The Solar Company, Philadelphia, PA, sells a high quality, high precision sun photometer.  With careful calibration (the instrument is initially calibrated at Mauna Loa, Hawaii, against Dobson 83, the world ozone standard),  accurate measurement of stratospheric ozone is routinely attainable. This calibration may drift slightly over time, and must be validated against subsequent accurate ozone measurements to verify the calibration.
A close-up of the instrument shows a digital readout and a simple menu for entering set-up information like the exact UT time, GPS location, and the calibration constants which allow the instrument to calculate total ozone as the measurements are taken and stored in Microtops.  800 separate measurement can be stored. Alternately, the data can be downloaded into a spreadsheet and ozone calculated independently using the same equations.  A recent article in the Journal of Geophysical Research describes the specifications of the instrument and the equations used for calculating ozone [M. Morys, F.  Mims  III, S. Hagerup, S. E.  Anderson, A.  Baker, J.  Kia,  and T.  Walkup, "Design, Calibration, and Performance of MICROTOPS II Handheld Ozone Monitor and Sun Photometer," J. Geophys. Res., 106, 14573 (2001)].	Experimental Determination of Ozone 1.  Collect data at 3 l's - 305, 312 and 320 nm  2. Find latitude and longitude - USGS map or GPS  3. Know date and local time  4. Determine the angle of the sun (measure or calculate)  5. Calculate Ozone
	To calibrate Microtops, choose a clear sky day between mid-May and mid-September, so that it is possible to obtain ozone measurement in an airmass range of 1 - 4.5 Find a location away from urban pollution.  A mountain peak is ideal, although it is difficult to completely avoid haze due to humidity.  The most ideal day for calibration follows a rainstorm which cleans the air of dust and is dominated by high pressure, provided that the weather front is past and ozone is very constant.
My students have done an excellent job calibrating Microtops instruments over a three year period.  It is easy to collect data.  Mornings are best, starting about 7 AM standard time and continuing to 10:30.  Data taken between 11 AM and solar noon shows a bit of scatter due to thermal effects in the atmosphere and should be avoided.
Here is how we calculate ozone:                         Ozone (DU) =   [ln(Io1/Io2) - ln(I1/I2) - (b305-b312). (Pm/1013)]/(a305-a312). m                                                                                          where             b = Rayleigh scattering cross sections = 1.787 x 1010.l-4.25              m = airmass                m = mu              a = ozone absorption cross sections              P = pressure in millibar             l = filter wavelength
	This graph shows a Langley plot (ln(I) vs mu) of Mauna Loa data for Microtops SN 5369.  These plot have correlation coefficients of 0.997 - 0.999.
If we do regression analyses for these plots, we can determine the slope and intercept of these lines.  The intercepts give the "extraterrestrial constants" needed to calculate ozone and the slope allows us to determine the best alpha and beta coefficients to match a standard ozone value, say from a Dobson spectrometer.  These are the "calibration constants" for the instruments.	Microtops Calibration Constants   305/312 312/320 DL 1.481 0.6819 Db 0.09494 0.08625 Da 2.863 1.122
Let's look at an inter-comparison of four different Microtops spectrometers calibrated at Mauna Loa in June, 2001 on two different September days in Santa Barbara, California. Note the agreement with one another and with the EPTOMS ozone earth satellite.  The standard deviation of each instrument is less than two percent for each channel pair; from experience the best channel pair to calculate ozone is the 305/312 nm pair.  Haze can effect the 320 nm channel, so it is usually not used. *Microtops #4708 was an independent standard instrument calibrated at Hohenpeissenberg, Germany in June, 2001. **EPTOMS data is obtained by downloading ASCII files for the date: see  ftp://jwocky.gsfc.nasa.gov/pub/eptoms/data/oz2001	Inter-comparison of Several Microtops II Spectrometers     9/1/01     9/8/01    Ser. No. 305/312 312/320 305/320 305/312 312/320 305/320 5369 284.2 284.3 285.2 290.2 290.2 290.3 5373 287.1 286.9 286.2 288.8 291.7 299.3 5374 282.3 283.7 287.0 284.9 286.9 291.8 5375 285.1 285.9 288.1 288.4 288.4 293.8 4708* 286.6 284.4 278.9 288.6 286.6 282.1 EPTOMS** 290     287
	Not bad!!