Chemical and physical characterizations of atmospheric aerosols over southern California

Abstract

In September 1994 the chemical and physical properties of aerosol particles over southern California were characterized. Concentrations of particle chemical speaes were higher near the surface than at higher altitudes. In these particles, measured organic and inorganic water soluble masses were 7 ± 1% and 75 ± 12%, respectively, of the dried total particle mass (TPM). Non-sea-salt (nss) SO=4, NO-3, NH+4, and H+ were the major contributors to the mass and ionic equivalence of the particles. The ratio of the soluble organics (SumOrg) to nss SO=4 was found to be a function of the TPM, which was similar to the simple relationship found over the northwestern Atlantic Ocean for altitudes <3 km. When the H+ ion was included, there was a good ionic balance between the cations and anions in the particles. The pH of the particles ranged from -2.4 to 0.20 (averaging -0.85). The particle volume size distribution dV/d log D shows distinct features according to the altitude and location of sample collection. In urban plumes, dV/d log D was significantly higher than at high altitudes and had a consistent accumulation mode peaking at 0.24 μm. For all samples, nss SO=4 and NH+4 comprised 59% of the accumulation mode particle mass on average. Samples showed two groups with high and low NH+4/nss SO=4 ratios of 0.85 and 0.01, respectively. For the firs group the low ambient relative humidities indicate that NH+4 and nss SO=4 were in crystallized ammonium sulfate and letovicite in equilibrium with solution, while the second group was probably close to H2SO4 droplets. On the basis of these compositions the refractive index η was found to be about 1.5 for the first group and <1.4 for the second group. The water contents of the NH+4 - nss SO=4 particles were estimated to be 35 ± 24%, leading to an average ratio of H2O/nss SO=4 = 0.9 ± 1.2.