Aerosol Size Distributions of Polycyclic Aromatic Hydrocarbons in Urban and Over-Water Atmospheres

Aerosol mass size distributions of 41 polycyclic aromatic hydrocarbons (PAHs) were measured during 20 different 12-h periods in urban Chicago and over Lake Michigan during July 1994 and January 1995. Geometric mean aerodynamic equivalent diameters (GMDs) range from 0.72 to 2.39 μm for particulate matter and from 0.33 to 9.85 μm for individual PAHs. GMDs of the less volatile PAHs are larger in the urban atmosphere than over the water during the summer. Geometric standard deviations of the particle size distributions, however, are larger at the urban location for many PAHs, indicating a broader mass size distributions. GMDs of unsubstituted PAHs (except perylene) are well correlated with their log subcooled liquid vapor pressures ( , Pa), following the form:  GMD = m_g log + b_g . Values for m_g and b_g range from 0.03 to 0.88 and from 0.83 to 8.80, respectively. Higher molecular weight PAHs are sorbed to the finest sized aerosols, but more volatile PAHs are associated with larger particles. The slope (m_g) and intercept (b_g) of these regressions are interdependent in these field data and follow the model:  b_g = m_hm_g + b_h, where m_h = 9.55, b_h = 0.61, and r² = 0.98, suggesting that all GMD vs log regressions for a class of semivolatile compounds tend to intersect at the same point (−m_h, b_h). This may allow the size distributions of the entire class of PAHs to be estimated by measuring the distribution of one PAH that is sufficiently removed from this intersection point. PAH size distributions change downwind of urban emission sources due to selective deposition of larger aerosols during atmospheric transport.