3.1.4. Long- Term Trends

Long-term trends in CN concentration, s_sp, s_ap, w0, and å are plotted in Figure 3.3a-d for the baseline observatories. The trends are plotted for the annual mean as well as for the averages for 2 months to highlight trends associated with different seasons. The aerosol properties at BRW exhibit an annual decrease in s_sp of about 2% per year since 1980. This reduction in aerosol scattering has been attributed to decreased anthropogenic emissions from Europe and Russia [Bodhaine, 1989] and is most apparent during March when the arctic haze effect is largest. The corresponding decrease in the Ångström exponent over the same time period points to a shift in the aerosol size distribution to a larger fraction of coarse mode seasalt aerosol. Stone [1997] noted a long-term increase in surface temperatures and cloud coverage at BRW from 1965-1995 which derive from changing circulation patterns and may account for the reduction in s_sp by enhanced scavenging of accumulation mode aerosols. In contrast to the reduction in s_sp at BRW, CN concentrations, which are most sensitive to particles with diameters <0.1 mm, have increased at a rate of 3% yr^-1 since 1976. Here, gas-to-particle conversion processes that produce these nucleation mode particles may be enhanced with respect to the decreased surface area of larger accumulation mode aerosols. The volcanic eruptions of El Chichón and Mt. Pinatubo in 1982 and 1991, respectively, seem to dominate any long-term trends in aerosol properties at MLO with peaks in CN, s_sp, and s_ap following these eruptions. This downward mixing of stratospheric sulfate aerosol from El Chichón and Mt. Pinatubo is also apparent at SPO with corresponding peaks in s_sp in these years. The step increase in CN at SPO in 1989 is due to replacement of the CN counter with a butanol-based instrument with a lower size detection limit. The reason for the decrease in CN and increase in å at SMO is not readily apparent, but it could stem from changes in long-term circulation patterns.

Fig. 3.3. Long-term trends for baseline stations showing months with the lowest and highest median values and annual averages for each year (ANN): (a) CN concentration, (b) s_sp at 550 nm, (c) å (550/700 nm), (d) s_ap and w0 at BRW and MLO.

The three regional stations, as well as BRW (since October 1997), have size-resolved sampling inlets with 1-mm and 10-mm diameter size cuts that permit the determination of the submicrometer aerosol contribution to the total aerosol optical properties. Figure 3.4 shows box and whisker plots for the different stations of the fractional contribution of submicrometer particles to scattering, backscattering, absorption, and single-scattering albedo. The size-resolved aerosol properties are part of a recent upgrade at BRW and contain data from only the last 3 months of 1997. The abundance of coarse mode seasalt aerosol at WSA is reflected in a smaller contribution of the submicrometer aerosol to the total aerosol scattering and absorption relative to the other continental sites. The spread of the WSA data suggest a highly variable air mass that changes between clean marine, polluted continental, and mixed regions. Only slight differences are apparent between the two regional continental sites.

Fig. 3.4. The fractional contribution of submicrometer particles to aerosol light scattering, backscattering, absorption, and single-scattering albedo at BRW, WSA, SGP, and BND.

Previous reports describing the aerosol data sets include: BRW: Bodhaine [1989, 1995]; Quakenbush and Bodhaine [1986]; Bodhaine and Dutton [1993]; Barrie [1996]; MLO: Bodhaine [1995]; SMO: Bodhaine and DeLuisi [1985]; SPO: Bodhaine et al. [1986, 1987, 1992]; Bergin et al. [1998a]; WSA: McInnes et al. [1998].

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