Low-Volume Filter Pack Air Chemistry

Overview
Aerosol concentrations, HNO₃ and SO₂ were sampled using Teflon filter packs. Three 47 mm diameter filters were 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 were Whatman 41, >20-25 µm particle retention, coarse porosity, ASTM, 12 sec, (Whatman number 1441-047). The cellulose filters were cleaned before coating by rinsing and soaking overnight in double-deionized water. The three filters were placed in line so that the Teflon filter was exposed to incoming ambient air first, the nylon filter second and the carbonate filter last. The filter packs were 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 was 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 was 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 was off line for any reason, either a needle valve or another mass flow controller regulated flow. During this time, flow was measured using a rotameter (Gilmont Instruments, Inc., or Scienceware®), and the flow measurements were corrected for instantaneous temperature and atmospheric pressure. Clean filter packs were exchanged for exposed ones every Tuesday.

The Teflon filters were 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 was then decanted into sample bottles and analyzed at the Cary Institute for pH, NO₃^-, SO₄^-2, NH₄⁺, PO₄^-3, Cl^-, Na⁺, Ca⁺², Mg⁺² and K⁺ for determination of aerosol chemistry (see Table 1 for analytical methods). Nitric acid vapor was 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 were sonicated for 15 minutes and then refrigerated for 24 hours before decanting the solutions and analyzing them for NO₃^- and SO₄^-2 (Table 1). Sulfur dioxide was determined by extracting the carbonate-coated filters in 50 ml of double deionized water with 2 drops of hydrogen peroxide. The filters were sonicated and extracted as described above and the resulting solution was analyzed for SO₄^-2 (Table 1). Concentrations of SO₄^-2 from the nylon and carbonate filters were added to determine total SO₂. 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 were calculated. For samples that returned less than detection limit concentrations, one half the detection limit was used to calculate air concentrations. Deposition velocities were 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 were 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.

top

• 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)

top

Low-Volume Filter Pack Air chemistry instrument make, model and 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)

top

Low-Volume Filter Pack Air Chemistry data quality assurance and quality control parameters and methods.
All filter packs and bottles were 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 bottle was dry. Every 12 months a filter pack with a set of filters was exposed under a vacuum of 3.00 lpm for 3 minutes. The filter pack and filters were handled and analyzed as regularly exposed filters. This was to ensure that the sample handling procedures introduced no contamination.

When analytical results were received from the Cary Institute analytical lab, data were examined and checked using two methods. First, ion balances and ionic strength were calculated using the following equations:

Ion Balance = ((ANIONS - CATIONS)/((CATIONS+ANIONS)/2))*100;
Ion Strength = CATIONS + ANIONS;

Where:
Cations = ZCA/20 + ZMG/12 + ZNA/23 + ZK/39 + ZNH4/18 + ZHA;
Anions = ZNO3/62 + ZSO4/48 + ZCL/35.5;

And:
ZHA was zefluor filter H+ conc (mg/l)
ZCA was zefluor filter Ca concentration (mg/L)
ZMG was zefluor filter Mg concentration (mg/L)
ZNA was zefluor filter Na concentration (mg/L)
ZK was zefluor filter K concentration (mg/L)
ZNO3 was zefluor filter NO3 concentration (mg/L)
ZSO4 was zefluor filter SO4 concentration (mg/L)
ZCL was zefluor filter Cl concentration (mg/L)
ZNH4 was zefluor filter NH4 concentration (mg/L)

ZHA was calculated as (10**ZPHA)*1000, where ZPHA=pH of zefluor filter extractant.

Ion balances and ionic strength were examined and samples were considered for reanalysis if the following criteria were met:

The second quality control step was to examine time series graphs of sample concentrations for each analyte. If any samples were obvious outliers, they were considered for reanalysis.

Instrument notes including calibration schedule, malfunctions and repairs, new instrumentation, anecdotal information etc. can be made available on request.

top

Low-Volume Filter Pack Air Chemistry Missing Value Codes
Missing values were represented by a single decimal point.

top