Urban dew: Its composition and influence on dry deposition rates

Abstract

The composition of dew collected from a Teflon surface was compared to summer rainwater concentrations at a site in Warren, MI. This comparison showed that natural dew is similar to rainwater with the exception that dew has much higher concentrations of Ca 2+ and Cl − and much lower acidity. Dry deposition rates of several species were measured to artificially-generated dew and a dry surface. It was found that deposition rates were 2–20 times greater to the artificial dew than to the dry surface, indicating that the presence of dew enhances both the retention of dry deposited particles and the absorption of water soluble gases. Measurement of the atmospheric concentrations of the depositing gases permitted the calculation of deposition velocities for particulate Cl −, NO − 3, SO 2− 4, Ca 2+, Mg 2+, Na +, K + and NH + 4. Deposition velocities for gaseous HNO 3, HCl, SO 2 and NH 3 were also determined after correction for particle deposition. These results indicate that acid dew is not a problem at this site. However, the ability of dew to increase the deposition rates of acids and acid precursors to some surfaces suggests that dew may be more acidic at sites with lower deposition rates of basic particles.