LOW-VOLUME FILTER PACK AIR CHEMISTRY DATA
Aerosol concentrations, HNO3 and SO2 are sampled using Teflon filter packs. Three 47-mm diameter filters are placed in each filter pack: a Teflon filter (1988-February 2004 Zefluor, 2 micron, Gelman Sciences, Inc.; February 2004-present Whatman, 1 micron) for collection of aerosols, a Nylasorb 1 micron nylon filter (Gelman Sciences, Inc.) for collection of HNO3 vapor and a potassium carbonate-coated cellulose filter for collection of SO2. The cellulose filters are Whatman 41, >20-25 µm particle retention, coarse porosity, ASTM, 12 sec, (Whatman number 1441-047). The cellulose filters are cleaned before coating by rinsing and soaking overnight in double-deionized water. The three filters are placed in line so that the Teflon filter is exposed to incoming ambient air first, the nylon filter second and the carbonate filter last. The filter packs are placed in an inverted stainless steel pot (from 1988 - July 1993 it was an inverted plastic funnel) on a tower approximately 10 m above a mowed grass surface. A continuous flow of air is drawn through the filter pack at 3.00 lpm using a Gast, Inc. oil-less vacuum pump (model 1031, upgraded to model 1531-107B-6288 in 1998), which is regulated by a mass flow controller (Tylan General Inc., model FC280V, upgraded to model FC2604S in 1998 and to Aalborg model GFC 17 in September 2002). When the mass flow controller is off line for any reason, either a needle valve or another mass flow controller regulates flow. During this time, flow is measured using a rotameter (Gilmont Instruments, Inc., or Scienceware®), and the flow measurements are corrected for instantaneous temperature and atmospheric pressure. Clean filter packs are exchanged for exposed ones every Tuesday. The sampling site is located in a flat, open field at an elevation of 128 m. GPS coordinates for the site are: N41.785823 W073.741447.
The Teflon filters are extracted in 50 ml of double deionized water for 24 hours in the dark at 2 degrees C after sonication for 15 minutes. The solution is then decanted into sample bottles and analyzed at the Cary Institute for pH, nitrate, sulfate, ammonium, phosphate, Chloride, Sodium, Calcium, Magnesium and Potassium for determination of aerosol chemistry (see Table 1 for analytical methods). Nitric acid vapor is determined by extracting each nylon filter in 50 ml of a mixture of NaHCO3 (0.28 M) and Na2CO3 (0.22 M) diluted 1:100 with double deionized water. The filters are sonicated for 15 minutes and then refrigerated for 24 hours before decanting the solutions and analyzing them for nitrate and sulfate. Sulfur dioxide is determined by extracting the carbonate-coated filters in 50 ml of double deionized water with 2 drops of hydrogen peroxide. The filters are sonicated and extracted as described above and the resulting solution is analyzed for sulfate. Concentrations of sulfate from the nylon and carbonate filters are added to determine total sulfur dioxide. After 1993 all sample extractions were preserved with 2 drops of chloroform.
Using the total amount of time that each filter was exposed and the average flow rate for the week, weekly concentrations of each component are calculated. For samples that returned less than detection limit concentrations, one half the detection limit is used to calculate air concentrations. Deposition velocities are estimated using a multi-layer dry deposition model (Meyers, T.P., Finkelstein, P., Clarke, J., Ellestad, T.G., Sims, P.F. 1998. A multilayer model for inferring dry deposition using standard meteorological measurements. Journal of Geophysical Research 103: 22645-22661). Weekly average deposition velocities are combined with weekly concentrations to estimate weekly fluxes. Data sets include monthly mean air concentrations, monthly mean deposition velocities and monthly total deposition. For a descriptions of variables, see variable list with units below.
LOW-VOLUME FILTER PACK AIR CHEMISTRY DATA VARIABLE DESCRIPTIONS WITH UNITS
- YEAR year during which sampling week ended
- MONTH month during which sampling week ended
- #WEEKS number of weeks included in calculation
- ACH mean air concentration of particulate H (ueq/m^3)
- ACCA mean air concentration of particulate Ca (ug/m^3)
- ACMG mean air concentration of particulate Mg (ug/m^3)
- ACNA mean air concentration of particulate Na (ug/m^3)
- ACK mean air concentration of particulate K (ug/m^3)
- ACNO3 mean air concentration of particulate NO3 (ug/m^3)
- ACSO4 mean air concentration of particulate SO4 (ug/m^3)
- ACCL mean air concentration of particulate Cl (ug/m^3)
- ACNH4 mean air concentration of particulate NH4 (ug/m^3)
- ACHNO3 mean air concentration of gaseous HNO3 (ug/m^3)
- ACSO2 total mean air concentration of gaseous SO2 (ug/m^3)
- VDSO2 mean deposition velocity for SO2 (cm/s)
- VDHNO3 mean deposition velocity for HNO3 (cm/s)
- VDFP mean deposition velocity for fine particles (cm/s)
- SO2FLUX total SO2 dry deposition for the month (mol/ha)
- HNO3FLUX total HNO3 dry deposition for the month (mol/ha)
- NO3FLUX total particulate NO3 dry deposition for the month (mol/ha)
- SO4FLUX total particulate SO4 dry deposition for the month (mol/ha)
- NH4FLUX total particulate NH4 dry deposition for the month (mol/ha)
LOW-VOLUME FILTER PACK AIR CHEMISTRY INSTRUMENT MAKE, MODEL & DATES USED
Pump: Gast, Inc. oil-less vacuum pump model 1031 (1988-1998), model 1531-107B-6288 (1998- present)
Mass Flow Controller: Tylan-Millipore model FC280V (1988-1998) FC2604S (1998-2002) Aalborg GFC17 (2002-present)
LOW-VOLUME FILTER PACK AIR CHEMISTRY DATA QUALITY ASSURANCE & QUALITY CONTROL PARAMETERS & METHODS
All filter packs and bottles are cleaned by rinsing in deionized water 7 times, allowing to soak overnight in deionized water, rinsing again 4 times in deionized water and finally either dried in a drying oven at no more than 60 degrees C or allowed to sit with caps loosened (bottles) until dry. Every 12 months a filter pack with a set of filters is exposed under a vacuum of 3.00 lpm for 3 minutes. The filter pack and filters are handled and analyzed as regularly exposed filters. This is to ensure that the sample handling procedures introduce no contamination.
When analytical results are received from the Cary Institute analytical lab, data are examined and checked using two methods. First, ion balances and ionic strength are calculated using the following equations:
Ion Balance = ((ANIONS - CATIONS)/((CATIONS+ANIONS)/2))*100;
Ion Strength = CATIONS + ANIONS;
Cations = CA/20 + MG/12 + NA/23 + K/39 + NH4/18 + HA;
Anions = NO3/62 + SO4/48 + CL/35.5;
HA is teflon filter H+ conc (mg/l)
CA is teflon filter Ca concentration (mg/L)
MG is teflon filter Mg concentration (mg/L)
NA is teflon filter Na concentration (mg/L)
K is teflon filter K concentration (mg/L)
NO3 is teflon filter NO3 concentration (mg/L)
SO4 is teflon filter SO4 concentration (mg/L)
CL is teflon filter Cl concentration (mg/L)
NH4 is teflon filter NH4 concentration (mg/L)
HA is calculated as (10**PH)*1000, where PH = pH of teflon filter extractant.
Ion balances and ionic strength are examined and samples are considered for reanalysis if the following criteria are met:
- Ionic Strength (ueq) andIon Balance (%)
- Less than 50greater than 40
- Between 50 and 100greater than 20
- Greater than 100greater than 10
The second quality control step is to examine time series graphs of sample concentrations for each analyte. If any samples are obvious outliers, they are considered for reanalysis.
Analytical methods, instrument notes including calibration schedule, malfunctions and repairs, new instrumentation, anecdotal information etc. can be made available on request.
MISSING VALUE CODES
Missing values are represented by a single decimal point.