EXPERIMENT # 2

Precipitation Sampling

 

OVERVIEW

 

The purpose of this experiment is to gain experience sampling precipitation and analyzing its composition. You will become acquainted with the operation of a state-of-the-art modern automated precipitation collector similar to those used in the National Acid Deposition Program (NADP) network.  As in NADP network operation, we will collect weekly integrated samples.  Each student will have an opportunity to care for the sampler for one week during the semester.  As for all lab experiments, be sure to document your actions thoroughly in your lab book.  Information about samples from all class weeks will be assembled for use in preparing your lab report.

 

 

Materials Needed:

 

Nanopure Water and Wash Bottle

Automated Sampling Apparatus (stationary)

2 Sampling Buckets

Sample Bottles (for Sample Storage)

Permanent Marker and Labels

pH Meter Kit

Pipettes and Tips

Aluminum Foil

Lab book

Palm pilot for downloading event data (at the end of your sampling week)

Precipitation sampling log sheet

 

 

Procedure:

 

Each student will be responsible for one week of sampling during the semester. Plan on retrieving your sample before or after class on Wednesdays.

 

Wash all sampling equipment that will be in contact with the sample 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 in the class laboratory. Weigh your sample bottles before sampling and note the empty weight (with cap) on the bottle with a marker.

 

Collect precipitation samples, weigh, and measure pH

 

On Wednesday you will retrieve the sample bucket in the collector and replace it with a clean, dry bucket.  In order to trigger the automated collector to open, wave your hand quickly through the light path of the extinction sensor for ca 30 seconds, note down the time in your lab book (the data log of the instrument doesn’t know if your hand or rain drops triggered an opening) – it will close by itself after ca 1 min which gives you enough time to swap the collection buckets. Be sure to prevent contamination; do not touch the interior or rim of the buckets.  Describe your method and actions in detail in your lab notebook.

 

Bring the collected bucket into the lab.  If no sample is observed in the bucket, be sure to record that.  If the bucket was overfilled with snow, note that as well.  If the collected sample is frozen, cover the bucket with aluminum foil and allow the sample to melt completely.  Pre-weigh an appropriate size Nalgene bottle then pour the collected sample into it and weigh again.  Record the pre- and post-sample bottle weights on the log sheet.  Label your sample with “AT560” and the collection date.

 

Pipette 3 1 ml aliquots of your sample into separate vials for pH measurement.  Allow to come to room temperature.  Calibrate the pH meter and electrode, following procedures from Lab 1.  Rinse the electrode first with DI water then in the first sample vial.  Use the 2nd and 3rd sample vials to make 2 measurements of the sample pH.  Record these on the logsheet.

 

Refrigerate the remainder of your sample for later analysis by ion chromatography.

 

Precipitation event data

 

During your assigned week of precip sampling, keep notes about the occurrence of major and minor precipitation events.  If you can, record onset and termination of precipitation events in your lab book (remember that it might rain or snow at the Department but not in downtown Fort Collins at the same time…). You may want to record observations about precipitation intensity and ice crystal habit (shape and degree of riming) as well.  In addition to these event data, you will also download a precipitation event log from the automated collector using the palm pilot provided. Have the TA show you how to operate it and how to download the data to the computer in the lab.

 

Items to include in report/discussion:

 

All precipitation event logs will be available on the PC in the AT560 lab.  A copy of the precip sampling logsheet will also be posted there.

 

1.      Construct plots of the concentrations of major ions (sulfate, nitrate, ammonium) and pH in precipitation over the 9 weeks of measurement.  How do your sample compositions compare to historical values measured at one or more nearby NADP monitoring sites?

2.      Calculate the total deposition flux (µg/m2) for each of the three major ions (nitrate, sulfate, ammonium) for the study period. Hint: you will need the bucket diameter, ion concentrations, and associated sample volumes to make these calculations.  Was the deposition flux dominated by one week or spread across the study?

3.      How do our sample pH values compare to those expected in a pristine atmosphere?  What species do you believe may have influenced the pH of your samples? What sources might contribute to the presence of these species in the local atmosphere?