GAS-TO-PARTICLE CONVERSION OF TROPOSPHERIC SULFUR AS ESTIMATED FROM OBSERVATIONS IN THE WESTERN NORTH PACIFIC DURING PEM-WEST-B

Aircraft observations during the Pacific Exploratory Mission in the we stern Pacific Ocean, phase B (PEM-West B), taken in February-March 199 4, have been used to constrain a numerical model that calculates local concentrations of gaseous H2SO4, rates of homogeneous nucleation, and concentrations of newly formed, nanometer-sized particles. The data w as selected from 13 flights over the western Pacific Ocean that covere d an altitude range from the boundary layer (BL) to the upper troposph ere (UT) and latitudes from 10 degrees S to 60 degrees N. The largest nucleation rates were calculated for the data from the flights over th e temperate latitudes (lambda>30 degrees N). Within these latitudes, h omogeneous nucleation rates averaged about 1-100 particles cm(-3) s(-1 ). Significantly smaller nucleation rates were calculated for the trop ical (lambda<20 degrees N and subtropical (20 degrees N<lambda<30 degr ees N) regions. In the tropics, average nucleation rates in excess of 10 particles cm s Mere limited to the UT. In the subtropics, large ave rage nucleation rates in excess of 1 particle cm(-3) s(-1) were obtain ed in the BL and in the UT, and average rates of about 10(-1) particle s cm(-3) s(-1) were obtained for the rest of the troposphere. The rela tively large nucleation rates calculated for the temperate latitudes c ould be largely attributed to the cold temperatures encountered in thi s region during the PEM-West B flights. For the data from the tropical and subtropical flights, little or no homogeneous nucleation was calc ulated for the average conditions encountered in the BL and midtroposp here (MT). Instead, significant nucleation was limited either to the U T or to several small-scale events. These enhanced nucleation events w ere generally characterized by spikes in relative humidity and low aer osol surface density. However, the strongest nucleation events, with h omogeneous nucleation rates of about 10 particles cm(-3) s(-1), were a ssociated with high concentrations of SO2, most likely as a result of pollution from the Asian continent. Our results imply that in regions in which homogeneous nucleation is dominated by small-scale fluctuatio ns, approaches that attempt to infer nucleation rates using average or typical conditions will grossly underestimate the actual average rate of nucleation.