The Size and Scattering Coefficient of Urban Aerosol Particles at Washington, DC as a Function of Relative Humidity

The relative humidity dependence of the size and scattering coefficient of atmospheric aerosol particles was measured at Washington, D.C., during the period July 26-31, 1979. Particle growth curves (i.e., curves of the ratio r/r sub(0) , of particle radius to particle dry radius, vs. relative humidity) were calculated by using measured values of the particle composition parameter B [idential] nu phi epsilon M sub(w) rho sub(0) / rho sub(w) M sub(s) , where rho sub(0) is the density of the particle in dry state; rho sub(w) and M sub(w) are the density and molecular weight of water; epsilon is the mass fraction of soluble material in the particle; M sub(s) is the mean molecular weight of the soluble material; nu is the mean number of moles of ions per mole of solute; and phi is the mean value of the practical osmotic coefficient. To determine B, a mobility analyzer was used to transmit dry particles of nearly uniform size to a thermal gradient diffusion cloud chamber where the supersaturation (S sub(c) ) necessary to activate the particles was measured. Then, a relationship between S sub(c) , r sub(0) , and B was used to determine B. The measurements were made for particles in the size range 0.03-0.08- mu m radius. The value of B ranged from 0.10 to 0.40 and averaged 0.23 for the period. For B = 0.23 and r sub(0) = 0.05 mu m, the computed values of r/r sub(0) are 1.22, 1.37, 1.57, and 1.90 at relative humidities of 80, 90, 95, and 98%, respectively. The light scattering coefficient as a function of (decreasing) relative humidity was measured directly by controlling the relative humidity of the air flowing to an integrating nephelometer. A smooth variation of scattering coefficient with humidity was observed. On the average, the scattering coefficient changed by a factor of 3.5 between 30 and 95% relative humidity. In general, the measured dependence of the scattering coefficient upon relative humidity was consistent with that computed from the particle growth curves.