Colorado State University Atmospheric Science Department

Goals

· Determine what form(s) aerosol sulfate is found in at Big Bend National Park (BBNP) (e.g., H₂SO₄, NH₄HSO₄, (NH₄)₂SO₄, Na₂SO₄, etc…)

· Determine whether previously observed correlations between BBNP sulfate and sodium reflect the presence of sodium sulfate aerosol

· Determine changes in sulfate content and speciation during transport of air from the coast to BBNP

Measurements

BBNP

· PM-2.5 aerosol (24 hr time resolution, collected by CSU July 1-October 31, 1999)

· Gaseous nitric acid (24 hr, CSU, July1-October 31, 1999)

· Gaseous ammonia (24 hr, CSU, July 1-October 31, 1999)

· MOUDI size-resolved aerosol (24 hr, CSU, 6 days per week, July 1-October 31, 1999)

· Bulk aerosol for Scanning Electron Microscopy (SEM) analysis (24 hr, CSU, July-October, 1999)

· Gaseous hydroperoxides (1 hr, CSU, collected periodically July 1-October 31, 1999)

· IMPROVE elemental composition and mass (Davis)

Other sites (coastal Texas and inland Texas)

· PM-2.5 aerosol (24 hr, Davis, late July – October 1999)

Measurement Methodology

· PM-2.5 aerosol composition, gaseous nitric acid, and gaseous ammonia will be obtained from samples collected at BBNP using a URG denuder/filter-pack sampler (see sketch below). Air is drawn in series through a 2.5 µm size cut cyclone, through two annular denuders, and through a filter pack.

§ The cyclone removes coarse aerosol.

§ The first denuder is coated with sodium chloride and collects gaseous nitric acid.

§ The second denuder is coated with citric acid and collects gaseous ammonia.

§ A Teflon filter is used for PM-2.5 aerosol collection. Extracts from this filter will be used for ion analysis and aerosol acidity measurements.

§ A nylon filter is used to collect nitric acid lost from the Teflon filter due to volatilization.

§ A citric acid impregnated glass fiber filter is used to collect ammonia lost from the Teflon filter due to volatilization.

· PM-2.5/denuder extraction and analysis procedures

§ All extractions are performed in an ammonia-free hood to prevent contamination by ambient ammonia.

§ The first denuder and the nylon filter are extracted and analyzed for nitric acid (measured as nitrate ion) by ion chromatography (IC). Water is used to extract the denuder and IC anion eluent is used to extract the nylon filter.

§ The second denuder and the impregnated glass fiber filter are extracted with water and analyzed for ammonia (measured as ammonium ion) by IC.

§ The Teflon filter is extracted using a 10^-4 N perchloric acid solution. A portion of the extract is set aside for IC analysis for anions (Cl^-, SO₄^2-, and NO₃^-) and cations (Na⁺, K⁺, NH₄⁺, Ca²⁺, and Mg²⁺). KCl is added to the remainder of the Teflon filter extract. This portion of the extract is used for H⁺ measurement, following EPA’s standard method for measurement of aerosol strong acidity.

· MOUDI size-resolved aerosol samples are being collected with 24 hr time resolution at BBNP. Samples are collected 6 days per week. The 7^th day is used for taking a blank and cleaning the impactor. Collection is made on aluminum foil impaction surfaces coated with silicone grease to prevent particle bounce. Collected substrates will be extracted followed by IC analysis for selected periods. Size cuts in use for the MOUDI span a range from 0.18 µm to several microns particle aerodynamic diameter.

· SEM aerosol samples will be collected using a low-flow (~5 lpm) filter sampler and polycarbonate filters. No upper size cut is required for this sampler; SEM analysis will provide both size and composition information for particles > 0.5 µm. Smaller particles probably cannot be accurately studied using automated SEM techniques. Test filters collected in July at Big Bend will be submitted for analysis by RJ Lee to determine whether a satisfactory particle loading is being obtained on the filters.
Notes

§ BBNP PM-2.5 aerosol and denuder samples are being extracted and analyzed on-site through at least August. This

§ minimizes potential changes in composition (especially neutralization) during sample storage

§ provides early feedback on aerosol composition and indicate whether appropriate measurement strategies are in use (e.g., are samples of sufficient duration to permit accurate quantitation of important ions). Chemical analysis results from the first week of July suggest current procedures are appropriate.

§ Results from BBNP PM-2.5 aerosol analyses will be used to select BBNP MOUDI and SEM samples and PM-2.5 and SEM samples from other sites for analysis

§ BBNP MOUDI aerosol samples will mainly be analyzed for periods where sodium sulfate appears to be important, although some additional “typical” sample periods will be analyzed to examine variations in aerosol composition with particle size.

§ PM-2.5 samples from other sites will be selected for analysis during periods when these sites are located upwind of BBNP and sodium sulfate is determined to be an important component of BBNP aerosol sulfate.

Expected information from planned measurements

· H⁺ measurements will reveal whether BBNP aerosol is neutralized or acidic

· PM-2.5 ion measurements will indicate whether sulfate is present in ammonium salts or whether a significant portion is probably associated with sodium ion.

· MOUDI size-resolved aerosol samples will reveal if sodium ion and sulfate are found in the same particle size range(s)

· SEM single particle measurements will reveal if sodium and sulfur are found in the same particles

· Measured aerosol and gas compositions at BBNP can be used to predict how PM-2.5 mass and scattering properties might change in the event of changes in ambient concentrations of sulfate, nitrate, or ammonium

· Comparison of BBNP results with results from upwind coastal and inland sites will reveal if sulfate is added to sea salt particles as they are transported to BBNP from the coast

· Peroxide measurements will reveal the potential for rapid oxidation of SO₂ to sulfate in the presence of clouds