In marine air masses

In this work we compare some of the major results from two field expeditions to the Southern Ocean region. The first expedition was a ship cruise between Punta Arenas/Chile and the west coast of the Antarctic Peninsula, which was conducted during the austral fall season (March 20 - April 28, 1986) as part of the United States Antarctic Research Program (USARP). Details of the ship cruise and a complete discussion of the results have been published elsewhere (12). The second expedition consisted of a series of flight measurements off the west and south coast of Tasmania during the austral summer season (December 3-18, 1986). We discuss preliminary results from 2 of 8 flights which were conducted in marine air masses behind cold fronts moving over the open ocean. 

Dimethyl sulfoxide (DMSO) has recently been detected in marine air masses. To date nothing is known about the atmospheric fate of DMSO in the gas phase. Reported here are product and kinetic studies on the reactions of OH, NO3 and Cl radicals with DMSO. The investigations were performed in a 420 1 reaction chamber at atmospheric pressure using long path in situ Fourier transform (FTIR) absorption spectroscopy for detection of reactants and products. 

Wet-chemical analysis and optical methods have been used to determine the abundance of formaldehyde in ambient air. Table 4-9 shows mixing ratios observed at various locations. The highest values are associated with urban air, especially under conditions of photochemical smog. Maximum values may then reach 70-100 ppbv. Mixing ratios in rural areas are of the order of a few ppbv, and still lower values are found in marine air masses. 

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... 

Aerosol surface area is likely to be variable even within a remote marine air mass. Previous MBL aerosol studies describe changes in aerosol concentration and composition due to entrainment from the free troposphere (Bates et al., 1998, 2001 Covert et al., 1998). Raes et al. (1997) found an observable link between vertical transport patterns and aerosol variability in the MBL specifically in the Aitken mode (<0.2/u.m). Hence entrainment of aerosol from the free troposphere appears to occur frequently, even in remote MBL air masses. In addition, aerosols have the capacity to travel great distances in the free troposphere, before being entrained into the MBL. 

Shaw, G., Aerosol Chemical Components in Alaska Air Masses. 2. Sea Salt and Marine Product, J. Geophys. Res., 96, 22369-22372 (1991). 

As expected from the earlier discussion of the Kohler curves, not all particles act as CCN. For example, only about 15-20% of the Aiken nuclei (see Chapter 9.A.2) in a marine air mass off the coast of Washington state acted as CCN at 1% supersaturation (Hegg et al., 1991b). Similarly, in a marine air mass in Puerto Rico, between 24 and 70% of the particles measured at 0.5% supersaturation before cloud formation led to cloud droplet formation (Novakov et al., 1994). 

Without understanding the actum mechanism of the artifact testing of scrubbers must be an empirical process. We have tested a variety of scrubbers in Miami air by using standard additions to ambient air samples. Two air samples were collected simultaneously, and a liquid standard containing DMS was added to the inlet of one channel. The standard additions were done at levels equal to or less than the amount of DMS collected in the ambient samples. Complete recoveiy of standards was achieved with all scrubbers when sampling marine air masses, i.e. in the case of onshore (easterly) winds. Losses... 

In the atmosphere peroxy radicals react with NO, NO2, HO2 radicals and other peroxy radicals (R 02). The importance of these reactions is dictated by the abundances of NO, NO2, and HO2 radicals and by the rates of the reactions of RO2 radicals with these species. In the troposphere the concentrations of NO, NO2, and HO2 vary widely, however, for the present purposes reasonable average concentrations are approximately (2.5—10) x 10s cm-3. Under atmospheric conditions, typical rate constants for the reactions of RO2 radicals with NO, NO2, and HO2 radicals lie in the ranges (8-20)xlO-12, (5-10) xlO 12, and (5-15)xl0 12 cm3molecule 1 s, respectively [4]. Hence, on average these reactions are of comparable importance in the atmospheric fate of RO2 radicals. On a local scale one reaction may dominate because of variation in the concentrations of NO (NO and NO2) and HO2 radicals. Thus, in remote marine locations with low NO levels, reaction of RO2 radicals with HO2 will be much more important than in urban air masses with high NO concentrations. 

Fig. 6-7. Concentrations of n-alkanes in pure marine air masses and in seawater observed at Loop Head, Ireland (filled symbols) and at Cape Grim, Tasmania (open symbols) according to measurements of Eichmann et aL (1979, 1980).

In Beirut, the elemental composition study of coarse and fine particles showed that crustal elements like Si, Ca, K, Ti, Mn and Fe were prevalent in the coarse fraction while in the fine fraction S, Cu, Zn and Pb predominated. All-time high Ca concentration was due to the abundance of limestone rocks, rich in calcite, in Lebanon, and increased Cl levels correlated with marine air masses. In PM2.5, sulfur concentrations were more prominent in the summer due to the enhancement of photochemical reactions. Sources of sulfur were attributed to local, sea-water and long-range transport from Eastern Europe, with the latter being the most predominate. Anthropogenic elements like Cu and Zn were generated from worn brakes and tires in high traffic density areas. Spikes of Pb were directly linked to... 

Atmospheric radon concentrations at Im from the ground are lower than those in soil by a factor of 1000. This sharp drop is due to fast mixing in the air and the relatively short half-life of Rn. Isotopes of radon gas and their decay products provide a unique set of tracers for the study of transport and mixing processes in a wide variety of atmospheric phenomena. The Rn isotope has been more widely used because of its longer half-life and the greater relative abundance in the free atmosphere. For near ground level, tracer studies have been based on thoron mainly because of its short half-life (55.6 s). Since the ocean represents a poor radon source compared with the soil and rocks of continental areas, marine air masses contain only 1% or less radon per unit volume than air over large land areas. Collection... 

A field study was conducted in March-April 1992 on El Yunque peak in Puerto Rico, a site that is influenced by both marine air masses and anthropogenic emissions, to determine the relative contributions of sulfate and organic aerosols... 

Relatively large deviations from the general equation are evident when monthly data for individual stations are considered (Table 3.1). In an extreme situation, represented by the St. Helena station, a very poor correlation between 8D and 5 0 exists. At this station, it appears that all precipitation comes from nearby sources and represents the first stage of the rain-out process. Thus, the generally weaker correlations for the marine stations (Table 3.1) may reflect varying contributions of air masses with different source characteristics and a low degree of rain-out. 

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. 

Approximate times of polluted (P), continental (C), and marine (M) air masses are indicated based on synoptic weather maps and consistent with aerosol composition measurements. Times of impactor samples, taken in duplicate concurrently with the streaker, are indicated at the top as midpoints of alternating 10- and 12-h sampling periods. 

In addition, the reaction of R02 with NO-, at night can become significant. For example, at H02 of 4 ppt and NOj of 10 ppt, which have been reported in a relatively clean marine region (Carslaw et al., 1997), the lifetimes of R02 relative to reaction with these species are comparable at 103 s. Kirchner and Stockwell (1996) predict that in more polluted areas, the R02 + NOj reactions may be important. For example, under conditions chosen to be representative of aged polluted air masses mixing with rural air masses, they calculate that 77% of the total R02 at night reacts with N03 overall (both day and night), however, only 0.66% is removed by reaction with N03. 

Figure 9.35 shows a typical set of mass size distributions for total suspended particles (TSP), Na, Cl, Al, V, NO-, S04, and NH4 at Chichi in the Ogasawara (Bonin) Islands, about 1000 km southeast of the main island of Japan (Yoshizumi and Asakuno, 1986). As expected for a marine site such as this, Na and Cl from sea salt predominate, and both the TSP and Na and Cl components peak in the coarse particle range. Al is also found primarily in the larger particles and is attributed to a contribution from soil dust. On the other hand, vanadium, non-sea salt sulfate (nss-S04 ), and ammonium are primarily in the fine particles. The vanadium levels are extremely low and likely reflect long-range transport of an air mass containing the products of combustion of fuel oil, which contains V because it is likely associated with a combustion source, it would be expected in the fine particle mode, consistent with Fig. 9.35. 

Platnick and Twomey (1994) have applied Eq. (KK) to marine clouds off the coast of California and southern Africa, to fogs in central California, and to ship tracks. Figure 14.42 shows a typical range of susceptibilities as a function of cloud droplet size. The measured susceptibilities in these studies covered three orders of magnitude, from 5 X 10-5 cm3 for fogs to 0.8 X 10-3 cm3 for marine clouds off south Africa and 2 X 10 2 cm3 for thin stratus clouds off the California coast. Similarly, Taylor and McHaffie (1994) report cloud susceptibilities in the range from 10-4 to >8 X 10-3 at various locations around the world. The highest susceptibilities were those with the smallest aerosol particle concentrations below the cloud base. As the particle concentration increased beyond 500 cm3, the susceptibility was relatively constant at 5 X 10"4 cm3. This means that the addition of new particles to a relatively clean air mass is far more effective than for a polluted one in terms of the effect on clouds. In short,... 

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). 

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