Atmospheric Enuironmemr Vol. 22. No. 4. pp. 683-694. 1988. 0@04-6981:88 $3.00+0.00 Printed in Great Britain. Pcrgamon Press plc zyxwvut A FIELD STUDY OF THE CLOUD CHEMISTRY AND CLOUD MiCROPHYSICS AT GREAT DUN FELL A. S. CHANDLER*, T. W. CHOULARTON*,G. J. DOLLARD$, M. J. GAY*, T. A. HILL*, A. JONES*, B. M. R. JONES& A. P. MORSE*, S. A. PENKETT~ and B. J. TYLER? *Department of Physics and Wepartment of Chemistry, WMIST, Manchester M60 lQD, U.K.; SEnvironmental and Medical Sciences Division, The Harwell Laboratory, Didcot OX1 1 ORA, U.K. and ADepartment of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. (First received 30 Sepre~er 1986 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK and received for publication 30 Sep~em~r 1987) A bstract-A comprehensive series of experiments designed to investigate the oxidation of SO2 to sulphate by H,OZ and O;, are being performed in the cap cloud at Great Dun Fell. In this paper results of the first set of experiments are presented. These took place during November 1985. The aim of these experiments was chiefly to monitor the HIOZ oxidation process by measuring its depletion with time within the cloud in the presence of SO1. Increases in sulphate content of the cloud water were not observed during this experiment because H,O1 levels were too low and oxidation by OS was inhibited by the low cloud water pH. The concentrations of aqueous phase HZ02 measured were typically 100 nmol and O3 gas phase concentrations 20 ppbv. It was found that, in the presence of SO*, the concentration of Hz02 declined much more rapidly with height above cloud base than predicted by simple dilution by liquid water. Assuming this to indicate reaction with SOI, a comparison was made with the predictions of the model of Hill, Choularton and Penkett (1986, Atmospheric Environment 20, 1763-1771). It was found that the rate of reaction was consistent with a value for the second order rate constant kHlol of 2 r 1 x 10’ s- ’ where kH,o, is defined by: d[Svt] kHto2C&Oz] CSOz.H,OI -= dt 0.1 +[H+] (Martin and Damschen, 1981, ~t~sp~eric ~n~iro~e~t IS, 1615-1621). This was determined with a cloud temperature of 8.5”C and a pH of 4.8. T’hisis about three times larger than the laboratory determined rate constant found in Martin and Damschen (1981) when corrected to the same temperature and pH. On some occasions microphysical measurements in the cap cloud indicated that tropospheric air from above the cloud top was being entrained into the cloud. Increases in H,O, concentrations with altitude within the cap cloud on these o&asions showed that extra H,O1 was beingsi&ltaneously introduced to the cloud system. It is suggested that this entrainment process may play a very important role in SO* oxidation in clouds when the reaction is oxidant limited. Key word index: Cloud chemistry/microphysics, H,Oz SO,/SO, conversion. I. INTRODUCTION A major collaborative experiment to investigate cloud chemical processes is being conducted at the Uni- versity of Manchester Institute of Science and Technology (UMIST) field station on Great Dun Fell in Cumbria, England. Great Dun Fell (GDF) is 847 m above sea level and forms part of the long ridge of the Pennine hills, which runs from the NW-SE down the centre of England. The prevailing SW winds blow almost perpendicular to the ridge, frequently forming cap clouds which envelop the site for parts of 250 days a- ‘. The dynamics and microphysics of the cloud have been extensively studied, see for example Carruthers and Choularton (1982) and Choularton et al. (1986). Additionally, these clouds make extensive contact with ground that is readily accessible, so that apparatus may be positioned at many different sites and the cloud examined at different stages in its history. These factors make the site ideal for the study of those aqueous phase chemical reactions which have a time constant of several hundred seconds or which produce measurable quantities of product on this timescale. The cloud is in effect a natural flow-through reactor which can be sampled at different stages of chemical state by collecting cloud water at different points on the hill in a line parallel to the prevailing wind. Experiments in wave clouds not in contact with the ground have been performed by Hegg and Hobbs (1982). Whilst these succeeded in detecting sulphate production, it was not possible to determine directly the mechanism of oxidation nor to monitor changes in chemical species within the cloud because of the great difficulty in making high resolution cloud chemical measurements from an aircraft. During the past 10 years laboratory-based evidence has accumulated to suggest that the oxidants H202 and O3 are responsible for much of the atmospheric sulphate produced by SOz oxidation and that there- fore aqueous phase oxidation contributes significantly to the acidity of precipitation (Penkett et al., 1979; Middleton et al., 1980; Chameides and Davies, 1982). 683