MOTIVATION: LABORATORY 5 AEROSOL AND CLOUD PARTICLE CHEMICAL COMPOSITION by Dr. Randy Borys Desert Research Institute Reno, NV The chemical composition of the aerosol and that of cloud droplets and ice particles are intimately linked by the physical processes which create them and transport them through the atmosphere. Here at Storm Peak Lab we are in a unique position to observe, collect and measure aerosol particles from a wide variety of sources; eg. upwind cities, power plants, etc. These particles, in turn, interact with the clouds and snow which envelope the lab. Thus, we can determine how aerosol particles of different types affect clouds and the cloud's ability to scavenge aerosol particles and to produce snow. The atmospheric aerosol prior to its involvement in a cloud is primarily found in the size range 0.1 um to 10 um diameter. Across this size range the chemical composition of the aerosol changes. This is due primarily to how the particles were produced. The production mechanisms such as crustal weathering and sea spray produce large particles. Small particles are produced by combustion sources or by gas-to-particle conversion. Particles from these sources will have different chemical compositions and, therefore, act as tracers of their ultimate source of origin. Once the particles are transported to Storm Peak Lab, and conditions are favorable for the formation of clouds and precipitation, the particles become incorporated into the cloud and, thus, play a role in determining the cloud's physical makeup. The first step in the process is the condensation of water vapor to form cloud droplets. This essentially proceeds on the largest particles first. This, then, results in the first droplets that form having the composition of the largest aerosol particles. For example, Lowenthal, et al. (2002, J. Geophys. Rsh.-Atmos.) have shown at SPL particles between 0.03 and 0.04 and 0.2 um serve as cloud condensation nuclei. Since the largest particles are the first to form droplets, they are likely the droplets which will grow to the largest sizes. This has important consequences for their removal. The next step in the process, which can actually occur concurrently with the first but at a slower rate, is the formation of ice particles. This happens primarily as a result of the freezing of the droplets that formed first. However, most of the droplets will continue to reside in the cloud as supercooled liquid. What causes the freezing of a very few of the droplets (approximately 10-5 to 10-6 m -3 of air) is usually a property of the aerosol particle in the droplet. One criterion is that they are insoluble. Another may be that they are crystalline like clays or rock particles. What happens next is the ice particles grow explosively compared to the cloud droplets and even at their expense because of the vapor gradients between the droplets and ice particles. The result of this growth by diffusion of water vapor to the ice is a strong enhancement of the original droplet's Laboratory 5, pg. 1
chemistry. That is the ice particle gets "cleaner" as compared to the cloud droplets. They also get larger, faster than the cloud droplets and begin to fall out in the form of snow whereas the cloud droplets essentially follow the air over the mountain. As some ice particles grow larger than others, their fall speeds become different and they begin to collide. Some stick together and form what are known as snow flakes or aggregates of many smaller ice crystals. The result is they fall even faster and enhance the precipitation rate and the removal of the aerosol particles contained in them. However, the net chemical composition of the snowflake is still relatively clean. Probably the most important mechanism for the removal of aerosol particles from the atmosphere occurs at this point in the evolution of the precipitation process. As the ice crystals grow and fall faster, they can also collide with the cloud droplets. When this occurs, the droplets freeze to the ice particle and contribute the aerosol particles contained in them to the snow. This can occur to the extreme case where the snow crystal has completely lost its identity and becomes a ball of frozen cloud water otherwise known as graupel. A graupel particle 3 mm in diameter may contain over a million cloud droplets. Thus it is clear that one crystal that grows by diffusion alone on a single frozen cloud droplet will be far less efficient than a crystal that grows by "riming" cloud droplets in removing aerosol particles from the atmosphere. Borys, et al. (2000, Atmos. Environ.) report that at SPL when pollution-derived aerosol particles are present in high numbers, cloud droplet sizes are reduced reducing snow crystal riming. Further, Borys, et al. (2003, Geophys. Res. Lett., 30(10), 1538) have shown the reduction in riming reduces the snowfall rate. This figure illustrates the aforementioned processes: Laboratory 5, pg. 2
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8. Take another tube of ph 7 solution. 9. Clean probe and place in ph 7 solution. Wait for READY signal and record measurement on data sheet. 10. Repeat procedure for ph 4 solution and record measurement (see page 5). C. Preparing sample to be tested (must be done in the cold room): 1. Weigh a clean beaker to be used for snow/cloud sample on scale. 2. Using metal spoon, place 4 grams of snow/cloud sample into beaker. Note the beaker number and sample number on the data sheet. Clean spoon thoroughly after each sample is placed in a beaker. 3. If the sample contains less than 4 grams, weigh beaker and weigh again with sample to determine sample weight. Note in data sheet and circle; there will not be enough sample for anion analyses at CCNY. 4. Once the sample has been measured into a beaker, place into transport box and cover with a paper towel. 5. Once all beakers have been prepared, take box containing beakers into the lab. Let the samples melt at room temperature D. Sample measurements: 1. Pour sample into a plastic tube. Rinse conductivity probe with deionized water. Place into tube for 20 seconds. Note reading and mark on data sheet. Repeat twice. Remember to wash out probe in between readings. 2. If the sample is less than 4 ml, measure the height of the water (inches) in the tube when the probe is immersed and note in data sheet. 3. Rinse off metal thermometer. Place in tube and write down temperature on data sheet. 4. Rinse off ph meter probe. Insert the temperature probe with ph probe into sample tube (Note: water will overflow a little). Wait for READY signal on display. Read ph and temperature and record on data sheet (pg. 5). 5. After all samples have been measured for conductivity and ph, RECHECK CALIBRATION of ph meter by measuring ph 4 and 7 solutions. Reminder: To avoid contamination of samples, wash everything (tubes, beakers, probes and spoon) with deionized water in between use. Store the conductivity probe in 100 µmmho solution and the ph probe in ph 7 buffer between measurement campaigns. But, always store ph probe in ph 7 buffer. Anion measurements: We will return the remaining cloud and snow samples, frozen, to CCNY. The concentrations of the sulfate, nitrate and chloride anions will be determined by liquid ion chromatography in collaboration with Prof. Phillip of CUNY/BCC. Anion content helps identify pollution sources: eg. high chloride indicates a sea-salt particulate source and high sulfate may indicate a coal-fired power plant particulate source. Laboratory 5, pg. 4
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