3.1.3. Annual Cycles

The annual cycles of aerosol optical properties for the four baseline and three regional stations are illustrated in Figures 3.1a-d and Figures 3.2a-e. The data are presented in the form of box and whisker plots that summarize the distribution of values. Each box ranges from the lower to upper quartiles with a central bar at the median value, while the whiskers extend to the 5^th and 95^th percentiles. The statistics are based on hourly averages of each parameter for each month of the year, as well as for the entire year.

Fig. 3.1. Annual cycles for baseline stations at BRW, MLO, SMO, and SPO; values representing the entire year period, for all years in the record, are also presented (ANN): (a) CN concentrations, (b) s_sp at 550 nm, (c) å (550/700 nm), and (d) Annual cycles of s_ap and w0 for baseline stations at BRW and MLO.

Fig. 3.2. Annual cycles for regional stations at Bondville, Illinois (BND), Sable Island, Nova Scotia (WSA), and Lamont, Oklahoma (SGP): (a) CN concentration, (b) s_sp at 550 nm, (c) å, (550/70 nm), (d) s_ap at 550 nm, and (e) w0 concentration.

In general, changes in long-range transport patterns dominate the annual cycles of the baseline stations. For BRW, the highest values of CN, s_sp, and s_ap are observed during the spring arctic haze period when anticyclonic activity transports pollution from the lower latitudes of Central Europe and Russia. A more stable polar front characterizes the summertime meteorology. High cloud coverage and precipitation scavenging of accumulation mode (0.1-1.0 mm diameter) aerosols account for the annual minima in s_sp and s_ap from June to September. In contrast, CN values have a secondary maximum in the late summer which is thought to be the result of sulfate aerosol production from gas to particle conversion of DMS oxidation products from local oceanic emissions [Radke et al., 1990]. The aerosol single-scattering albedo displays little annual variability with values between 0.90–0.98, which is indicative of highly scattering sulfate and seasalt aerosol. A September minimum is observed in å when s_sp and accumulation mode aerosols are also low but when primary production of coarse mode seasalt aerosols from open water is high. For MLO, the highest s_sp and s_ap values occur in the springtime and result from the long-range transport of pollution and mineral dust from Asia. However, little seasonality is seen in CN concentrations at MLO, indicating that the smallest particles (<0.1 mm diameter), which usually dominate CN concentration, are not enriched during these long-range transport events. Both the aerosol s_sp and Ångström exponent display seasonal cycles at SPO with a s_sp maximum and an å minimum in winter associated with the transport of coarse mode seasalt from the antarctic coast to the interior of the continent. The summertime peaks in CN and å are associated with fine mode sulfate aerosol and correlate with a seasonal sulfate peak found in the ice core presumably from coastal biogenic sources [Bergin et al., 1998a]. The aerosol extensive properties at SMO display no distinct seasonal variation.

Based on only 2-5 years of measurements, the annual cycles for the regional stations are less certain than those of the baseline stations. The proximity of the regional sites to North American pollution sources is apparent in the results, with monthly median values of CN and s_sp that are nearly two orders of magnitude higher than values from the baseline stations. The Bondville site (BND), situated in a rural agricultural region, displays autumn highs in s_ap and CN and a low in w₀ which coincide with anthropogenic and dust aerosols emitted during the harvest. As evident in the lower CN, s_sp, and s_ap values, the Southern Great Plains site (SGP) is more remote than BND. SGP has a similar but less pronounced annual cycle with late summer highs in CN, s_sp, and s_ap, and a corresponding minimum in w₀. Little seasonal variability is observed in WSA aerosol properties. CN and å values tend to be higher in the summer and likely result from transport of fine mode sulfate aerosol from the continent and lower coarse-mode seasalt aerosol production associated with lower summer wind speeds.

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