The respective interval widths are 0.025 and 0.125 in the case of the frequency distributions of the aerosol optical thickness and the Ångström

exponent. Histograms of AOT(500) vary from a sharp distribution ( Figure 3c) with a modal value of 0.050 for autumn to broader distributions with a modal value of 0.075 for the spring and summer seasons ( Figures 3a, 3b). The distributions are skewed towards higher values (right-skewed). All histograms of α(440, 870) are BIBF1120 skewed towards lower values (left-skewed) ( Figures 3d–3f). The most probable respective values for spring, summer and autumn seasons are 1.375, 1.750 and 1.625. The distribution of α(440, 870) for summer is sharper than during spring and autumn conditions. GSI-IX Many papers relate aerosol optical properties, e.g. the Ångström exponent, to a type of aerosol. However, the threshold for α(440, 870) usually used to distinguish marine aerosols varies depending on the author. Kuśmierczyk-Michulec et al., 2001 and Kuśmierczyk-Michulec et al., 2002 adopted a threshold of 0.26 (i.e. α(400, 865) ≤ 0.26) for those instances when sea salt controls aerosol optical thickness, whereas Smirnov et al. (2003) applied a much higher value of the Ångström exponent (α(440, 870) ≤ 1.0) and AOT(500) ≤ 0.15 to describe pure marine aerosols. Kuśmierczyk-Michulec (2009) concluded

that an Ångström exponent < 0.5 indicates the marine aerosol type, values of α(440, 870) between 1.0 and 1.5 represent the continental aerosol type, and values > 1.5 the industrial aerosol

type. Over Gotland, α(440, 870) ≤ 1.0 only make up 20%, 8% and 32% of observations in spring, summer and autumn respectively. In autumn, Ångström exponents Ketotifen < 1 are more frequently observed (32%) than in the other seasons, which indicates a higher contribution of marine aerosols. Even though the thresholds given above are approximate, the seasonal frequency distributions of the Ångström exponent with modal values ranging from 1.375 to 1.750 ( Figure 3) clearly indicate the high contribution of the mixed continental-industrial type of aerosols in the Baltic atmosphere throughout the year, but especially in summer. On the basis of the same Gotland AERONET station dataset from the period 1999–2001, Carlund et al. (2005) concluded that normally, the atmosphere over Gotland could be considered clear, with a daily median value of AOT(500) of about 0.08. The median value of α(440, 870) was 1.37, indicating that the dominant aerosol was more of a continental than of a pure marine type. Means of the seasonal distributions of AOT(500) and α(440, 870) are given in Table 2. The histograms of AOT(500) and α(440, 870) are skewed. Their longer tails contain extreme cases, with AOT(500) several times higher and α(440, 870) several times lower than the respective modal values.