Maritime air mass

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. 

Carbonic acid. Water in equilibrium with atmospheric CO2 at 25° C has pH 5.65. Natural acidification of precipitation to pH less than 5.65 can occur in remote maritime air masses due to the presence of NH4HSO4 aerosols (Charlson and Rodhe, 1982). In the soil, the partial pressure of CO2 may be 10-100 and up to 400 times greater than that of the atmosphere (Holland, 1978). Given a CO2 pressure 10 or 100 times that of the atmosphere, pH values of 5.15 and 4.65, respectively, are expected. If additional sources of protons are introduced into the soUs, say, from organic or inorganic acids, the pH is depressed further. 

Table 9-12 adds further observations of NOx in marine air masses and in the free troposphere. Measurements onboard ships and at coastal stations are made difficult due to self-contamination, and the original data must be carefully screened to eliminate conceivable effects of pollution. With these precautions it is then possible to show that the purest maritime air masses contain NOx mixing ratios less than 100 pptv. Broil et al. (1984) have... 

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. 

On an international scale, the Alps are a middle-sized chain of mountains which, due to their situation in the central latitude of Europe, are influenced by maritime as well as continental factors. Humidity is generally transported by the west and south winds flowing from the Atlantic or the Mediterranean towards the mountain chain. With altitudes of up to 4,500 m ASL, the Alps present an enormous barrier to the air masses being transported in this way, and this barrier effect reinforces European meridional temperature gradients [10]. 

Fig. 9.18 Isotopic compositions, as a function of altitude, for precipitation samples collected in a summer month (April 1976) and a winter month (October 1974), Maritime Alps. The data were used to establish local altitude effect equations (see text). Winter and summer <5lsO values fall on separate lines, reflecting the difference in the origin of the precipitating air masses from the Atlantic and Mediterranean, respectively. (After Bortolami et al., 1978.)...

Radon ( Rn) measurements made by the ARCAS provide a simple, reliable, real-time indicator of the relative maritime or continental nature of the air over coastal or oceanic areas (8). With a half-life of 3.8 days, Rn originates from the decay of Ra, a member of the decay chain. At least 98% of Rn originates from land masses (9). The radon flux at the surface depends on the radium content of the soils and rocks, the permeability of the source materials, atmospheric pressure, soil moisture, and vegetative cover (10). Relatively abrupt changes in the radon concentration over the ocean usually indicate changes in air masses and the passage of frontal systems. 

Over the European continent, the North-Atlantic cyclones weaken during their eastward propagation, but often they cause the weather to be quite variable. That means, a very diverse but usually heavy cloud cover with occasional precipitation prevails over the Baltic Sea. It is generally moist, cool, and rather windy. The global west-wind jet carries moist and cool air masses from the North Atlantic and the North Sea to central and northern Europe (Nehring et al., 1990). This leads to a predominantly maritime influence on the Baltic Sea region. This... 

Seasonal variations in the mixing layer affect the frequency of different air masses and hence of different pollutant concentrations a receptor site experiences. Figure 3.5(a) shows the frequency of different air masses experienced in January, at Kew in the UK. Polar maritime air (mP) is less pollutant-loaded than continental polar (cP) air, is unstable and has a frequency of 30% or more in all months, except March. True continental polar air only affects the British Isles between December-February, and is basically very stable. Similar air mass frequencies would be experienced in north west Europe. In Germany, the incidence of air mass movements from the south is very low. 

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