Continental air mass

Knowledge about the isotopic variations in precipitation is increased when single rain events are analyzed from local stations. Especially under mid-latitude weather conditions, such short-term variations arise from varying contributions of tropical, polar, marine, and continental air masses. 

Average ratios for Si, Al, K, Ca, and Ti to Fe approximate earth crust ratios, indicated by arrows at left. These elements occur mainly in coarse particles (Figures 3 and 4). The Mn/Fe ratio is somewhat greater and Zn/Fe is 20 times greater than earth crust ratios. These two elements have appreciable fine-mode concentrations (Figure 4). All 8 elements may occur characteristically in continental air masses. 

Similarly, Martin and co-workers (1994) measured aerosol particles in the size range from 0.05 to 1.5 /rm below the base of stratocumulus clouds, along with cloud droplet number concentrations in maritime and in continental air masses. Figure 14.46 shows the relationship between cloud droplet number concentration and the aerosol particle concentration for a set of flights carried out in the vicinity of the British Isles and in the South Atlantic (Martin et al., 1994). There is an almost linear relationship between the two for maritime air masses. Given that the cutoff for particle measurements was 0.05 /xm, these concentrations may have been underestimated, so that the slope of the line for maritime air masses can be taken as unity. That is, essentially all of the maritime particles at the cloud base could act as CCN under the range of supersaturations in these studies. 

However, this relationship did not hold true for continental air masses. The fraction of aerosol particles that lead to cloud droplet formation is clearly less than one, in agreement with the studies of Gillani et al. (1995) discussed earlier. In addition, the relationship is much more scattered, indicating that the chemical... 

FIGURE 14.46 Average cloud droplet number concentration as a function of subcloud aerosol particle concentration (0.05-1.5 fj.m) in marine ( ) and continental ( ) air masses (adapted from Martin et al., 1994). 

Tritium in water vapor has also been utilized in the study of meteorology. Kigoshi and Yoneda observed daily variations in the tritium content of atmospheric moisture (water vapor) collected in Tokyo. High tritimn contents indicate the arrival of continental air masses from the north, and the low contents in tropical low-pressure air masses are nearly equal to that of surface ocean water. 

Surface ozone measurements from 1992 to 1997 of Cape Hebdo (26°N, 128°E) at the northern tip of Okinawa Island were analyzed in order to stndy ozone trends in northeast Asia. The measurements from October to March were selected, because during this time the site was often in the ontflow of the continental air masses. The cases which represent polluted air masses advected from the continent were selected by trajectory analyses. From this collective an increasing trend of ozone of 2.6 2.0% per year was calculated in the polluted air of the continent. An increasing trend of the annual mean valnes was also found in the background air of Japan (see previous discussion). A further increase in ozone concentrations in east Asia is predicted becanse the NOx emissions of the Asian continent are expected to increase by a factor of 3 from 1990 to 2010 if no measnres are taken for emission reduction. 

Distinct air masses are a major feature of the troposphere. These air masses are uniform and horizontally homogeneous with respect to temperature and water vapor content. These eharacteristics are determined by the nature of the siuface over which a large air mass forms. Polar continental air masses form over cold land regions polar maritime air masses form over polar oceans. Air masses originating in the tropics may be similarly classified as tropical continental air masses or tropical maritime air masses. The movement of air masses and the conditions in them may have important effeets upon pollutant reactions, effects, and dispersal. 

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