Recent ice core measurements show lead concentrations increasing since 1970, suggesting new nonautomobile-related sources of Pb are becoming important worldwide (1). Developing a full understanding of the major sources of Pb and other metals is critical to controlling these emissions. During the March, 2006 MILAGRO campaign, single particle measurements in Mexico City revealed the frequent appearance of particles internally mixed with Zn, Pb, Cl, and P. Pb concentrations were as high as 1.14 µg/m 3 in PM 10 and 0.76 µg/m 3 in PM 2.5 . Real time measurements were used to select time periods of interest to perform offline analysis to obtain detailed aerosol speciation. Many Zn-rich particles had needle-like structures and were found to be composed of ZnO and/or Zn(NO 3 ) 2 · 6H 2 O. The internally mixed Pb-Zn-Cl particles represented as much as 73% of the fine mode particles (by number) in the morning hours between 2-5 am. The Pb-Zn-Cl particles were primarily in the submicrometer size range and typically mixed with elemental carbon suggesting a combustion source. The unique single particle chemical associations measured in this study closely match signatures indicative of waste incineration. Our findings also show these industrial emissions play an important role in heterogeneous processing of NO y species. Primary emissions of metal and sodium chloride particles emitted by the same source underwent heterogeneous transformations into nitrate particles as soon as photochemical production of nitric acid began each day at ∼7 am. IntroductionParticulate air pollution is correlated with increased morbidity and mortality through cardiovascular and pulmonary effects (2). Although relatively little is known about the specific chemical constituents responsible for the adverse health effects, metal-containing particles are implicated in a number of studies (3-5). The solubility of metal ions present in particles affects their mobility in the human body while their oxidation state greatly affects their toxicity (5). Other factors such as particle size and shape determine how deep into the respiratory tract a particle may travel. Smaller particles with a compact morphology penetrate deeper into the lungs where they are more likely to be retained by the body (6-8). Anthropogenic particles created by high temperature processes (e.g., combustion and ore processing) possess many of the properties responsible for adverse health effects.In urban areas, anthropogenic sources of submicron metal-containing particles are plentiful. For example, the burning of fossil fuel leads to the association of Ni and V within particles (9). Prior to 2000, tetra-ethyl-lead was used as a gasoline additive in many countries and resulted in traffic related emissions of submicron lead particles (10). In industrial areas, smelting produce particulate emissions rich in heavy metals (11). Combustion of municipal waste produces submicron particles composed of Zn, Pb, and Cl as well as numerous other metals (12). Zn and Pb are often found to be internally ...