A.G. Clarke

Seasonal variation in atmospheric aerosol concentration and composition at urban and rural sites in northern England

Abstract Seasonal variations in atmospheric aerosol concentration and composition have been determined at two nearby sites, one urban and one rural, near Leeds, W. Yorkshire. Aerosols, sampled on a daily basis and collected in the size ranges 2− 4 , NO − 3 , Cl − and NH + 4 . Dark smoke and SO 2 were also measured at both sites. Results are given covering the period October 1982–September 1983. The average concentration of particles was higher at the urban site. The urban-rural difference in coarse particle concentration, which was about a factor of 2, was more significant than the difference in the fine particle concentration, which was only 1.3. Smoke and SO 2 concentrations showed strong wintertime maxima and summertime minima. Fine NO − 3 and Cl − concentrations also had pronounced wintertime maxima and summertime minima attributed to the variation in volatility of their ammonium salts. Total mass, SO 2− 4 and NH + 4 did not show any clear seasonal variations. Anti-cyclonic conditions in summer resulted in elevated mass concentrations of secondary pollutants, e.g. SO 2− 4 . The fine fraction contained ca 50% water-soluble inorganic ions at Leeds and slightly more at the rural site. These proportions showed little seasonal variation.

Heterogeneous reactions of sulphur dioxide and SO2NO2 mixtures with a carbon soot aerosol

Abstract The surface reaction rates of SO2 and SO 2 NO 2 mixtures with a freshly generated carbon soot aerosol have been measured at room temperature and gas concentrations of a few ppm using radiotracer techniques. The soot had a surface area of 24 m2 g−1 and the limiting surface SO3 formation was 3–4 mg g−1. The initial rate of reaction increased with SO2 concentration up to about 150 μg SO3 g−1 min−1 at 7.3 ppm, the exact value depending on the method of data analysis. The addition of NO2 accelerated SO3 formation via SO2 + NO2 → SO3 + NO. However at high NO2 concentrations the final amount of SO3 is depressed. This is interpreted as active site poisoning by surface NO3 formation. A Langmuir-Hinshelwood model incorporating these reactions is developed and shows good agreement with the experimental results. The new data show the importance of including NO2 surface reactions in considerations of gas-aerosol interactions in the atmosphere and in plumes.

Chloride ion effects on the aqueous oxidation of So2

A survey of electrolyte effects on the oxidation rate of sodium sulphite solutions has been carried out covering Na+, K+, NH4+, Ca2+, SO2−4, HCO3−, NO3−, F−, Cl− and Br− up to 0.1 M concentration. No significant cation effects were revealed but significant anion catalysis by Cl− and Br− was found. The levels at which the reaction rate was affected were above 10−3 M. For solutions of water containing 10−5−10−4 M sulphite the reaction rate constant was found to be first order in SO2−3 and half-order in H+consistent with previous workers. For chloride solutions the results are best represented as −d[s(IV)]dt∝[S(IV)]2[Cl−]1.3[H+]−0.5pH 3−7. Possible mechanisms involving either ClO− ions or Cl atoms are discussed. Liquid aerosols or droplets in the environment which contain significant chloride levels will show a greater ability to oxidize adsorbed sulphur dioxide than pure water droplets.

Oxidation of SO2 in rainwater and its role in acid rain chemistry

Abstract A limited survey of rainwater composition covering 80 precipitation events was carried out to establish the levels of soluble metal ions which may influence the oxidation rate of SO 2 in the aqueous phase. For 40 of the samples sodium sulphite was added at low concentrations and the rate of oxidation of S(IV) to S(VI) established. The first order rate constant k s(IV) was found to vary over three orders of magnitude, from 10 −5 to 10 −2 s −1 . pH and iron concentrations were found to be the most important factors affecting the rate. There was poor correlation between the rate of oxidation and the manganese, copper and zinc concentrations. The absolute values of the rate constants are lower than those used in recent modelling studies of atmospheric droplets and the importance of iron relative to manganese also differs from earlier studies.

A comparison of urban and rural aerosol composition using dichotomous samplers

Abstract The concentrations and composition of atmospheric aerosols have been measured on a daily basis at two sites, one urban and one rural, near Leeds, W. Yorkshire. The aerosols were collected by automatic dichotomous samplers and particles in both size ranges (⩽ 2.5 and 2.5–15 μm) were analysed for SO42−, NO3−, Cl− and NH4+. Simultaneous measurements of dark smoke and SO2 were taken at each site. Preliminary results for the first 3 1 2 months of the survey are given covering the period mid-June to the end of September 1982. The differences between the two sites were generally small during this summer period although under certain meteorological conditions the urban particulate pollutant levels can become considerably higher than the rural levels. Detailed attention is given to the sulphur chemistry and the degree of neutralisation of the aerosols. The fine particle fraction is generally found to be well neutralised by ammonia but acidic aerosols do occur under conditions that lead to low NH3 concentrations.

The oxidation of sulphur dioxide in electrolyte droplets

Abstract The rate of absorption of sulphur dioxide into deliquescent aerosols of MgCl 2 , NaCl and (NH 4 ) 2 SO 4 has been studied using a radioactive tracing technique. The amount of SO 2 absorbed was approximately linear in the calculated liquid water content of the aerosols and reached about 12 mg g −1 water after a reaction time of 145 min. Over the range 1–20 ppm the reaction was zero order in gas phase SO 2 concentration. Additional metal catalysts (Mn 2+ , Fe 3+ ) had relatively small effects on the oxidation rates whilst NO 2+ (0–20 ppm) had a large effect on the initial rate but did not significantly increase the final amount of SO 2 absorbed. The rates of reaction are shown to be far higher than in pure water and significant reaction continues down to pH 1–2. A detailed model of ionic activities and equilibria in the droplets is developed to aid the interpretation of the results and the implications for natural aerosols such as sea salt assessed.

Dissolution of trace metals from particles of industrial origin and its influence on the composition of rainwater

Abstract The dissolution of trace metals (Fe, Mn, Cu, Zn, Ca and Mg) from pulverized fuel ash and steel dust particles in aqueous suspensions was studied as a function of solution pH and aerosol mass loading. The rate of dissolution, the final concentration and the fraction of the metal leached increased with decreasing pH over the range of pH 2.5→ 5. The leaching of aerosol particles is an important source of trace metals in rainwater, and the results of the leaching experiments were found to be consistent with the observations of these metals in urban rainwater. Alkaline materials such as Ca and Mg leached from particles may be important in the electroneutrality balance of rainwater and they could affect the solution pH. The dissolution of metals such as Fe and Mn may provide catalysts which could be important in the aqueous oxidation of SO 2 , and the leaching of Ca and Mg may have a ‘buffering’ effect in reacting cloud and aerosol droplets.

OXIDATION RATES OF SO2 IN SEA-WATER AND SEA-SALT AEROSOLS

Abstract The oxidation rates of sodium sulphite in bulk sea water solutions have been measured and found to be second order in [S(IV) ]. The reaction is much faster than in pure water due to chloride ion catalysis. The rate of absorption of SO2 into sea-salt aerosols at various relative humidities has also been studied using a radioactive tracing technique. The reaction was found to be zero order in gas phase SO2 concentration over the range 0.1–2 ppm. The reaction rate in the aerosols is several orders of magnitude faster than in sea water presumably due to the higher concentrations of Cl− and other ions. The reaction rates both in the bulk solutions and in the aerosols were found to be faster for artificially prepared ‘sea water’ than for natural sea water, probably due to the absence of organic inhibitors. The conversion rates of SO2 in marine or coastal atmospheres due to this reaction are estimated for various salt concentrations and relative humidities. In favourable circumstances it could be competitive with other mechanisms. A possible effect of this reaction on the deposition rate of SO2 to the sea is noted.

Chloride aerosols in central Northern England

Concentrations of chloride aerosols in the atmosphere have been measured on a daily basis using dichotomous samplers at one urban and one rural site near Leeds, West Yorkshire. For two periods during the 18 month sampling programme major marine cations (Na+, Mg2+) in the aqueous extracts of the filters were analyzed in addition to the other common ions (Cl−, NO3−, SO42−, NH4+). The contributions of marine aerosol to the coarse and fine mass fractions were 10–20% and 2–3%, respectively even though the sites were some 100 km from the sea. The displacement of Cl− from marine aerosols was clearly demonstrated. Road salt can dominate the coarse mode in winter under certain meteorological conditions. Fine chloride, essentially NH4Cl aerosol derived from combustion-generated HCl and atmospheric ammonia, shows a marked seasonal dependence attributed to the thermodynamics of the NH3 + HCl = NH3Cl equilibrium and the relative concentrations of atmospheric NH3 and HCl. Of the total Cl− aerosol measured at Leeds, about 23 was of marine origin.

Electrolyte solution theory and the oxidation rate of sulphur dioxide in water

Abstract A theoretical assessment of the effect of ionic strength on the thermodynamics and kinetics of sulphite ions in aqueous solution is made. It is concluded that in the range up to 0.1 M ionic strength substantial effects on the equilibria and reaction rates would be anticipated, especially at low pH, but the available experimental data are inadequate to confirm or to deny these possibilities.