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

Abstract

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 Δ17O = δ17O − 0.52 × δ18O 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 Δ17O anomaly indicating either uptake or in situ formation of secondary sulfate on sea spray. Non‐sea‐salt sulfate Δ17O in the coarse fraction is consistent with (1) either primarily coagulation of finer sulfate particles, when Δ17O is low in all size fractions, or (2) ozone‐driven oxidation of SO2 within the sea spray, as observed in the relatively higher Δ17O in coarse particles compared to fine. It is proposed that triple‐isotope measurements of sulfate oxygen can be used to quantify the budget of in situ sea spray nss‐SO4 formation. The Δ17O 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 Δ17O, 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.