Evolution of anthropogenic aerosols in the coastal town of Salina Cruz, Mexico: part III size-segregated elemental composition analysed by total-reflection X-ray fluorescence spectrometry

Abstract

Heavy metals in various size modes of the atmospheric aerosol are a concern for human health. Their and other elements' concentrations are indicative for anthropogenic and natural aerosol sources. Si, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Hg, and Pb were determined as a complementary contribution to a study on aerosol cycling during the wet season, June 2004, in a humid, subtropical climate, i.e. in the city of Salina Cruz, situated on the Pacific coast of the Isthmus of Tehuantepec (16.2 N, 95.2 W), Mexico. For mass (gravimetry) and elemental analyses, particles were collected by a Berner low-pressure round nozzle cascade impactor using four stages corresponding to 0.1-0.25, 0.25-1.0, 1.0-4.0, and 4-16 mm of aerodynamic particle size. The impaction plates were modified such that approx. 1/6 consisted of a plastic support (Persplex Õ ) for total reflection X-ray fluorescence spectrometry (TXRF). The elements' total content was determined by TXRF without any further sample pretreatment. Limits of quantification (LOQ) for elemental content in individual impactor stages corresponded to 25-60 ng m À3 for Si; 0.8-4 ng m À3 for Cl, K, Ca, Ti, and V; 3-20 pg m À3 for Cr, Mn, Fe, Cu, Ni, and Zn; and 7-50 pg m À3 for As, Se, Br, Rb, Sr, Hg, and Pb. In some samples, however, high blank values for the supports gave an LOQ ¼ 6-19 ng m À3 for Cl; 3-7 ng m À3 for Ca; 3-7 ng m À3 for Fe, Ni, Cu, and Zn; and 60-70 ng m À3 for Pb. The influence of local natural, industrial, and vehicle traffic sources for heavy-metal mobilization was obvious. Heavy-metal abundances did not coincide with regionally distributed pollutants. V and Ni were found at particularly elevated levels advected with the sea breeze, which points to ships as sources. Br and Pb were found at particularly low levels. The concentrations of Br, Rb, Sr, and Pb were found below LOQ at least in some, As, Co, Se, and Hg in all of the samples. The elements' characteristic differences in mass size distributions were obvious despite the coarse size resolution. During the cycling of air masses from land to sea and back again, enrichment of super-micrometre particles in the near ground aerosol was observed under dry weather conditions. Rain preferentially removed the large particles with which heavy metals have been associated. *Corresponding

Atomic spectrometry update. X-ray fluorescence spectrometry

“…XAFS data linked Cr and S to the ultra-fine ash particles and confirmed TEM data linking carbon with transition metal complexes. Lammel et al 162 used a Berner low pressure round nozzle cascade impactor with four stages corresponding to 0.1-0.25, 0.25-1.0, 1.0-4.0 and 4-16 mm aerodynamic particle sizes for the investigation of anthropogenic aerosols in the coastal town of Salina Cruz, Mexico. The impaction plates were modified to accommodate a Perspex Ò support for subsequent analysis by TXRF without further sample pre-treatment.…”

Atomic spectrometry update. Environmental analysis

“…TXRF techniques can offer sensitivities on a par with ICP-MS analysis but, to date, have been applied mostly to liquids such as those from sample digestions. Lammel et al 80 used a low pressure Berner impactor with four particle sizing ranges of 0.1-0.25 mm, 0.25-1.0 mm, 1.0-4.0 mm and 4-16 mm to study anthropogenic pollution sources. Modification of the impactor plates with a Perspex support allowed for the direct determination of elements by TXRF without any sample pretreatment and consequently very low detection limits were attainable, e.g.…”

“…In contrast, anionexchange separation coupled to an octapole ICP-MS reaction system 132 has been reported for the determination of Se oxidation states. The reaction cell utilized H 2 to overcome the argon polyatomic interferences and allow for the detection of the most abundant 80 Se isotope. With a pH 6.5 buffer system, both oxidation states of Se could be determined in natural water samples with LODs of 0.4 for Se IV and 0.6 mg L À1 for Se VI .…”

“…Once again a collision/reaction cell instrument was used, but in this case with a mixture of He and H 2 as the collision gas. The main advantage of a VG interface was the elimination of the BrH + interference, which allowed for the determination of the 80 Se isotope and gave species specific LODs of between 7 and 15 ng L À1 .…”

Quantitative total reflection X‐ray fluorescence analysis of directly collected aerosol samples

“…Further compounds associated with higher morbidity are sulfates and nitrates as well as organic and elemental carbon . Furthermore, Pb and Hg are well known biotoxic elements and can be easily analyzed by TXRF …”