Roy M. Harrison

Field measurements of the dissociation of ammonium nitrate and ammonium chloride aerosols

Abstract Atmospheric concentrations of HNO3, HCl and NH3 have been measured at Essex, UK and Cabauw, The Netherlands, using both filter pack (Essex) and denuder (Cabauw) sampling methods and integration times of 3, 12 and 24 h. Comparisons of the concentration products [HNO3] [NH3] and [HCl] [NH3] with the predictions of equilibrium chemical thermodynamics for a non-interactive external mixture of NH4Cl and NH4NO3 show generally good agreement at temperatures above ca 5°C, and indicate that both aerosol salts are normally present at these sites. The ratio [HCl] [HNO 3 ] agrees well with theory at lower temperatures (

Field intercomparison of filter pack and denuder sampling methods for reactive gaseous and particulate pollutants.

Abstract Atmospheric sulphate, nitrate, chloride and ammonium species have been measured with colocated filter pack and denuder samplers. In general the total amount of a species collected by the two types of sampler was almost the same, although there was evidence of better inlet efficiency for particles in the filter pack sampler. The filter pack gives slightly higher measurements of the volatile species HNO3, HCl and NH3 than the denuder, with a corresponding lower measure of particulate NO3−, Cl− and NH4+, attributable to volatilization of ammonium salts from the filter pack pre-filter. In the context of most ambient measurements, the divergences between the techniques are small, and it is argued that differences in these measurement techniques are inevitably site, operator and apparatus-specific and data from one site or research group cannot readily be extrapolated to other sites.

The assessment of air and soil as contributors of some trace metals to vegetable plants I. Use of a filtered air growth cabinet

The sources of heavy metals in a number of consumer crops were investigated in the laboratory by growing plants in a dual growth cabinet supplied with both clean and ambient air. Under these conditions, plants were exposed separately to filtered and normal ambient air to assess the influences of soil and atmosphere on the accumulation of Cd, Pb, Zn, Cr and Ni. Radish, carrot, pea, spinach and lettuce plants were successfully grown in the cabinet. Analysis of the metals in the plant tissues showed that the foliar route is potentially of similar importance to the soil-root pathway as a route of transport to the exposed parts of the plants. Whilst the exposed parts showed the highest metal accumulation from the air, the levels of metals in fruits and storage roots resulting from foliar translocation of the airborne component appeared to be low generally. The metal which achieved highest translocation from foliar deposition was Pb. The effect of spraying plants with rain-water was to enhance slightly the total content of all trace metals analysed.

Vapour pressure of ammonium chloride aerosol: Effect of temperature and humidity

Abstract The effect of relative humidity (RH) on the constant for dissociation of ammonium chloride into gaseous HCl and NH 3 has been estimated for different temperatures, using thermodynamic data. At RH over 75–85% the ammonium chloride aerosol exists in the liquid phase, with the dissociation constant two orders of magnitude lower at 98% RH than for solid aerosol at the same temperature. It is predicted that ammonium chloride aqueous aerosol forms predominantly in fogwater and cloud droplets, and in regions where local emissions of NH 3 are important.

The equilibrium of ammonium chloride aerosol with gaseous hydrochloric acid and ammonia under tropospheric conditions

Abstract Using thermodynamic data, the equilibrium constant for dissociation of solid ammonium chloride aerosol into gaseous hydrogen chloride and ammonia has been calculated. The existence of solid ammonium chloride aerosols is thermodynamically possible at temperatures and concentrations observed in the troposphere, but significant gas phase concentrations of hydrogen chloride and ammonia will also be present.

Nitrous and nitric acid measurements at sites in South-East England

Measurements of HONO and HNO3 have been made using annular denuder samplers at sites in south-east England. Whilst concentrations of HNO3 exhibited a diurnal variation, with a maximum in mid-afternoon nitrous acid shows the opposite diurnal cycle with maximum levels at night due to daytime photolysis. Concentrations of HONO increase with those of NO2, and elevated nighttime HONO level appear to be followed by high levels of HNO3 the following day. Average concentrations of HONO (0.45± 0.26 ppb in 24 h samples are comparable to those of HNO3 (0.56±0.36 ppb in 24 h samples), each representing about 5–10% of the concentration of NO2. Although NO2 oxidation provides the source of HNO3 concentrations of the two compounds are not related, presumably since the formation of NH4NO3 aerosol limits HNO3 concentrations at out site.

Land-surface exchange in a chemically-reactive system; surface fluxes of HNO3, HCl and NH3

Abstract Vertical gradients from 0.25 to 2 m of NH 3 , HNO 3 and HCl and associated aerosol components have been measured in the field above various surfaces in eastern England. The data have been examined to identify the effect, if any, of chemical reaction processes upon the observed vertical profiles. It is concluded that chemical transformations are too slow to influence concentration gradients and thus the surface exchange process. Assuming chemically conservative behaviour, deposition velocities for HNO 3 and HCl have been calculated; these lie within the range 0.4–7.7 cms −1 and 0.4–6.9 cms −1 for HNO 3 and HCl, respectively. Estimation of resistances to deposition indicates a negligible surface resistance for both species. Fluxes of ammonia were predominantly upward from the ground with a mean value of 0.031 μg m −2 s −1 which is consistent both with an emission inventory of the U.K. and with the measured atmospheric concentration of NH 3 .

Measurements of atmospheric HNO3, HCl and associated species on a small network in eastern England.

Abstract Measurements of gaseous HNO3, HCl and NH3 and particulate NO3−, SO42−, Cl− and NH4+ have been made at a small network of sites in eastern England using sampling intervals from 3 h to 7 days. Both HCl and HNO3 are spatially rather uniform, with some variation apparently due to spatial variations in NH3, which stoichiometrically exeeded the sum of both gaseous acids. Mean concentrations of NH3, HCl and HNO3 between February 1987 and January 1988 were 1.90, 0.67 and 1.01 μg m−3, respectively. Pollution roses revealed low NH3 concentrations, and high associated HCl and HNO3 with winds from the North Sea. HCl, but not HNO3 showed an appreciable elevation in concentration on the sector NW from our site, which we speculate may be due to the large capacity of coal-fired power stations in this upwind sector. Three-hourly data have been examined for diurnal effects and its is concluded that nocturnal formation of NO3− is occurring.

The optical properties and morphology of cloud-processed carbonaceous smoke

The effects of high relative humidity and low supersaturations of water vapour on the morphology of fractal clusters of carbonaceous smoke are examined. Under supersaturation conditions, characteristic of those found in the atmosphere, smoke still consists of fractal clusters, although the fractal dimension has been substantially increased. The result of cloud processing (repeated compression/expansion cycles) upon the optical properties of the clusters indicates little change for small clusters (N 103 primary spherules). This is qualitatively consistent with the predictions of theory if the particles change from clusters of fractal dimension D < 2 to compact spheres as a result of such processing.

Semi-quantitative x-ray diffraction analysis of size fractionated atmospheric particles

Abstract We describe the development of a simple standardization technique for the x-ray diffraction (XRD) analysis of atmospheric particulate matter, using aluminium oxide as an internal standard. Accuracy and precision were better than ± 25% for nearly all the typical atmospheric crystalline compounds studied. Good agreement was found with elemental analyses. Detection limits were around 40–75 μg, depending on the compound, for scan times of about 1-h. This represents a considerable improvement in speed of analysis over previous studies. The suitability of various filters for XRD analysis was assessed. Efforts were made to minimize potential sample modification during and after sampling, and tests made to examine hydration effects. The representativity of the collected samples to actual atmospheric conditions is discussed. The technique was applied to size-fractionated atmospheric samples collected in suburban Toronto, Canada. Around half the collected mass was XRD identifiable, of which about half evidently originated from blown dust (soil, building materials, road salt, etc.) with the remainder comprising some eight pollution derived compounds of NH4+, SO42−, NO3−, Cl− and Pb. The distribution of the major ions between different compounds was determined. XRD identifiable lead was related to total Pb concentrations. We conclude by considering our results in a wider context by examining the similarities and discrepancies between other related studies worldwide.