POST-INDUSTRIAL SOURCES OF LEAD IN URBAN AIR: A CASE STUDY USING LEAD ISOTOPES IN YEREVAN, ARMENIA

*Robert Kurkjian, Earth Sciences Department

University of California at Santa Cruz, Santa Cruz, CA 95064

Charles Dunlap, Environmental Research and Management Center

American University of Armenia, Yerevan, Armenia

A. Russell Flegal, Environmental Toxicology

University of California at Santa Cruz, Santa Cruz, CA 95064

ABSTRACT

We have measured lead concentrations and isotope compositions in air, gasoline, and soil in Yerevan, Armenia in order to trace the sources of lead in urban air during the transition from a heavily industrialized city with a vehicle fleet burning leaded gasoline to a post-industrial city with vehicles consuming primarily unleaded gasoline. In Yerevan this transition occurred during the 1990s when Armenia’s industrial output decreased by more than 80% following the break-up of the Soviet Union. Sometime in the late 1990s Armenia began importing unleaded gasoline. Our measurements of lead concentrations in Yerevan’s air in 1995, 1996, and 1998 indicate that the shift to unleaded gasoline consumption occurred by 1998. Lead concentrations in gasoline samples collected in 1997-1999 were <0.001 g/L, much lower than the range of 0.15-0.84 g/L that is typical of leaded gasoline.

Using stable lead isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb), we have discriminated among the potential sources of lead in air. In 1995 and 1996, the source of lead in Yerevan air was the combustion of leaded gasoline. By 1998, leaded gasoline combustion had ceased, lead concentrations in air had dropped by more than one order of magnitude, and the isotopic composition of air had de-coupled from that of gasoline. In samples taken at 30 meters above street level in 1998, lead was a binary mixture of 50% soil dust lead and 50% industrial emissions. In 1999, the industrial emissions of lead had abated, and lead 30 meters above street level exhibited a still larger component of soil dust in comparison to 1998 samples. Samples taken less then 3 meters above street level in 1999 derived their lead from emissions of unleaded gasoline combustion before appreciable mixing with soil dust occurred.

We conclude that despite leaded gasoline phase-out and dramatic decrease in industrial emissions, lead in urban air will continue to be dominated by human sources. In addition to the increased prominence of lead from industrial emissions, lead accumulated in city soils from past emissions will be a dominant source of lead in city air. In addition, the re-suspension of soil dust will further limit the already slow removal of past lead emissions from the near surface soil layer.

* Corresponding author: kurkjian@es.ucsc.edu

An expanded manuscript has been submitted for publication to Atmospheric Environment

INTRODUCTION

Armenia, the smallest republic (29,000 km²) of the former Soviet Union (FSU), has been in the process of building a new economic and political system since it gained its independence in 1991. Difficulties during the transition were aggravated by a regional conflict coupled with an embargo by neighboring countries. As a result, Armenia experienced a severe energy shortage and an economic crisis in the early- to mid-1990s leading to a decrease in industrial production by more than 80%. To date, this industrial output has not increased substantially.

Consequently, the release of lead to the atmosphere from stationary sources decreased (Kurkjian, 2000). Following the period of industrial decline, consumption of unleaded gasoline has increased, resulting in reductions of lead emissions from mobile sources. Although, many cities world-wide have phased out leaded gasoline, few have experienced a simultaneous and significant reduction of lead from stationary sources as well.

By tracing the movement of lead through the environment in Yerevan, we have gained insight into lead sources that may exist in other urban centers in the future. It is particularly important to pay attention to the resuspension of past emissions that are bound to the city’s soil. As air quality standards become more stringent over the next few decades and stationary source emissions are reduced, soil-bound lead may contribute a significant portion of lead to the atmosphere.

However, resolving the historic sources and projecting the future levels of lead in Armenia is complicated by several factors. Armenia has imported its gasoline from eastern and western Europe and from several countries of the FSU, and the lead content in gasoline has not been regulated. While lead concentrations in Yerevan’s air have not been reliably quantified by the Armenian Government, their reports have stated that levels of lead in air exceed international guidelines (Ministry of Environmental Protection, 1998). In order to quantify the levels of lead in Yerevan’s air, distinguish between different sources of lead, and determine the cycling mechanisms of lead in a post-industrial setting, we collected air, gasoline, and soil samples between 1995-1999.

METHODS

Air samples were collected in Yerevan, Armenia from 1995-1999. Two sets of samples were collected using different protocols. The Armenian Ministry of Environmental Protection collected nine air samples in 1995 and 1996 in accordance with existing guidelines. The samples were collected using 5.0 µm pore filter paper at a pumping rate of 2.42 L/s. The sampled air volume ranged from 63 to 179 m³. UCSC researchers collected samples (air, soil, and gasoline) from 1996-1999 utilizing trace metal clean techniques (NRC, 1993; Patterson and Settle, 1976). The samples were collected on acid-clean, 37 mm diameter Teflon filters with a 0.45 µm pore size with a portable air pump operating at 0.25 L/s. Air was sampled from 8 and 72 hours, and the sampled air volume ranged from approximately 7-21 m³. In 1999, the sampling height was decreased from 30 m to 3 m above street level to quantify changes in concentration and to determine shifts in isotope composition.

Gasoline and soil samples were also collected in Yerevan. Gasoline was purchased throughout Yerevan from 1997-1999. Aliquots (1 mL) were evaporated in acid-clean Teflon vials in order to be transported. Twenty-four soil samples were collected throughout Yerevan in 1999 at a depth of 1-5 cm. The samples were sealed in poly-bags and transported to UCSC for processing and analysis.

Sample processing was conducted in a trace metal clean HEPA filtered (class 100) laboratory at UCSC. All acids used were high-purity (Optima or Seastar). Air filters were leached in 8N HNO₃. Samples were reconstituted with 10 mL of 1% HNO₃. Residue from evaporated gasoline samples was dissolved in concentrated HNO₃, reconstituted in HCl and evaporated. Soil samples were weighed (0.5 g) and leached in HCl, centrifuged, and evaporated to dryness. The samples were then treated with concentrated HNO₃, dried, and reconstituted in 20 mL of 1% HNO₃.

The samples were split for lead concentration and isotope composition analyses. Concentrations were measured on a Finnigan MAT Element magnetic sector inductively coupled plasma source mass spectrometer (ICP-MS). In order to measure the lead isotope compositions, sample aliquots (0.5 mL) were passed through Teflon microcolumns to separate lead from the sample using AG1-X8 75-150 mesh anion exchange resin. The columns were flushed and eluted with HCl and HBr. The samples were then loaded onto rhenium filaments with silica gel and H₃PO₄. The isotope compositions were measured on a VG Sector 5430 thermal ionization mass spectrometer (TIMS). Calibration and mass fractionation corrections were determined by concurrent analyses with a 100 ng/mL NIST SRM 981 (common lead isotope standard). Lead concentrations of the procedural blanks were ~200 ng lead. Average errors (± 2 sigma) were ±0.000065 for ²⁰⁶Pb/²⁰⁷Pb and ±0.000048 for ²⁰⁸Pb/²⁰⁷Pb.

RESULTS AND DISCUSSION

The lead isotope compositions for air samples collected in Yerevan in 1995-1996 fall within the range of soil isotope compositions (Figure 1). We estimate that at least 98% of the lead in soil is from the deposition of leaded gasoline emissions since Armenian government records document the use of 42-52 tons of lead per year by mobile sources and less than 1 ton per year from stationary sources during the Soviet period (Kurkjian, 2000). Thus, the soil isotope compositions provide a time-averaged value for previous gasoline lead. The overlap of soil and air isotope values in 1995-1996 suggests that the combustion of leaded gasoline continued to dominate lead in air for those years. Further, the lead concentration in air for 1995-1996 was ~0.5 µg/m³ which is typical of urban areas where leaded gasoline is consumed (EPA, 1986), supporting the conclusion that leaded gasoline combustion was dominant in Yerevan’s air through 1996.

In contrast to the predominance of gasoline lead in air apparent for 1995-1996, the 1998 lead isotope compositions and concentrations show a decoupling of the gasoline and air (Figure 2). All gasoline samples were unleaded gasoline ([Pb]<0.0001 g/L), and the concentration of lead in air in 1998 ([Pb]<0.01 µg/m³) was orders of magnitude lower than in 1995 and 1996. Trace amounts of lead were emitted to the atmosphere by the combustion of this gasoline, but the isotopic composition of the air is clearly controlled by sources other than gasoline.

The partial overlap of the air and soil isotope compositions suggests that the re-suspension of lead-contaminated soil was one of the sources of lead to air in 1998. The lead in Yerevan soil is markedly elevated above natural concentrations of 10 ppm with an average concentration of 290 ppm (n=24). The high concentration of suspended particulate matter in Yerevan’s air suggests that re-suspension of soil dust is common, and observations of dust clouds from abandoned construction sites and other vacant lots in Yerevan’s dry climate confirms this. Consequently, we estimate that lead from soil is one member of a binary mixture and accounts for 50% of the lead in air.

Two clusters of isotope values for air (²⁰⁸Pb/²⁰⁷Pb values of ~2.44 and ~2.45-2.47) suggest a source in addition to city soil, and one possible source is the smelting of copper ore in Armenia’s two mining regions, or the import of lead from that ore for use in the few remaining active industries (including a lead crystal factory). Five galenas from Armenia’s two massive sulfide deposits (Cu-Ni-Zn-Mo-Pb) were measured for isotope composition in order to determine their potential contribution to lead in Yerevan’s air. The ore bodies are located more than 100 km from Yerevan, and in the past the relatively small quantities of lead produced in Armenia are reported to have been exported. However, the isotope compositions of the lead ores (2.49<²⁰⁸Pb/²⁰⁷Pb>2.46) overlap the cluster of air compositions of unknown origin suggesting that it may have been used in Yerevan as well. The likelihood of this may be increased due to the collapse of the historical (Soviet) export markets. The other 50% of lead in urban air in 1998, therefore, is attributed to a small amount of local lead used in industry that is observed in the absence of the dominant leaded gasoline emissions of the previous years.

Air samples collected in 1999 were collected at a height of 30m (as in previous years) and, for the first time, at 3m (i.e. near street level). The isotope values of the near street samples overlap soil and gasoline values, while the samples taken at 30m fall outside of the gasoline field (Figure 3). We interpret the samples taken at 3 m (²⁰⁶Pb/²⁰⁷Pb ~1.15) to contain lead dominated by vehicle tailpipe emissions before they have had a chance to mix with other lead sources in city air. At 30 m, the separation of air lead from gasoline lead suggests mixing of near street emissions with other sources. In the 30m samples, we conclude that the lead (²⁰⁶Pb/²⁰⁷Pb ~1.16) is a mixture of gasoline lead and the resuspension of contaminated soils. The gasoline consumed was unleaded ([Pb]< 0.0001). The concentrations of lead in air at both sample heights were low ([Pb]< 0.05).

The concentration of lead in Yerevan’s air is currently orders of magnitude lower than during times of industrial activity and leaded gasoline combustion, and is below both the Armenian standard of 0.3 µg/ m³ and the World Health Organizations guideline of 0.5 µg/ m³. However, the removal of these past sources has only revealed new sources of anthropogenic origin. As is the case with air over the European continent (Dunlap et al., 1999) lead in Yerevan air will continue to dominated by the persistence of past emissions in soils and newly detectable emissions from industry. In the air near street level, the small amounts of lead in unleaded gasoline control the lead in air. We conclude, therefore, that a return to natural lead levels will not occur in cities that make a transition to unleaded fuel combustion and dramatically reduce the operations of (or emissions from) heavy industry. We also suggest that the residence time of lead in soils will be increased by its re-suspension into air in dry climates.

This study was partially supported by a grant from the North Atlantic Treaty Organization, Scientific and Environmental Affairs Division (ENVIR.LG974677).

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