Long-term record of nss-sulfate and nitrate in aerosols on Midway Island, 1981–2000: Evidence of increased (now decreasing?) anthropogenic emissions from Asia

[1] Increasing anthropogenic emissions from Asia, especially from regions undergoing rapid industrial development, have raised interest in the outflow of chemically and radiatively important gases and aerosols. Previous studies have shown that high concentrations of Asian pollution spread over a broad region of the North Pacific every spring. Here we report on studies of aerosol concentrations at Midway Island (28 130N, 177 220W) in the central North Pacific over the period 1981–2000. Using a relatively simple procedure we estimate the natural and anthropogenic fractions of sulfate and nitrate aerosol and show that the estimated anthropogenic component almost doubled from 1981 to the mid-1990s. This increase closely parallels estimates of increased emissions of SO2 from China. However, measurements in the late 1990s suggest that sulfate and nitrate concentrations have stabilized and perhaps decreased. Thus over the longer term pollution emissions from Asia and concentrations over the North Pacific may be less than earlier projections, a factor which has implications for the assessment of future climate trends.