Detailed observations have been made of the seasonal variation in concentration of a wide range of nonmethane hydrocarbons over the North Atlantic Ocean in air with a predominantly polar maritime origin. The results bear out many of the findings of our previous studies [Za 'ghtman et al., 1990] with the observation of a concentration maximum in the winter and a minimum in the summer. The more recent results indicate that the amplitude of the seasonal cycle is remarkably constant. Also the total concentration of reactive carbon in the various forms of nonmethane hydrocarbons probably exceeds 20 ppbC. This constitutes a substantial reservoir of material which could take part in ozone production in the relatively remote troposphere, if sufficient nitrogen oxides are copresent. The hydrocarbon composition of air in winter is strongly dependent on its origin, with tropical air having a composition similar to polar air in the spring months. The relative magnitude of the seasonal cycles for some hydrocarbons is proportional to their rates of reaction with hydroxyl radicals. This is true for straight chain paraffins up to C5, acetylene and benzene, and it suggests that the removal of these molecules from the atmosphere occurs predominantly by reaction with hydroxyl radicals. For other molecules, particularly branched chain paraffins and substituted aromatic molecules, there is evidence that other removal processes are operating in competition with the hydroxyl radical, especially in winter. Arguments are advanced that in the case of the branched chain paraffins this may be caused by reaction with nitrate radicals. 1988; Liu et al., 1987].Nonmethane hydrocarbons are known to have a controlling influence on the production of ozone in the polluted boundary layer of the atmosphere. It is quite possible, however, that their influence is more widespread, particularly because in winter time, photochemical degradation is much less efficient. In winter conditions, many nonmethane hydrocarbons can escape from the boundary layer in source areas and disperse into the free troposphere over large parts of the northern hemisphere.In a recent paper [Lightman et al., 1990] we have drawn attention to the buildup in wintertime of a substantial concentration of hydrocarbons in the free troposphere over the North Atlantic Ocean. Here we wish to extend our observations to the seasonal cycles of a much wider range of hydrocarbons, from (22 to (28, which have been obtained with more sophisticated analytical techniques. The measurement programme which produced the data on the •Now at Department of Chemistry, University of California, Irvine. composition of the free troposphere also examined the detailed enow at Atmospheric Chemistry Research Unit, Imperial College, composition of hydrocarbons in urban plumes, spreading from Silwood Park, Ascot, England. London. Interesting comparisons can be made between the composition of "clean" background air in the northern hemisphere in winter and that recently contaminated with pollutant emissions Copyrig...

An analytical system has been developed which allows the confident identification and measurement of 35 hydrocarbons of different reactivities in air samples collected in many locations. This paper describes the application of the technique to follow the differential decay of hydrocarbons in urban plumes spreading from London during the summer. The data have been used to determine atmospheric hydroxyl radical concentrations, averaged over several hours, on the assumption that the decay of the hydrocarbons is entirely due to reaction with these radicals. Hydroxyl radical concentrations were derived from the measured decay of several alkenes which agreed with theoretical estimates. There are strong indications, however, that substituted aromatic molecules decay much faster than could be accounted for solely by reaction with hydroxyl radicals; this may indicate the presence of a further chemical removal mechanism. Indirect estimates of globally averaged OH concentrations havealso been made using the measured distribution of methyl chloroform in the atmosphere [Singh et al., 1979] and the seasonal variation of •4CO [Volz et al., 1981]. Indirect estimates of OH concentrations in chemically well defined situations, such as urban plumes, can be extremely useful in validating models of regional ozone production and in understanding atmospheric photochemical theory. The plume study reported here used a high quality sampling and analytical system to collect time-resolved data over several hours on a large number of hydrocarbons, covering a wide range of atmospheric reactivity with OH radicals. OH concentrations were then derived from the decay of both aliphatic and aromatic hydrocarbons in combination with the comprehensive kinetic data base on hydrocarbon reactions with hydroxyl radicals recently published by Atkinson [1990]. 2. EXPERIMENTAL PROCEDURE 2.1. Collection of Air Samples A twin turboprop Jetstream aircraft owned and operated by the Cranfield Institute of Technology was employed for this study. Flown unpressurized, the Jetstream has an operational altitude ceiling of about 3300 m and a maximum endurance of about 4 hours at 120 knots (222 km/hr). The procedure for the collection of air samples has been described extensively in a previous publication [Lightman et al., 1990] and is therefore only briefly presented here. Discrete air samples were collected through stainless steel tubing from an intake manifold located on the aircraft roof, forward of the engines. Two metal bellows MB 158 pumps in series were employed to fill 1.6-L electropolished stainless steel sample bottles. Each bottle was fitted with a dip tube to facilitate flushing for 4 min before pressurizing to 4 atmospheres (60 psig) over a period of up to 1 min. The effect of this high pressure is to 2851 carbons in the atmosphere, Environ. Sci. Technol., 15, 113-119, 1981.

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Measurements of atmospheric ozone at concentrations typical of the free troposphere have been compared for ECC sondes and a UV-absorption photometer, using a Bendix chemiluminescent analyser as a transfer standard. Comparisons were conducted in the laboratory and in the tropospheric part of the atmosphere. It was found that the measurements agreed to within 4% provided that the background current for the ECC sonde was measured before exposure to ozone in the preparation procedure, and was assumed to be constant throughout the sonde flight. These results confirm those of earlier experiments and mean that the methods currently used to correct for the background current in the troposphere need to be revised.

The free tropospheric concentrations of several light hydrocarbons and ozone have been measured throughout most of the year in air flows approaching the UK from unpolluted areas of the Northern Hemisphere. The measurements were made using an aircraft at altitudes between 1500 and 3000 m and concurrent measurements of halocarbons confirmed that the air had not passed recently over potential sources of hydrocarbons. Most of the hydrocarbons show a clear seasonal pattern with a summer minimum and winter maximum, but propene and ethene, which are very short‐lived in the atmosphere and have oceanic sources, show no seasonal pattern. The halocarbon concentrations also exhibit no seasonal pattern but ozone shows a spring maximum. The results for ethane and propane, which have a common anthropogenic source, are consistent with a summer to winter average OH radical concentration ratio of about 4 to 7 and a travel time for source area to sampling point of about 20 days. It is suggested that the presence of large concentrations of non‐methane hydrocarbons on an extensive scale will influence the seasonal pattern of ozone observed in large parts of the northern hemisphere.