CLIMATE CHANGE AEROSOLS, BLACK CARBON AND ADVERSE HEALTH EFFECTS
by James Grainger , PhD

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One of the most controversial contemporary scientific issues has emerged from divergent and conflicting views concerning the gradual warming of the Earth’s atmosphere over the past half century.  Climate change challenges have been related to changes in the concentrations of greenhouse gases (water vapor, CO₂, CH₄, N₂O, and CFCs) which trap light from the Sun and infrared radiation from the Earth’s surface resulting in the greenhouse effect.  While this is a natural phenomenon which helps maintain a stable climate and temperature on Earth, anthropogenic (human) activities such as fossil fuel combustion, deforestation and a spectrum of industrial processes have generated increases in greenhouse gas concentrations.

Careful observations of global temperature records over the past 50 years has revealed less global warming than predicted by computer models that only include greenhouse gas accumulations (Chuang, 2001).  An explanation for this apparent discrepancy is that the greenhouse gas effect may be counteracted by an atmospheric cooling effect caused by increasing concentrations of anthropogenic aerosols (microscopic particles directly suspended in the atmosphere or trapped in clouds) may be changing the climate of the Earth.

Aerosol particles may range in diameter from 0.01 micron (10^-6 meter)  to several tens of microns. Particles enter the atmosphere from a variety of natural and anthropogenic (Figure 1) sources – sulfate aerosols from volcanoes, salt aerosols from sea spray, dust aerosols from desert ecosystems and carbonaceous aerosols from volatile organic compounds (VOCs) emitted from plants.  Increasing percentages of aerosols are generated by human activities resulting in the ubiquitous hazesthat characterize industrialized regions around the globe.  Anthropogenic aerosols include sulfuric acid, carbonaceous materials (soot) and smoke from combustion of fossil fuels in factories, power plants, motor vehicles and incinerators.  The burning of grasslands and forests to clear agricultural land is an additional aerosol source.

Increased morbidity and mortality have been associated with inhaled airborne particulate matter (PM).  In 1997, the EPA issued revised National Ambient Air Quality Standards for airborne PM by focusing on PM with aerodynamic diameters of < 2.5 microns (PM_2.5) with the potential for deeper penetration into the respiratory system.  Small increases in levels of  PM_2.5 result in increases in cardiovascular and respiratory mortality (Penn, 2005, Schwarze, 2006).  PM contains a mixture of carbon-centered combustion particles (including higher molecular mass, more biologically reactive PAHs), secondary inorganics and crustal derived particles. Although the soot component is a small percentage of aerosol composition, the high reactivity and penetration potential indicate that its constituent high molecular mass PAHs can function as biological markers for climate change aerosols.

Black carbon (BC) particulates result from incomplete combustion where the quantity produced depends upon the efficiency of the combustion process as well as the quantity of fuel.  Over the past century, coal utilization in industrial countries has become more efficient and less polluting.  This is not the case for industrially developing countries where similar changes in fuel efficiency have not transpired or is in early development.  As a result there are strong regional and temporal variations (Figure 2) in quantities of NO₂ , SO₂ and BC emissions (Navokov, 2003).

Researchers conclude that 25 -35% of black carbon in the global atmosphere originate in India and China (Hadley, 2007) and that more than 75% of the BC transported over the West Coast of the US in spring derives from Asian sources.

A validation of aerosol transport estimates based on simulations by the Chemical Weather Forecast System (CFORS) was conducted by tropospheric and surface measurements (Hadley, 2007) using vertically integrated aerosol mass retrieval (Figure 3).  The time frame aerosol trajectory from East Asia to the West Coast of the U.S. is well defined.  Surface measurements were conducted using a site network in Alaska , Washington and California established through the Interagency Monitoring of Protected Visual Environments (IMPROVE).

The composition of a PM_2.5 carbonaceous particulate from Seattle , Washington evaluated in a Joint EPA/Environment Canada pollution assessment by characterization of the Georgia Basin /Puget Sound Airshed is presented in Figure 4.  The carbon content (Maykut, 2001) was found to be 60%, with 43% attributed to organic carbon (OC) and 17% attributed to elemental carbon (EC).  The carbon content includes an array of PAHs that are transferrable to the biosphere through the food chain and through inhalation of fine particles with high potential for penetration of the pulmonary system.  The higher molecular weight, more reactive PAHs, such as benzo[g]chrysene and dibenzo[a,l]pyrene are found exclusively in the particulate phase because of low vapor pressures.

While BC is a small component of greenhouse gases, it is a more significant component of carbonaceous particulate content.  PAHs integrated with particulate matter are highly penetrating and reactive and are associated with a spectrum of adverse health effects that include cancer, cardiovascular and pulmonary ailments (Sullivan, 2003; Penn, 2005; Veneis, 2005; Sauvain, 2006; and Schwarze, 2006).

An ultra-carcinogen, dibenzo[a,l]pyrene (Table 1), can potentially be utilized as a marker for particulate exposure since its low vapor pressure (~10^-10 torr) restricts it to the particulate phase. Relative Potency Factors (RPFs) and air concentrations of PAHs found in air samples in Eurasian cities are displayed in Table 1. The Relative Potency Factor (RPF value) established by Muller (Muller 1997) for dibenzo[a,l} pyrene is at the high end of the range reported by several investigators (Bostrum, 2002).  The PAH air concentrations in Xuan Wei, China are extraordinarily high compared to other Eurasian cities and indicate the potential impact of trans-Pacific aerosols and particulates by the dynamic resultant trajectories of the polar and subtropical jet streams.  PAH metabolite profiles from protein adducts referencing unreactive and highly reactive PAHs across a range of structural types can provide additional information on human health effects (Lodovici, 1998; Goldman, 2001; Hecht, 2005; Ragin, 2008).  Correlating human exposure data with aerosol particulate projections and determinations can potentially translate a direct human health dimension to climate change where the adverse effects to human health from aerosol particulates may be quantified more directly than resolution of abstract obfuscations clouded by economic and political agendas.

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