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Table of Contents
Zenith Sky Chart Coefficient Generation SOP for the Brewer Spectrometer
Introduction
The Brewer spectrophotometer utilizes two main methods for determining column ozone, direct sun (DS) and zenith sky (ZS). The DS measurements are processed using Beer’s Law however the ZS measurements are processed using an empirical parametric equation with 9 parameters. These parameters can be calculated using large data sets of near simultaneous DS and ZS measurements that were conducted over a wide range of ozone values and air mass factors. The nine parameters used in the equation describe the relationship between the total ozone column (TOC) and the ZS measured values. These relations include effects from the instrument and from the sky. By modeling the effects from the sky, it is possible to separate the two effects and then the effects from the instrument can be represented as a linear correction to the sky effect. This process is described in detail in Fioletov et al, 2011.
Method
Data Processing
New zenith sky coefficients should be done with data after a transfer or absolute calibration has been done with up to 1 year of data but can be performed with a few months of processed, cleaned DS O3 with ZS data. It is important that the files must contain both the cleaned DS and ZS data for creating new constants as the calibration needs to fit the ZS data to the DS.
You will have to merge the .dat files using the merge function under ‘tools’ drop-down menu in the matlab software. Select the files you wish to combine and press okay. Upload the file to be used for the calibration in the 'clean o3 data file' section of the 'Sky Calibration tab'.
Select which instrument you wish to calibrate and then select the appropriate region that the instrument operates in to use the correct albedo for creating the new coefficients.
You then select the calculation parameters. Most of the preset options are standard and what you will use to create the new ZSF file, you will need to select the correct date range. Use a date that starts close to the most recent calibration to insure the most accurate DS data is used to create the new coefficients.
Select 'Run ZSF fitting' to begin the program. After you've selected to run the program you will jump to the 'Sky Calibration Plot' page of the software. Once the process is complete the graphs will populate with data to show how the new ZSF calibration will change the fitting of the ZS data.
