Isotopic analysis of aerosol sulfate and nitrate during ITCT-2k2: Determination of different formation pathways as a function of particle size

The triple isotopic composition of oxygen in sulfate and nitrate, and the sulfur isotopic composition of the sulfate fine fraction, have been measured on size- segregated aerosol samples collected at Trinidad Head, coastal California, alongside the ITCT- 2k2 campaign in April - May 2002. The isotopic anomaly Delta O-17 = delta O-17 - 0.52 x delta O-18 has been determined in both sulfate and nitrate and was used as a specific tracer of the formation pathways of these species. Coarse mode sulfate in all samples exhibited a small but significant Delta O-17 anomaly indicating either uptake or in situ formation of secondary sulfate on sea spray. Non- sea- salt sulfate Delta O-17 in the coarse fraction is consistent with (1) either primarily coagulation of finer sulfate particles, when Delta O-17 is low in all size fractions, or ( 2) ozone- driven oxidation of SO2 within the sea spray, as observed in the relatively higher Delta O-17 in coarse particles compared to fine. It is proposed that tripleisotope measurements of sulfate oxygen can be used to quantify the budget of in situ sea spray nss- SO4 formation. The Delta O-17 measured in size- resolved nitrate revealed, for the first time, differences in the nitrate formation budget as a function of particle size in a given air mass. The coarse particle nitrate possessed a higher Delta O-17, suggesting a relatively larger N2O5 hydrolysis contribution to the nitrate formation budget compared to fine particles where homogeneous formation is more important. We conclude that the complete isotope ratio analysis may provide a basis for future modeling of the formation and transformation processes of the soluble aerosol, based on direct observation of the mechanisms.