AEROSOL PARTICLE MEASUREMENTS

Transcription

1 LABORATORY 2 AEROSOL PARTICLE MEASUREMENTS MOTIVATION: Atmospheric aerosol particles are submicron and supermicron in size. The submicron particles form primarily from combustion and gas-to-particle reactions. The supermicron particles form from evaporation of sea-spray and wind-erosion of soil. Volcanoes are a major source of both kinds of particles. Human production of these particles leads to increased haziness, sometimes in remote areas such as the Arctic. Many of these particles serve at the nuclei for the micron-size cloud droplets and millimeter-size snow crystals. It is important to determine the relationship between the numbers of aerosol particles and cloud particles because the increased numbers of human-caused particles may be causing clouds to contain more droplets and crystals. This is a major research topic at SPL (Borys, et al., 2000: Atmos. Environ., Hindman, et. al., 2002: J. Wea. Modif.) Such increases may affect our climate because clouds are a major influence on the radiative balance of the atmosphere. Storm Peak Laboratory is an ideal location to study the interactions of aerosol and cloud particles because the aerosol particles can be measured prior to cloud formation and the cloud particles can be measured after cloud formation. The cloud droplets are readily collected because they exist in a supercooled state and freeze upon striking a surface. The resulting rime ice deposit is easily removed from the surface for analyses as detailed in Laboratory 3. The snow crystals also are readily collected as described in Laboratory 4. During cloud-free periods, aerosol particles flow past SPL. We measure their numbers with a suite of instruments that measure both the submicron and supermicron particles. The flow of air past SPL during a cloud-free period is as follows: air is forced to rise over the north-south oriented Park Range upon which SPL is perched. The air rises and cools (due to adiabatic expansion) but does not cool to the dew-point temperature, hence a cloud does not form, the air remains clear. The air contains too little water vapor to condense and form a cloud. During cloudy periods at SPL, cloud droplets and snow crystals form on many of the aerosol particles. We measure the sizes and numbers of the droplets with the FSSP. The numbers of particle which do not form droplets are measured with the suite of instruments. We estimate the sizes, numbers and types of the snow crystals by preserving them with a plastic coating and observing them under a microscope (see Laboratory 4). The mountain induced cloud in which the droplets and crystals form is like a "blanket" draped over the Park Range (you estimated cloud base in Lab 1): Laboratory 2, pg. 1

2 The air that is forced to rise over the barrier is now moist enough for condensation to occur. But, after the air flows over the range, it sinks down into North Park, a large valley directly east of SPL. Descending air warms adiabatically and hence, the cloud droplets and snow crystals evaporate. The annual precipitation is about 16 inches in Hayden (the airport where you landed), is about 60 inches at the crest of the range (where SPL is) and is, once again, 16 inches at Walden just 20 miles downwind of SPL in North Park (Hindman, 1986: J. Appl. Meteor.). From these numbers, you can see the tremendous influence the Park Range has on the weather. At SPL, the DRI has installed a suite of instruments that measures the size distribution and number concentration of aerosol particles with diameters between and 20 µm: The aerosol particle size distribution is divided into three modes: the nucleation, fine and coarse. The nucleation mode is populated by particles that formed by gas-toparticle reactions or by combustion, the fine mode is populated by particles that coagulated and grew from the nucleation mode and the coarse mode is populated by particles produced by mechanical processes (eg. road dust). It can be seen in the diagram, cloud droplets form on the smaller particles while the crystals form on the larger particles. The fate of the aerosol particles in cloud droplet and snow crystal formation at SPL has been described by Borys, Hindman and DeMott (1988, J. Atmos. Chem., 7, ), Borys, Lowenthal and Mitchell (2000, Atmos. Environ. 34, ), Lowenthal, Borys and Wetzel (2002, J. Geophys. Res., 107, 4345) and Borys, et al. (2003, Geophys. Rsh. Ltrs. 30, 1538). The operating principles of the suite of instruments can be found on-line at First, there are the particle counters, the TSI3025A ultrafine condensation particle counter (UCPC) and TSI3010 condensation particle counter (CPC). Briefly, particles greater than a minimum size are caused to grow to lightscattering sizes by condensation of alcohol vapors in a rapid expansion chamber as Laboratory 2, pg. 2

3 described by Agarwal and Sem (1980, J. Aerosol Sci., 11, ). Then, there is the particle sizer, the TSI3936 scanning mobility particle sizer (SMPS). Particles entering the instrument and are electrically neutralized. Then, a known charge is applied to the particles and they flow through an annulus in which an electric charge is applied to a center probe. All particles smaller than a size specified by the probe migrate to the probe. The particles just larger flow through the annulus and into a TSI3022 ultrafine CN counter to be counted. The charge on the probe is increased and a larger size-cut occurs and those particles are counted. The sizer steps through a particle diameter rage of 8.6 to 300 nm in about 5-minutes. The minimum particle diameter detected by the TSI3022 counter is about 8.6 nm and so this is the minimum diameter detected by the SMPS. Finally, there is the particle sizer and counter. The TSI3321 aerodynamic particle sizer (APS) is a time-of-flight spectrometer that measures the velocity of particles in an accelerating air flow through a nozzle to size the particle. A count is added to the size-bin encompassing the particle size. Then, the next particle in the air stream is sized and counted and so on until a statistically significant number of particles are counted. This takes about 5-minutes. So, a size spectrum and number concentration is obtained every five minutes. Both the SMPS and APS instruments have cyclones on the inlets to remove particles, respectively, greater than 1 and 20 micrometers in diameter. The SMPS and APS instruments both have "cyclones" on the inlets to remove particles, respectively, with diameters greater that 1 and 20 micrometers. The mass concentration (micrograms/cubic meter) of aerosol particles with diameters less than or equal to 10 micrometers, called PM 10, is a standard air pollution measurement. The Routt County Environmental Health Dept. measures hourly PM 10 values downtown. Thus, we can compare our DustTrak data with theirs to develop a pattern of PM 10 in the valley. Also, in January 2000, the acidic cloud episode detected at SPL occurred concurrently with elevated PM 10 concentrations at SPLB as reported by Hindman, et al. (2002, J. Wea. Modif.). The operating principles of this instrument can be found on-line at An Met-One AEROCET 531 PM 10 will also be used. Details are at OBJECTIVES: Learn to measure the concentrations of aerosol particles using the PortaCount Plus (a variation of the CPC), the CPC, the Ultrafine CPC, the DustTrak and Scanning mobility particle sizer and Aerosol particle sizer. INSTRUMENTS: SPL: TSI 3010 CPC for CN (Dia > 0.01 µm) TSI 3025A Ultrafine CPC for ultrafine CN (Dia > µm) TSI 3936 Scanning mobility particle sizer (Dia = to 0.3 µm) is connected to a UCPC TSI3022 (Dia> 7 nm) TSI 3321 Aerodynamic particle sizer (APS) (Dia = 0.37 to 20 µm) for PM 10 SPLM: TSI 3010 CPC and absolute air filter to zero CPC TSI DustTrack (DT) for PM 10 SPLB: TSI 8020 PortaCount Plus (PCP) and absolute air filter to zero PCP AEROCET 531 particle mass monitor for PM 10 Laboratory 2, pg. 3

4 PROCEDURES: SPL: The CPC and Ultrafine CPC data are continuously recorded with the weather data on the CR10 data logger. Every three hours, record on the CPC data sheet CN and UFCN concentration values from the real-time SPL weather display on the SPL web-site (stormpeak.dri.edu). The screen updates every 5-min and the values are 5-min averages. SMPS and APS data display screen Weather display screen The Aerodynamic particle sizer (APS) and Scanning mobility particle sizer (SMPS) data are displayed on the Gateway desktop monitor. These data are recorded onto a Laboratory 2, pg. 4

5 computer and preliminary analyses are automatically performed (make sure the clock on the Gateway computer is identical to the clock on the CR10 to synchronize the data). We will display cumulative mass concentrations values: the mass of aerosol particles with sizes greater than or equal to a minimum particle diameter. The SMPS measures the mass concentration of the submicron particles and the APS measures the mass concentration of the supermicron particles. Record values on the APS/SMPS data sheet (displayed are 5-minute-average mass concentration): Note: SMPS mass concentration in μg/m 3 SPLM: Automatic, unattended operation CPC DustTrak SPLB: TSI PortaCount Plus (PCP) to measure CN: At the 0800, 1400, 2000 and 0200 MST observations, record the time and three CN values in the CN data sheet. Note, the PCP screen is blank and the data are scrolling on the PC screen attached to the PCP. At the 0500, 1100, 1700 and 2300 MST observations, record the time and three CN values and replace the wick with the wick that has been soaking. : Laboratory 2, pg. 5

6 Met-One AEROCET 531 to measure PM 10 : At each observation time, just record three mass concentration values in the data book: Laboratory 2, pg. 6

7 Laboratory 2, pg. 7

8 Laboratory 2, pg. 8

9 Laboratory 2, pg. 9

10 Laboratory 2, pg. 10

11 Instructions on manipulating the filter and filterholder 1. Remove the red lid form the filter-holder 2. Unscrew the screw-cap of the holder 3. Lift the Key 4. Use the clipper to pick the filter from the filter box. Notice that there is a sheet that separate the filters; remove the separator and take the filter. The shiny side of the filter is faced downward. 5. Place the filter on the top of the filter-holder base. Notice, the shiny side of the filter must face upward 6. Do not wrinkle the filter. Avoid the electrostatic caused by the clipper and/or your finger, which results in folding and sticking the filter in the clipper and/or finger 7. Make sure the filter is flat; return the key and make sure it is stable does not twist 8. Screw on the screw-cap 9. Cover the apparatus by the red lid to protect the filter 10. The gas meter is attached to the trailer tongue. There is a tube, which is attached to the gas meter. Insert the filter-base into the tube. 11. Remove the red lid and let the filter collect particles for six hours. The filter must be changed every six hours; no more, no less. 12. The tube leaving the gas meter is attached to the air pump. It sucks the air and allows particles that have diameter greater than about 0.1 microns to be collected. Instructions on opening the Petri dish 1. Hold the top cover with your thumb and index in the indented area 2. Lift the cover gently Instructions on removing the filter from the holder and placing in Petri dish 1. Remove the red cover gently 2. Unscrew the screw cap carefully without touching the filter 3. Lift the Key upwardly 4. Use the Clipper to remove the filter; do not allow the filter to fold. Keep it flat 5. Place it in the Petri dish shiny-side up 6. Close the Petri Dish 7. Label it as SPLB xx (e.g. SPLB 01, SPLB 02, etc) or SPLxx, depending on the location of the filter. Laboratory 2, pg. 11

12 Laboratory 2, pg. 12

13 Laboratory 2, pg. 13