Pergamon J. Aerosol Sci. Vol. 28, Suppl. I, pp. $333-$334, 1997 ©1997 Elsevier Science Ltd. All fights reserved

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Coarse nitrate formation in a coastal area of the North Sea

E. Plate, M. Schulz

Institute of Inorganic and Applied Chemistry, University of Hamburg Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany

KEYWORDS: Nitric acid, coarse nitrate, sea salt, chloride loss, coastal atmosphere

Nitric acid and nitrate, a secondary aerosol originating mainly from automobile traffic, belong to the nutrient cycle. Their deposition on the sea surface can cause eutrophication in the upper water layer, which is a sensitive part of the marine ecosystem with high biological productivity. The deposition of nitric acid is enhanced by its reaction with sea salt: I-INO3 + NaCI ---} NaNO3 + HCI, which is suggested to be a particle surface reaction (Ottley and Harrsion, 1993). For the 1-2 ~tm diameter size fraction the chloride evaporation was found to be almost complete (Pakkanen 1995). Other acidifying compounds (H2SO4, SO2) can contribute to the depletion of chloride but our fmdings indicate this to be of minor importance in the German Bight.

To study this reaction 12 hourly ambient air samples were taken during three field experiments (27.7.-19.8.1994, 28.4.-9.5.1995 and 23.2.-3.3.1996) within the German KUSTOS project in the German Bight on the RV ,Heincke", respectively RV ,,Alkor" and in parallel at the coastal station Westerhever (Plate et al. 1995). Sampling at the two stations was co-ordinated with regard to actual wind speed, wind direction and position of the ship. At the downwind station sampling time was delayed so that the same air mass could be investigated twice (Lagrangian approach). At both stations the Denuder Difference Method (DDM) was used to determine separately gaseous nitric acid (HNO3) and particulate nitrate (NO3) beside sea salt and other major components of the aerosol. The method, analytical procedure and possible artefacts are described in Shaw et al. (1982) and Behlen (1996). In addition, a 10-stage low pressure impactor (LPI 25-14, BERNER) was used on the research vessel during the first two experiments to measure the size distribution of the major aerosol compounds.

The reaction of HNO3 with sea salt results in coarse sodium nitrate (NaNO3) and volatile hydrogen chloride (HC1). Due to the chloride loss, the CI/Na + ratio decreases in sea spray aerosol particles (table 1). We found, that for onshore blowing winds the mean replacement of chloride was 20.6 % in the German Bight and even 33.6 % at the coastal station Westerhever. The different percentages reflect high sea salt loadings and small HNO3-concentrations in the inner German Bight. The absolute NaNO3-concentrations were slightly enhanced in the German Bight compared with Westerhever. For offshore blowing winds with low sea salt concentrations 13.6 % of the sea salt in the German Bight and 31.5 % at Westerhever were found to be transformed. Despite deposition of coarse NaNO3 the absolute concentrations increased from Westerhever to the reasearch vessel.

Sea salt compounds are generally found in the coarse mode (>1 I~m). The reaction between HNO3 and sea salt therefore results in a nitrate size distribution shifted to larger particle sizes, figure 1. For particles between 1 and 10 ~tm a frcation of 45 % of the sea salt were transformed to NaNO3 (16.6 nmol m'3), table 1. This higher transformation rate in the impactor is due to coarse sea salt losses in the impactor inlet. Highest NaNO3 fractions (64%) were determined between 1-2 lam. Photolysis of NOx and subsequent neutralization of HNO3 by ammonia, is responsible for the fine particle ammonium nitrate (<1 ~tm) of continental origin.

S333

$334 Abstracts of the 1997 European Aerosol Conference

Table 1. Separated for on- and offshore blowing winds, the ratio of coarse sodium nitrate in the German Bight in 1994-96 is compared with impactor derived data from the German Bight in 1994-1995.

Na+(particulate)

NO3-(partieulate)

HNO3 (gaseous)

[~nolm -3]

onshore blowing winds

RV Heincke / coastal station RV Alkor Wegerhcver 372 hours 384 hours

73.4

102.6

25.5

offshore blowing winds

RV Heineke / coastal stalion RV Alkor Westerhever 348 hours 336 hours

all winds

iwoac~ > lom RV Heincke

422 hours

atmospheric concentrations I

42.7 I 45.3 17.0

93.1 [ 106.5 101.4

38.9 27.8 19.1

transformation of sea salt to sodium nitrate

36.9

32.7

26.5 (DDM)

CI'/Na + (seawater: 1.17) 0.93 0.77

NaNO3 fromNaCl [%] 20.6 33.6

tr,.olm 15.1 14.3

1.01 0.80

13.6 31.5

6.2 5.3

0.64

45.1

16.6

E

8 ' - 6

120 : . . . . . . . . . . . . . . . . . . . . . . . . . ,

10o~ i

8O

601

40

20

0 lO ,~o 10 " * 10 *o 10 ' *

[~ml AD

Figure 1. Size distribution of sodium and nitrate measured with a low pressure impactor on RV Heincke in the German Bight (1994/95 ), 422 hours.

The scavenging of nitrate species by the reaction with sea salt seems to be an important atmospheric cleaning process in coastal areas, which results in enhanced nitrogen input to coastal ecosystems. Due to atmospheric pollution sea salt particles are no pure alkaline chlorides.

ACKNOWLEDGEMENTS We would like to thank Prof. Dr. W. Danneeker and Andreas Behlen for their most appreciated support and advice. Thanks are also due to Andreas Rebers, Thomas Stahlschmidt, Holger Gerwig, GGtz Steinhoff and Susanne Tamm for help in taking the samples, Kirsten Michaelsen for providing us with meteorological data and the BMBF for funding this work (project grant No. 03F0111A, ,,Ktistennahe Stoff- mad Energiefltlsse", KUSTOS).

REFERENCES Behlen A. 1996. Ph.D. thesis, University of Hamburg, Germany. Ottley C. J. and Harrison R. M. 1992. Atmos. Environ., 26A, 1689-1699. Plate E. et al. 1995. In B. Jahne and E.C. Monahan (eds.) Proc. of Third Int. Syrup. on Air-Water Gas Transfer. Pakkanen T. A. 1995. Atmos. Environ., 30, 2475-2482. Shaw 1L W. et al. 1982. Atmos. Environ., 16, 845-853.