R. A. Burgess

Observations of the Nitrate Radical in the Marine Boundary Layer

A study of the nitrate radical (NO3) has been conducted through a series of campaigns held at the Weybourne Atmospheric Observatory, located on the coast of north Norfolk, England. The NO3 concentration was measured in the lower boundary layer by the technique of differential optical absorption spectroscopy (DOAS). Although the set of observations is limited, seasonal patterns are apparent. In winter, the NO3 concentration in semi-polluted continental air masses was found to be of the order of 10 ppt, with an average turnover lifetime of 2.4 minutes. During summer in clean northerly air flows, the concentration was about 6 ppt with a lifetime of 7.2 minutes. The major loss mechanisms for the radical were investigated in some detail by employing a chemical box model, constrained by a suite of ancillary measurements. The model indicates that during the semi-polluted conditions experienced in winter, the major loss of NO3 occurred indirectly through reactions of N2O5, either in the gas-phase with H2O, or through uptake on aerosols. The most important direct loss was via reactions of NO3 with a number of unsaturated nonmethane hydrocarbons. The cleaner air masses observed during the summer were of marine origin and contained elevated concentrations of dimethyl sulfide (DMS), which provided the major loss route for NO3. The box model was then used to investigate the conditions in the remote marine boundary layer under which DMS will be oxidised more rapidly at night (by NO3) than during the day (by OH). This should occur if the concentration of NO2 is more than about 60% that of DMS.

Simultaneous observations of nitrate and peroxy radicals in the marine boundary layer

This paper describes the most extensive set of simultaneous measurements of the concentrations of nitrate (NO3) and peroxy (sum of HO2 + RO2, R = alkyl and acyl) radicals to date. The measurements were made in the coastal marine boundary layer over the North Sea, at the Weybourne Atmospheric Observatory on the North Norfolk coast during the spring and autumn of 1994. In spring the average nighttime concentration of NO3 measured by differential optical absorption spectroscopy, was about 10 parts per trillion (ppt) (maximum 25 ppt). The corresponding peroxy radical concentration, measured by the chemical amplifier technique, averaged about 2 ppt (maximum 6 ppt), although this is likely to be an underestimate of the total radical concentration. There is a significant positive correlation between the two sets of radicals, which has not been reported previously. A box model of the marine boundary layer is used to show that this correlation arises from the processing of reactive organic species by NO3. During spring the relatively long lifetime of NO3 (up to 18 min) at night is controlled by reaction with dimethyl sulfide (DMS), and the model predicts significant production of HNO3, methyl tiomethylen (CH3SCH2O2) and other peroxy radicals, HCHO, and eventually sulfate. A nighttime production rate for the hydroxyl (OH) of about 2 x 10(4) molecules cm(-3) s(-1) is estimated. During one night in autumn the NO3 lifetime of about 3 min is too short to be explained by reaction with unsaturated hydrocarbons, but is satisfactorily accounted for by the heterogeneous loss of N2O5 on deliquesced aerosols in relatively polluted conditions.

The magnitude and extent of elevated ozone concentrations around the coasts of the British Isles

Abstract The marked decreases in ozone concentrations that occur over level ground at night do not occur over open water surfaces. Thus during on-shore flow coastal regions experience higher concentrations than further inland. The magnitude and spatial extent of this enhanced exposure are quantified by analysis of data from monitoring sites in the coastal zone during periods of on-shore flow and by application of a multi-layer, Lagrangian, diffusion model. Observations show that, with on-shore winds, the daily-mean concentration at the coast is 5–7 ppbv greater than at inland sites. Combining these increased concentrations with statistics on flow direction indicates enhanced daily-mean concentrations at the coast typically of 3 ppbv, but up to 5 ppbv in some locations, compared to locations inland. Model studies indicate that the spatial extent of these increased concentrations is restricted to a coastal band of only a few kilometres in width.

Studies of oxidant production at the Weybourne Atmospheric Observatory in summer and winter conditions

Detailed studies have been made of the behaviour of gases and radicals involved in the production of oxidants at the Weybourne Atmospheric Observatory in both summertime and wintertime conditions. In June 1995 the range of meteorological conditions experienced varied such that ozone destruction was observed in clean northerly air flows reaching Weybourne down the North Sea from the Arctic, and ozone production was observed in varying degrees in air with different loadings of nitrogen oxides and other precursors. The transition point for ozone destruction to ozone production occurred at a nitric oxide concentration of the order of 50 pptv. Plumes of polluted air from various urban areas in the U.K. were experienced in the June campaign at Weybourne. Quantitative studies of ozone production in a plume from the Birmingham conurbation on 18 June 1995 showed that the measurement of ozone production agreed well with calculated production rates from the product of the nitric oxide and peroxy radical concentrations (r2=0.9). In wintertime conditions (October–November 1994) evidence was also found for oxidant production, defined as the sum of O3+NO2. At this time of year the peroxy radical concentrations (RO2) were much lower than observed in the summertime and the nitric oxide (NO) was much higher. There was still sufficient RO2 during the day, however, for a slow accumulation of oxidant. Confirmatory evidence for this comes from the diurnal co-variance of (O3+NO2) with PAN, an excellent tracer of tropospheric photochemistry. The same type of covariance occurs in summer between PAN and ozone. The results obtained in these series of measurements are pertinent to understanding the measures necessary to control production of regional photochemical air pollution, and to the production of ozone throughout the northern hemisphere in winter.

Measurement of the Diurnal Variation of the OH Radical Concentration and Analysis of the Data by Modelling

Daily variations of the hydroxyl radical concentration have been measured during a campaign at the Weybourne Atmospheric Observatory (WAO) in June 1995. These measurements are compared with box model calculations, based on a slightly modified, second generation Regional Acid Deposition Model (RADM2). Results from eight days of the comparison are presented. A detailed analysis and discussion of the different source and sink terms is given for two days: Julian Day (JD) 170 (19 June, and 178 (27 June). In both cases excellent agreement between the measurements and the calculation is obtained, indicating that the model describes the OH chemistry sufficiently well. Furthermore, the analysis of these days demonstrate that JD 170 is dominated by the NOx catalysed OH production, whereas JD 178 is influenced by OH formation via ozone photolysis.

Observation on great dun fell of the pathways by which oxides of nitrogen are converted to nitrate

Two field experiments to investigate the formation of nitrate as an airstream passes through a hill cap cloud have been performed at the UMIST field station on Great Dun Fell. Techniques chosen for the measurement of various nitrogen species are described. The results of the second field experiment are discussed and compared with those of the first. Evidence is found in support of the hypothesis that under the range of conditions studied, the dominant pathway for nitrate production is the solution of N2O5 formed from the reaction of NOx with O3 upwind. The effectiveness of this pathway by night and by day is observed to be a function of the NOx mixing ratio. A surface reaction rate constant of around 300 cm3 cm−2 s−1 for the hydration of N2O5 is inferred from the observations. These results are shown to be consistent with recent laboratory measurements of the rates of reaction of nitrogen species. It is suggested that pathways other than via N2O5 may be significant sources of nitrate under certain conditions that merit further investigation.

The Emission and Distribution of Ozone Precursors over Europe

The increasing concentration of ozone (03) in the northern hemispheric troposphere is a matter of great concern. The tropospheric content of ozone is partly the result of a downward transport from the stratosphere [1, 2], but ozone is also formed in the free troposphere and in the polluted boundary layer by photo dissociation of nitrogen dioxide (NO2) followed by the recombination of the oxygen atom formed (03P) with an oxygen molecule (O2) [3]. Net production of ozone is only possible if nitrogen monoxide (NO) is converted to NO2 by reacting with species other than ozone itself. In the free troposphere NO reacts with free radicals formed by oxidation of methane or carbon monoxide (CO). In the polluted atmospheric boundary layer free radicals are formed by the oxidation of VOC (volatile organic compounds) [4-7], and the explanation to the post industrial increase of ozone is most likely the enhanced antropogenic emissions of NOx and other precursors [8-10]. Episodic occurrence of elevated ozone is mainly controlled by NMHC (non-methane hydrocarbon) emissions, but NO„ emissions are also important [11-13]). In less polluted areas like Scandinavia NO„ rather than VOC is regarded as the limiting factor in ozone formation [14-16]. The basic mechanisms of the production of tropospheric ozone are thus well known [17-19]. The quantitative role of various compounds on different scales, especially individual hydrocarbons, is however not yet well established. Since the ozone concentrations do not respond linearly to precursor controls [20], the influence of various processes on the production of ozone and other secondary pollutants can best be investigated by a combination of measurements, emission inventories and modelling.