Environ. Scl. Technol. 1983, 17, 169-174
Major Ion Composition and Chemical Associations of Inorganic Atmospheric Aerosols Roy M. Harrison* and Casimiro A. Plot
Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4Y0, England Atmospheric aerosols were sampled and analyzed for H+, NH4+,Ca2+, Mg2+, Na+, K+, S042-, C1-, and NO3-. Gaseous ammonia was also measured. The analyses demonstrated that these ions are the main constituents of the particulate water-soluble fraction of the aerosols sampled. Ion balance studies showed that the main compounds present were (NH4)2S04,NH4N03,and NH4Cl in polluted aerosols and NaCl and MgClz in maritime aerosols. Backward air mass trajectories for collected samples indicated that NH4Clis primarily produced locally, NH4NO3 has a local to medium-distance origin, and (NH4)2S04has been transported over long distances.
Introduction The problems caused by the deposition of acidity on soils, plants, and water bodies are important and wellknown (1). Environmental acidity is caused mainly by the emission of SOz and NO, produced from industrial and transport activities, which after oxidation in the atmosphere are wet or dry deposited as sulfuric and nitric acids or their partially or totally neutralized ammonium salts. Neutral ammonium sulfate deposited on plants is taken up with release of hydrogen ions and acts from the biological point of view as a strong acid (2). It has been proposed that HC1 contributes significantly to the acidity of rain and aerosols. In the U.K. an important source of HC1 is the burning of coal. According to Gorham (3),HC1 accounted for most of the rainwater acidity in central parts of England in the decade previous to 1958. For Martin ( 4 ) ,HC1 still plays an important role in atmospheric acidity in certain areas of the U.K. The present research work has been developed with the aim of assessing the amount of acidity in aerosols in rural England, identifying the chemical compounds responsible for that acidity, determining the chemical associations within the aerosol, and obtaining information on the mechanisms of formation, transport, and removal of the pollutants. The aerosols were collected at a rural site ca. 3 km from the outskirts of the small city of Lancaster, close to the coast of north west England. Details of the site (Hazelrigg) have been reported elsewhere (5). Measured concentrations are therefore characteristic of background levels and frequently represent aged polluted atmospheres resulting from medium- and long-range transport of pollutants. Experimental Section The amount of acidity in aerosols is difficult to evaluate with accuracy, due to the easy contamination during sampling and possible interferences in the analytical procedure. The aerosols were sampled for 24 h with a Hi-Vol air sampler onto 20 X 25 cm rectangular filters. The filter material consisted of treated and extensively washed Whatman GFA ( 6 ) or of Pallflex 2500 QAO, neutralized and cleaned according to the methods reported by Tanner et al. (7) to prevent artifact formation. These treatment processes have been recognized as producing a substrate 'Present address: Departmento de Ambiente, Universidade de Aveiro, 3800 Aveiro, Portugal. 0013-936X/83/0917-0169$01.50/0
inert to adsorption and oxidation of SOz and the neutralization of sulfuric acid aerosols and gaseous HN03 (8). The treatment had also the advantage of lowering the level of blank concentrations of measured compounds (see Table 1). A portion of the exposed air filter was immersed in distilled water, the water-soluble compounds were extracted in an ultrasonic bath, and after filtering, the solution was analyzed for the following components: S042by an adaptation of the chloranilate method (9);NOs- by a modification of the Saltzman method after reduction to nitrite with hydrazine (IO);C1- by spectrophotometry, through the measurement of absorbance of the complex formed with ferric mercuric thiocyanate (11); NH4+ by catalyzed indophenol blue spectrophotometry (IO);Na+ and K+ by atomic emission flame photometry; Ca2+and Mg2+by flame atomic absorption. An aliquot of the water extract was used for the determination of acidity by use of the Gran titration method reported by Brosset and co-workers (12,13). In some samples aerosol acidity was measured instead by using a selective solvent extraction technique (14). In these samples a portion of the exposed filter was washed with isopropanol and another portion with freshly distilled benzaldehyde, with subsequent measurement of extracted S042-. This procedure permitted the determination of amounts of sulfuric acid and bisulfates existent in the aerosols. Gaseous ammonia was collected from the atmosphere by adsorption on Whatman No. 41 filter material impregnated with KHS04, after removal of ammonium particles on a prefilter (15),and the collected material was analyzed as NH4+. For some sampling intervals the total suspended-particle concentration in the atmosphere was determined by weighing a separate fiiter in dry conditions before and after sampling. Filters were dried over silica gel in a desiccator at a relative humidity of
