Virus-sized Pollutants

Posted on | October 12, 2009 | Comments Off

Small but Mighty: Virus-sized Pollutants Affect Mexico City Human Health and Climate
By Rachel Hauser

Air pollution causes significant human health and environmental problems; in megacities, with populations in excess of 10 million, these impacts are magnified. A growing body of evidence suggests that atmospheric nanoparticles – particles between 3 and 50 nanometers, about the size of a simple virus – can pose a major health threat. Once inhaled, nanoparticles can translocate from the lungs to the rest of the body. By understanding the genesis, growth, and concentrations of such particles, scientists and decision makers gain vital information on local and regional threats to human health by urban pollutants. In addition to health concerns, the rate of nanoparticle formation also influences cloud formation by increasing the concentrations of the nuclei upon which cloud droplets form, which in turn affects global climate.

Nanoparticles can form in the atmosphere when aged pollutant gases cluster together to form a nascent particle, or they can be directly emitted from the pipes of motor vehicles and industries. The distinctions between these two processes are important: nucleation can occur spontaneously over hundreds of square miles and thus affect large regions, whereas directly emitted nanoparticles primarily affect those close to the sources.

Mexico City provides a unique and important location to study nanoparticle formation. Its distinctive geography (located 7,400 feet above sea level, the city is encircled by mountains that effectively trap pollutants), large human population (18 million), and significant number of cars (about 3.4 million) all contribute to making it among the world’s most polluted cities. Until recently, little information was available about the distribution and dynamics of nanoparticle formation in and around Mexico City. Seeking to gain a better understanding of how nanoparticles form in this region, a team of scientists from the National Center for Atmospheric Research (NCAR) and the University of Minnesota collected this information as part of the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign.

Using several instruments, including the recently developed Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), the team found sub-10-nanometer particles developing at unexpectedly dramatic rates. Recent analysis of the MILAGRO data, published in Geophysical Research Letters in 2008, shows that 84% of the nanoparticles captured during Mexico’s almost-daily pollution surges were organic in nature; this discovery sheds new light on nanoparticle formation.

For decades, researchers believed that gaseous sulfuric acid was critical for nanoparticle formation and subsequent particulate growth. This study shows that sulfuric acid plays a relatively minor role in the nanoparticle growth once they are formed. Based on the MILAGRO results, current models that do not include the role of organics in processes may be dramatically underestimating the impact of aerosols on human health and climate. In addition, the frequency of these particle formation events – often occurring every day for more than a week – underscores the importance of understanding this phenomenon for protecting the growing populations in our megacities.

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