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.

To begin making the new zenith sky coefficients you must first create a test file directory and then a unique file in that directory for each brewer being analyzed. You can host the test file wherever is most convenient for you, ie /Desktop or /Documents. Populate the new brewer file with the following;

• ds2zs.exe
• ds2zs.cfg
• ICF file for the Brewer being analyzed
• Relevant B-files for the brewer being analyzed

Open the ds2zs.cfg file in the test directory you created with notepad or another text editor. Edit the file with the following parameters:

• files: change the b file naming to reflect the brewer being analyzed as displayed above
• icf: enter the icf file to use in re-processing
• zsf: refers to one of the following 4 models based on location. Set to &II
• dtime: maximum time difference between ZS and DS (default 10)
• min mu: minimum air mass factor for DS/ZS pairs (default 1.2)
• max mu: maximum air mass factor for DS/ZS pairs (default 4.2)
• max std: max DS/ZS ozone standard deviation within the measurement (default 2.5)
• 03 height above station (km): ozone layer height above the station (default 22km)
• Rayleigh height (km): effective air height for Rayleigh calculations (default 5km)
• Difference between 03ETC and SLR6: the difference between O3ETC and SLR6 at calibration. This number can be found in the brewer’s icf file near the bottom of the text file. Refer to figure 3.

Save the newly updated cfg file to the test directory and rename it ds2zsxxx.cfg.

Highlight and drag the newly created cfg file and drag it into the ds2zs.exe file.

The ds2zs.exe program will execute its routine and create two files in the test directory.

• ds2zs.dat: file contains the ozone data that was used in the processing and reprocessed ZS data with the new calculated coefficients.
• ds2zs.zsf: file contains the newly calculated coefficients and statistics for the ZS processing. This file is used by the operation Brewer software once it has been renamed.

If the program does not run or create the two files above run the program in a command line. To do this type ‘cmd’ into the navigation bar in the ‘Test’ directory you created or navigate with a new command terminal. Once in the correct directory write ‘ds2zsxxx.cfg ds2zs.exe’ and the program will run in your command terminal. From here you can see why the program failed. In cases where it does not provide an explanation it is possible that the program did not create the two new files because there is not enough data for the coefficients.

Rename the ds2zs.zsf file to zsfJJJyy.xxx. Once this is done repeat for every brewer you wish to analyze in the respected folder.

Once a new zsf file is created it is important to make sure that the newly created coefficients and statistics for the ZS processing improves the results for ZS measurements. To do this the old zsf file and new zsf file must be compared in BFilePro or another data processing software. For this example we will be using BFilePro and Brewer #039.