Seasonal cycle of surface ozone over the western North Atlantic

Abstract

THE possible impact of pollution from North America and Europe on tropospheric ozone throughout the Northern Hemisphere is a major environmental concern^1–4. We report here continuous measurements of ozone from Bermuda (32° N, 65° W) and Barbados (13° N, 60° W), which suggest that despite their proximity to the eastern US seaboard, natural processes rather than pollution control surface ozone in these regions. Although springtime daily average ozone concentrations at Bermuda are greater than 70 parts per billion (10⁹) by volume (p.p.b.v.) and hourly values in 1989 sometimes exceeded the Canadian Air Quality limit of 80 p.p.b.v., trajectory analyses indicate that these high levels of ozone are transported from the unpolluted upper troposphere >5 km above the northern United States and Canada⁵. During the summer, when surface ozone concentrations over the eastern United States can exceed 70 p.p.b.v. owing to pollution⁶, typical values at Ber-muda are between 15 and 25 p.p.b.v. At Barbados, both the seasonal and diurnal variations in surface ozone are nearly identical to those at Samoa in the tropical South Pacific, where the isolation from anthropogenic sources⁷ and low levels of NO_X (ref. 8) ensure that natural processes control surface ozone^9–11.