Measuring Tropospheric Ozone

1. Schönbein Paper Preparation

1. Place 100 mL of distilled water in a 250-mL beaker.
2. Add 1¼ teaspoon of cornstarch.
3. Place a stir bar in the beaker, and place the beaker on a hot/stir plate. Heat on a medium to high setting, and stir the mixture slowly until it gels near approximately 90 °C. The mixture is gelled when it thickens and becomes somewhat translucent.
4. Remove the beaker from the heat source, add ¼ teaspoon of potassium iodide, and stir well.
5. Cool the solution before applying to the filter paper.
6. Lay a piece of filter paper on a glass plate or hold it in the air, and using a small paint brush, carefully brush the paste onto the filter paper. Turn the filter paper over and do the same on the other side. Try to apply the paste as uniformly as possible.
7. Set the paper out overnight and away from sunlight, or place in a low temperature (20 °C) drying oven to dry.
8. Once the paper is dry, use scissors to cut the filter paper into 1-inch wide strips. If storing the paper for later use, place the strips in a sealable plastic bag or glass jar, out of direct sunlight.

2. Ozone Measurement

1. Spray strips of test paper with distilled water and hang a minimum of three strips at each data collection site out of direct sunlight. Hang by securely taping or tacking one end of the strip to a structure (e.g. wall, parking blocks, light poles). Strips can also be hung using wire with one end driven into the ground and the other end secured to the strip. Ensure the strips hang unobstructed.
2. Expose the paper for approximately 8 h. Note where each strip was hung, and use weather data to record relative humidity location during paper exposure. Alternatively, a psychrometer can be used to measure relative humidity of each site.
3. If the results will not be recorded immediately, seal the strip in an airtight container after exposure.
4. To observe and record test results, spray the paper with distilled water.
5. Observe the color by comparing it to the provided color scale and recording the corresponding Schönbein number. Calculate the average Schönbein number for each site.
6. Use the relative humidity data for each site and the Relative Humidity Schönbein Number Chart (Figure 3) to convert Schönbein site averages to ozone concentration (ppb).

Figure 3. Relative Humidity Schönbein Number Chart