Marine biogenic and anthropogenic contributions to non‐sea‐salt sulfate in the marine boundary layer over the North Atlantic Ocean

As a part of the Atmosphere/Ocean Chemistry Experiment (AEROCE), daily aerosol samples were collected in the marine boundary layer at Barbados, West Indies (13.17°N, 59.43°W), Bermuda (32.27°N, 64.87°W), and Mace Head, Ireland (53.32°N, 9.85°W), and in the free troposphere at Izaña, Tenerife, Canary Islands (28.30°N, 16.48°W; 2360 m asl). In this report, we use multiple variable regression analyses with methanesulfonate (MSA) and Sb and/or NO 3 − as the independent variables to assess the relative contributions of the marine biogenic and anthropogenic sources to the total non‐sea‐salt (nss) SO 4 2− concentrations at the AEROCE sites. On the basis of 2 years of data at Bermuda and Barbados, the marine nss SO 4 2− /MSA mass ratios (19.6 ± 2.1 and 18.8 ± 2.2) were consistent throughout the year and comparable to those at American Samoa in the tropical South Pacific (18.1 ± 0.9). At Mace Head (based on 1 year of data), this ratio was about 3.01 (±0.53). An analysis of the residuals and an assessment of the root mean square deviations indicate that the ratio at Mace Head can also be reasonably applied throughout the year. However, there is enough uncertainty during the winter that we cannot rule out a significant increase (to about 20) during periods with low concentrations of both MSA and NO 3 − . The results from 4 years indicate that the marine contribution is too low to permit a reasonable assessment of the biogenic nss SO 4 2− /MSA ratio at Izaña. The continental nss SO 4 2− /Sb mass ratio varies significantly from one location to another. At Bermuda, where North American sources are expected to dominate, the ratio is about 29,000, about a factor of 2 higher than the average of 13,500 at Mace Head where European sources dominate. Intermediate values occurred at Barbados (18,000) and Izaña (24,000) where both European and North African sources are significant. Estimates based on these ratios indicate that, on an annual basis, the contributions from anthropogenic sources account for about 50% of the total nss SO 4 2− in aerosols at Barbados, 70% at Bermuda, 85–90% at Mace Head, and about 90% at Izaña. If the same biogenic nss SO 4 2− /MSA ratios are applicable to rainwater, then the relative contributions in precipitation at Barbados and Bermuda are comparable to those in aerosols.