Jul 10, 1996 - The increase of gas transfers (probably the dimethyl sulfide) induced by the whitecap coverage upon the sea surface or a specific production of ...
Pergamon PII: S 1352-23 10 (97)
Atmospheric Environment VoL 31, No. 18, pp. 2991 3009, 1997 X~ 1997 Elsevier Science Lid All rights reserved. Printed in Great Britain 00088-5 1352 2310/97 $17.00 + 0.00
CONTRIBUTION OF MARINE AEROSOLS IN THE PARTICLE SIZE DISTRIBUTIONS OBSERVED IN MEDITERRANEAN COASTAL ZONE J. P I A Z Z O L A a n d S. D E S P I A U L.E.P.I, B.P 132, 83957 La Garde cedex, France (First received 10 July 1996 and in final fi)rm 24 January 1997. Published July' 19971 Abstract--Results from analysis of aerosol size distributions measured in a Mediterranean coastal zone are presented. This paper focuses on aerosol particles smaller than 10/~m because they specifically represent a mixed contribution of aerosols in a coastal zone. The interpretation is first based on low wind speed periods to identify the background aerosol in the study area. We can note the influence of wind direction on both aerosol concentrations and size distributions that have been measured. The contributions of the various particle sources, continental or marine, from anthropogenic or natural origin have been demonstrated regardless of each aerosol size distribution. The influence of solar irradiation has been observed for low wind speed periods. It induces a bimodal size distribution characterized by the classical fine particle peak around 0.04 ~m and an accumulation mode at 0.1 era. Aerosol size distributions measured during high continental wind periods (which correspond to a 25 km fetch) show a strong concentration peak around 0.02/~m. The good correlation obtained between this particle concentration and wind speed shows that the smallest particles measured during high continental wind speed periods are probably of marine origin. The interpretation of this large contribution of very small particles has focused on two main hypotheses. The increase of gas transfers (probably the dimethyl sulfide) induced by the whitecap coverage upon the sea surface or a specific production of fine particles as previously suggested by Despiau et al. (Journal of Aerosol Science 1996, 27(3), 403-4151. The specificities of the smallest particles are also observed if we compare background aerosol and concentrations recorded during high wind speed periods. In contrast with the sea-surface-generated aerosols which decrease during an extended period of low wind speed (Smith et al., Atmospheric Environment 1991, 25A, 547-555), we have observed larger concentrations of 0.1-0.3/~m particles during the background conditions. :~5 1997 Elsevier Science Ltd. Key word index: Aerosol particles, size distribution, coastal, air sea interface
INTRODUCTION Aerosols are important to a large number of physical and chemical processes in the atmosphere. These particles affect climate by scattering and absorbing radiation (Charlson et al., 1992), and as cloud condensation nuclei, they influence cloud albedo (Twomey, 1974). Thus, they affect the heat budget and may induce changes in our life conditions. In particular, from the environmental point of view, they may transport pollutants and bacteria. The number, the size distribution and the composition of particles vary in time and space in response to their origin, arising from a variety of production sources and meteorological conditions. Each different origin corresponds to an aerosol component and superposes to a quasiconstant particle concentration, independent of meteorological conditions, called the "background aerosol" (Withby, 1978; Smith et al., 1991), which also refers to aged aerosol. This definition of background aerosol largely corresponds to submicronic particles because of their longer residence time in the
atmosphere. It was generally assumed, until a recent date, that the background aerosol is essentially constituted by a continental contribution (Gathman, 1983), since marine production of aerosols at the air-sea interface concerns particles of a diameter roughly larger than 0.5 #m at 80% of relative humidity (Monahan et al., 1983; Woolf et al., 19871. However, in addition to the direct emissions of particles through the air-sea interface, an indirect production of very small particles ( 80%) and leads to cloudy conditions. The second one is of continental origin, is dryer and results in very sunny conditions. However, in addition to the local wind direction, we also need trajectory analysis for a better information about the origin of particles transported by the wind. Thus, during the period of the experimental campaign, we used calculation of back-trajectories issued from the French National Meteorology. Figure 3 shows 4 d back-trajectories, started at 950 hPa. Figure 3a shows the air mass trajectory during a high wind speed period ( > 9 m s -1) and a northwest direction in the
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Fig. 1. Experimental site. On the general map, the experiment point is marked T.
Particle size distributions
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Porquerolles island
Fig. 2. Detailed view of the experimental site. The full circle reported on the western tip of the Porquerolles island is the experimental station. The pie chart represents the time percentage of each direction recorded during all low wind speed periods analyzed during the whole campaign. When the wind speed is high ( > 9 m s 1) the local direction of winds are mostly NW (so called "Mistral") or E SE in the study area as pointed by the arrows.
study area, which are specifically studied in the paragraph concerning high NW wind periods (Fig. 6). Air sampled at Porquerolles island had spent the last 2 d over the continent, but we can note that it was originally transported from the Atlantic. In fact, a depression system generally located over north of Italy transports air masses in the Rhone valley (located in the north of Marseille) with a north-south trajectory (Fig. 3). So, the air masses can originate from very different locations. Air sampled in Mistral conditions is sometimes near the Groenland 4 d before, as well as over east Europe. Figure 3b shows that in east direction wind, the sampled air had spent the last 4 d over the ocean, along the coast. East or southeast winds occur when the depression is situated near the south of Italy. They generally blow through a large portion of ocean, as shown by the back trajectory (Fig. 3b), which is issued from experiments led in 1995 in the same area (Piazzola and Despiau, 1997).
B A C K G R O U N D AEROSOL
When the wind speed is low (V ~ 14 m s-1) whatever the season is, i.e whatever the solar irradiation values are (Fig. 11). This would mean that more gas is transferred into the atmosphere during such periods and thus the probabilities for the gas to be photo-oxidized are more important. The correlation between wind speed and 0.01 0.045/~m aerosols would be, in that case, an indirect correlation. As the whitecap cover also varies with atmospheric stability, water temperature and wind speed (Monahan and O'Muircheartaigh, 1986; Wu, 1986), Erickson (1993) has proposed a model of the transfer velocity for trace gases which takes into account the thermal stability at the air-sea interface. In this model, the wind speed, air sea temperature difference, water temperature and moisture were introduced in the local drag coefficient used in the following expression of the whitecap fraction:
W = 0.2C3D/zV 3
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Fig. 12. Transfer velocity for CO2 function of wind speed and thermal stability, by Erickson (1993). when the wind speed is larger than 14-15 m s -1 Below this wind speed value, the fine mode concentrations are not really larger than those recorded in background conditions (Fig. 10). It is important to note that even though the Erickson model produces estimates of the transfer velocity for CO2, the data are easily convertible to the DMS gas. Since the transfer velocity for a certain trace gas, kx, is related to the Schmidt number, as follows:
kx = krt. \SCR,/
(4)
where kR. is the transfer velocity of radon and
(2)
where V is the wind speed (m s-1) and CD the local drag coefficient, CD
H U M = 0 . 8 5 S S T = 25* C 1100
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for V < 3 . 6 m s - 1 for V > 3 , 6 m s - 1 .
So
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(5)
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(3)
where Z = 20 m, k is the yon Karman constant (0.4), CN is the neutral drag coefficient and Ym(Z/L) is a function of the temperature and moisture gradients at the air-sea interface (see Erickson, 1993 for further details). The computed values of the transfer velocity for CO2 are reported in Fig. 12. This model shows that the stability dependence is most important for wind speeds larger than 10 m s- 1. Moreover, Fig. 12 shows that the transfer velocity increases dramatically when the wind speed is higher than 15 m s - 1. We observe this threshold value concerning the size distributions evolution with the wind speed experimentally, as shown in Fig. 6, where it is clear that the 0.01-0.04 #m particle concentrations also increase considerably
Thus, the transfer velocity for DMS may be simply deduced from the CO2 data by multiplying the CO2 transfer velocity by the ratio (ScoMs/Scco~)". The model of Erickson (1993) predicts a strong enhancement of the transfer velocity of CO2 (and thus of DMS) at the interface, at approximately the same wind speed value. So it would mean that the highest concentrations of DMS are injected into the atmosphere when the wind speed is larger than 14-15 m s-1 with a great possibility to be converted into particles owing to solar conditions characteristic of the N W wind in the study area. The hypothesis of atmospheric DMS photo-oxidation is also supported by results issued from the aerosol formation. Photo-oxidation of DMS was studied
Particle size distributions in air at 35% of humidity by Hatakayema et al. (1985). The time variation of particle size distributions they obtained by means of EEA (TSI, model 3030), from photoirradiation of a mixture containing DMS, showed the occurrence of an aerosol concentration peak after 25 min of irradiation, as reported in Fig. 13. This would be in accordance with our data since Despiau et al. (1996) show that there is a time lag smaller than 1/2 h between the high wind speed occurrence and the increase of the fine mode concentration for high NW winds. Furthermore, this peak is centered around 0.02/~m (Fig. 13), as observed in the size distribution reported in Fig. 6, for a wind speed of 18 m s - 1 and 40% of humidity. However, if numerous studies led to the conclusion that DMS is the major source of the sulfate over the ocean, the relationship between the number concentration of CCN and the flux of DMS in the atmosphere is not quantified. In
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