MEASUREMENT OF EXPECTED NUCLEATION PRECURSOR SPECIES AND 3-500-NM DIAMETER PARTICLES AT MAUNA-LOA-OBSERVATORY, HAWAII

Atmospheric measurements of expected homogeneous nucleation precursors and aerosols were made at the Mauna Loa Observatory, Hawaii, from 28 June to 27 July 1992. Large molecular clusters and gas phase specie in cluding sulfuric acid (H2SO4), methane sulfonic acid (MSA), and OH wer e measured with a mass spectrometer. Aerosol measurements consisted of ultrafine particle concentrations (similar to 3-4 nm diameter) and fi ne particle size distributions (15-500 nm diameter). The altitude of t he observatory (average pressure 680 mb) and the presence of mountain- valley winds permitted sampling of both up- and downslope air. Downslo pe air was found to have the highest concentrations of MSA but low ult rafine concentrations. whereas, upslope air had the highest H2SO4 and ultrafine concentrations. Episodes of substantial increase in ultrafin e particles were observed during periods of rapid increase in H2SO4 co ncentrations. Total aerosol surface area and H2SO4 concentrations had the strongest influence on ultrafine particle concentrations. It is co ncluded that for the conditions at Mauna Loa, H2SO4 was a vapor precur sor of the newly formed particles, and MSA contributed little to new p article formation. Low concentrations of ultrafine particles were ubiq uitous in upslope air and may indicate widespread, low nucleation rate , source of new particles. The data were also used for estimating part icle nucleation and growth rates. Measurements taken suggest that nucl eation rates exceed values predicted by the classic theory for binary H2SO4-H2O nucleation.