface. It is the mechanism through which pollutants (chemically altered or not) return to the earth.
 
 

In the majority of cases it is efficient enough but there are some pollutants such as CO2 which show signs of a global increase due to the large emissions of pollutants from both natural and anthropogenic sources. There are two types of deposition mechanisms differentiated by the state of

the pollutants:
 
 

• Dry deposition (or deposition by the direct transfer of the gas or particle), when the pollutants are transferred to the earth's surface by gravity or turbulence. Dry deposition can occur through three different mechanisms; diffusion, impactation or sedimentation.

• Wet deposition. When the pollutants are dissolved in water they are absorbed into drops which then precipitate, impacting on the earth's surface as snowflakes or raindrops.

There are two methods for calculating deposition flow from data measured by analyzers:
 
 

• Gradient method; the flow is derived from the vertical concentration gradient and is based on "slow" measurement instruments.

• Eddy correlation method; flows are derived from the covariance of the concentration fluctuations and are based on "fast" measurement instruments .

OBJECTIVESThe principal aim of the project is to develop a piece of software with which to record valid data for the calculation of dry deposition flows for NO-NO2-NOx and SO2.
 
 

The software will be developed based on an existing system which has already been used for several measuring series.
 
 

In order to develop the software it is necessary to previously carry out a series of studies:
 
 

• Firstly, it is necessary to carry out a study of the theory behind the process of dry deposition of pollutants.

• Secondly, it is necessary to investigate the instruments with which we are to perform the measuring. Their theorical principles, performance and interface mechanisms will be studied.

• A study will also be made of the pollutants the software will measure, the main reactions that occur with them in the atmosphere and their effects on animals and plants.

Other subsequent objectives which could follow on from the project would be to carry out a parameterization of the dry deposition flows using the data collected as well as the testing and validating of existing deposition models comparing their predictions with the data produced by the developed software.
 
 

THEORY
 
 

The friction velocity u* quantifies the fluctuations of turbulence velocity in the air, the scale concentration c* quantifies the fluctuations of a pollutant's concentration. From the friction velocity and the concentration scale we define the flow of a pollutant as:
 
 

In neutral conditions, wind speed increases logarithmically above the zero plane, obtaining the standard equation of wind profiles which, adding the stability function FM gives us:
 
 

where k is von Karman's constant, established experimentally at 0.4, z the height above the zero plane and L Monin-Obhukhov's longitude.
 
 

In the same way, the concentration scale c* can be defined as:
 
 

The stability functions FH and FM have been experimentally estimated after much work and can be found in specialist literature.
 
 

Although the flow could be calculated from the equations described above, given the non linearity of the gradient with height, the differential method is particularly unsuitable in certain cases. The correct application of the differential method requires a vertical concentration profile on

three or more points above the surface. If we can only measure at two heights we have an alternative procedure consisting of integrating equations (2) and (3) between points z1 and z2, both above the surface. The resulting equations are:
 
 

in which YM and YH are the integrated stability functions for momentum and heat respectively.
 
 

Therefore, having wind and concentration measurements at two heights (z1 and z2) and knowing L, we can calculate both u* and c* and thus through equation (1) obtain the flow of a pollutant.
 
 

HARDWARE AND SOFTWARE
 
 

In this project we have used the following instrumentation:

• Slow response sulphur dioxide analyzer.
• Slow response NO-NO2-NOx analyzer.
• Industrial PC.
• DT23-EZ data acquisition board (analogic-digital conversion).
• Air suction system.

Physical structure:

In particular, our software records pollutant measures at two heights since u* and L are obtained from a piece of software, developed in parallel, which uses "fast" measurement instruments.

At a fixed time interval, called the cycle time, the software gives the mean of the pollutant measurements recorded at the two levels. The software manages an air suction system at two heights and alternates between the two levels during the cycle time.
 
 

Software structure:

The software requires timing parameters wich are read from a setup file:

• Cycle time: During this time measures are taken from each level. After this period the data received during this cycle are used to calculate the results.
• Wait time: During this time the measures are not used because of the change from a measurement level to another.
• Measurement time: During this time the data from the measurement instruments are added to calculate the results.
• Break time: Time betwen consecutives measures.

Example:

• 2 measurement level.
• Cycle time: 10 minutes.
• Wait time: 3 minutes.
• Measure time : 2 minutes.
• Time betwen measures : 24 seconds.

The software calculations can be obtained through output files, although we can see the data which the software receives at any point in time or the data calculated in the last cycle, through a graphical web interface.
 
 

The software allows to calculate SO2, NO, NO2 and NOx concentration fluxes from the data stored in the output files. Calculated data:

• for each measurement level,
• for each component: SO2, NO, NO2, NOx,
• arithmetic mean, maximum, minimum and standar desviation.

Posible ampliations for the software are:

• Increase the number of measurement level simply configuring a setup parameter.
• Addition of other calculations not yet included.
• Remote software output data download via Modem-GSM.