Toxic metals in the atmosphere in Lahore, Pakistan

Abstract

Aerosol mass (PM₁₀ and PM_2.5) and detailed elemental composition were measured in monthly composites during the calendar year of 2007 at a site in Lahore, Pakistan. Elemental analysis revealed extremely high concentrations of Pb (4.4 μg m^− 3), Zn (12 μg m^− 3), Cd (0.077 μg m^− 3), and several other toxic metals. A significant fraction of the concentration of Pb (84%), Zn (98%), and Cd (90%) was contained in the fine particulate fraction (PM_2.5 and smaller); in addition, Zn and Cd were largely (≥ 60%) water soluble. The 2007 annual average PM₁₀ mass concentration was 340 μg m^− 3, which is well above the WHO guideline of 20 μg m^− 3. Dust sources were found to contribute on average (maximum) 41% (70%) of PM₁₀ mass and 14% (29%) of PM_2.5 mass on a monthly basis. Seasonally, concentrations were found to be lowest during the monsoon season (July–September). Principle component analysis identified seven factors, which combined explained 91% of the variance of the measured components of PM₁₀. These factors included three industrial sources, re-suspended soil, mobile sources, and two regional secondary aerosol sources likely from coal and/or biomass burning. The majority of the Pb was found to be associated with one industrial source, along with a number of other toxic metals including As and Cr. Cadmium, another toxic metal, was found at concentrations 16 times higher than the maximum exposure level recommended by the World Health Organization, and was concentrated in one industrial source that was also associated with Zn. These results highlight the importance of focusing control strategies not only on reducing PM mass concentration, but also on the reduction of toxic components of the PM as well, to most effectively protect human health and the environment.

Introduction

Rapid growth and industrialization, especially in developing nations, has resulted in severe air pollution problems in many of the world's megacities (Mage et al., 1996). The use of fuels, such as biomass, coal, diesel oil, and furnace oil in residential combustion contributes to regional air quality issues in many of these developing countries (Waheed et al., 2006). Lahore, Pakistan has a population of approximately ten million and is undergoing rapid population growth. This megacity is located in northeastern Pakistan, and with northern India, forms one of the most industrialized areas on the Asian subcontinent with severe air pollution issues (Rattigan et al., 2002, Ghauri et al., 2007). In addition to the local industry, Lahore is a hub on the national highway system with an influx of heavy traffic (Waheed et al., 2006). Extremely high levels of particulate matter (PM) concentrations for Lahore were recorded by Smith et al. (1996) in the mid-1990s. A more recent study by Ghauri et al. (2007) established year-long air quality baselines for urban centers in Pakistan and showed that Lahore had the highest total suspended particulate (TSP), and PM₁₀ concentrations, as well as SO₂, NO_x, and O₃ concentrations, of the six urban centers studied. Additionally, fog episodes with elevated levels of pollutants, causing detrimental effects to human health and the economy, have been documented in Lahore, and are expected to increase and worsen due to growing regional economies and lack of pollution controls (Hameed et al., 2000, Biswas et al., 2008).

It is well documented that air pollution contributes to increases in mortality (Dockery et al., 1993 and references therein). A number of trace metals, such as As, Cd, and Cr, have been identified as carcinogenic and human exposure to these metals through aerosols represents a serious concern (International Agency for Research on Cancer, IARC website, 2009). Earlier work by Parekh et al. (1987) documented elevated levels of toxic metals in ambient aerosol from Karachi, Pakistan. Later work documented elevated levels of toxic metals, including Pb, Cd, and Ni, in human blood samples for various locations in Pakistan (Younas et al., 1998, Kadir et al., 2008).

Effective and cost-efficient regulation of toxic metals in the atmosphere (e.g. lead) is not always achieved with a general reduction in aerosol concentrations. An understanding of sources, as well as the water solubility and the size fraction in which the metal occurs, can guide control efforts to exact the greatest benefit in terms of reduction in human exposure to air toxic metals in the atmosphere. In this paper we present the results of a one year study of PM₁₀ (particulate matter 10 μm and smaller) and PM_2.5 (particulate matter 2.5 μm and smaller; fine mode) in Lahore, Pakistan, with a focus on the individual elements, including toxic metals. Principle component analysis was carried out to gain an understanding of sources of metals to the atmosphere in Lahore, in comparison with other megacities.