Experiment # 6 - Precipitation Sampling

The purpose of this experiment is to gain experience sampling precipitation and analyzing its composition. Because the likelihood of precipitation occurring within the timeframe of our regular class schedule is small, you are asked to complete this project independently. You may choose to work alone or in groups. As for all lab experiments, be sure to document your actions thoroughly.

You also are provided with a choice of sampling methods. You may choose to obtain sequential precipitation samples during a precipitation event or you may conduct snow pack sampling. If you sample falling precipitation, keep in mind that the highest concentrations and the most interesting temporal changes usually occur during the first couple hours of the event. You should attempt to obtain 4-6 samples that span that timeframe. If you conduct snowpack sampling, you may choose to sample the snowpack at a few locations (perhaps varying in elevation), or you may choose to attempt to sample separate events (layers) within a given snowpack. Keep in mind that material can accumulate on the surface of the snowpack following an event (through dry deposition of particles and gases) and that partial melting of the snowpack may "flush" many of its ions out the bottom.

Materials Needed (varies depending on your approach):

· Nanopure Water and Wash Bottle

· Sampling Apparatus (perhaps scoops, large plastic bottles, plastic funnels)

· Sample Bottles (for Sample Storage)

· Permanent Marker and Labels

· pH Meter Kit

· Pipettes and Tips

Procedure:

1. Choose your sampling approach (and partner if desired). Make arrangements for a sampling campaign (perhaps in conjunction with the weather). When planning your strategy, keep in mind the points made at the top of the page.

2. Wash all sampling equipment in Nanopure water. Rinse all sampling equipment with a small portion of Nanopure water to obtain blanks. Label and store the blanks in the refrigerator.

3. Collect precipitation samples. It is best to do your sampling away from tall objects (e.g., buildings, trees). There really are no special tricks. Just be sure to prevent contamination. Describe your method in detail in your lab notebook. If you are collecting a time series of samples, you may want to record observations about precipitation intensity and ice crystal habit (shape and degree of riming) if relevant. These may help you explain changes in precipitation chemistry. Store and label your samples appropriately. You should probably melt your samples (assuming they are snow) and refrigerate them in plastic vials/bottles. You need to melt the entire sample to ensure a representative measurement of pH (some species tend to partition between the liquid and solid phases in a mixture).

4. pH measurement should be completed within 24 hours of sampling (this is less critical if you sample the accumulated snowpack and you keep your samples frozen). Naturally occurring microorganisms can consume organic acids and raise pH within a couple of days. Remove small portions of your samples for measurement of pH (you don't want the pH probe to contaminate your primary sample). Measure and record the pH of your samples. Be sure your samples and calibration buffers are at the same temperature.

5. Refrigerate the rest of your samples in case you have an opportunity to analyze them by ion chromatography. Consult with the TA about possibilities for sample IC analysis.

Discussion:

1. How do your sample pH values compare to those expected in a pristine atmosphere?

2. What species do you believe may have influenced the pH of your samples (if you have an opportunity to have the ionic composition of your samples measured you can of course be more authoritative about this)? What sources might contribute to the presence of these species in the local atmosphere?

3. How did your sample compositions vary in time or space? What explanation can you offer for the observed patterns?