Environmental Chemistry Volume 2

A Review of the Literature Published Up to Mid-1980

By H. J. M . Bowen

The Royal Society of Chemistry

Copyright © 1982 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-765-6

Contents

Chapter 1 Inorganic Particulate Matter in the Atmosphere By P.A. Cawse, 1,
Chapter 2 The Elemental Content of Human Diets and Excreta By H.J. M. Bowen, 70,
Chapter 3 The Elemental Constituents of Soils By A. M. Ure and M. L. Berrow, 94,
Chapter 4 Mycotoxins By D. S. P. Patterson, 205,
Chapter 5 Occurrence, Distribution, and Chemical Speciation of some Minor Dissolved Constituents in Ocean Waters By J. D. Burton and P. J. Statham, 234,
Author Index, 267,

CHAPTER 1

Inorganic Particulate Matter in the Atmosphere

BY P. A. CAWSE

1 Introduction

Inorganic particulate matter that is raised to the atmosphere by both natural and artificial (anthropogenic) sources is mainly distributed in the lower troposphere. The upper boundary of the troposphere, the tropopause, is found at 11-17 km above the earth’s surface depending on latitude. Volcanic eruptions may inject particles through the tropopause boundary to the stratosphere, which extends to the stratopause at ~5O km altitude. Interplanetary debris from micrometeors may also provide a small contribution to particle loads in the atmosphere.

Particulates in the stratosphere will be subject to a global distribution, whereas material injected to the troposphere will be transported in the zonal circulation before returning to the earth’s surface. These transport pathways from sources to sinks determine the local, regional, and global nature of pollution, and eventually, possible effects on biological targets.

The inorganic fraction generally comprises 80 — 90% of the total suspended particulates (TSP) in the atmosphere; of the remainder, benzene-soluble organic compounds may amount to 5% and biological debris, including bacterial and fungal spores, are also present. In the Antarctic aerosol practically all of the mass is attributed to SO2-4,2 but in a non-urban area at Chilton (Oxon) for example, 50% of TSP was accounted for by SO2-42, NO-3, NH+4, and NaCl with a further 3% from Ca, K, and Mg:3 Fe and Al can typically account for 2-3% of the particulate mass Potentially toxic metals such as Pb can constitute 1% in urban environments, but 0.1% in remote areas. Concentrations of suspended particulates in urban environments throughout the world show annual mean values between 60 and 500 μg m-3.

The changing quality of the atmosphere in England since medieval times6 and public concern towards increasing industrial pollution demonstrate the historical importance of anthropogenic inputs to the atmosphere and disturbance to the natural background of airborne particulates. Today we are faced with changes in systems of energy production and manufacturing processes that will affect both the output and nature of particulate emissions. Trends in atmospheric composition must therefore be recorded to assess the impact of these changes on the environment and to decide on the need for controls on emissions. The annual consumption of coal in England and Wales is now approaching 80 million tonnes, a record level, and activities of this magnitude demand very careful assessment of environmental consequences.

Total particulate concentrations in the atmosphere and levels of associated elements of potential toxicity such as As, Cd, Pb, and Se may present a nuisance or serious hazard to man and animals by inhalation and ingestion of contaminated food. Deposition of aerosols may induce a response in soil and water ecosystems, for example by acidification and accumulation of metals. Deterioration in visibility caused by suspended particulates and their role in modification of the world climate by disturbing the transfer of radiant energy are of major concern. The presence of increased concentrations of water soluble ions such as SO2-4 and NO-3 in polluted aerosols may influence natural processes of nucleation. Hence, pollution is now being recognized as a complex international as well as a national problem.

This report is aimed at broad coverage of recent advances in research on inorganic particulates in the atmosphere, mainly from 1975 onwards. Studies on the fate and effects of such materials are included. Within this remit, some 1500 references have been identified of which a selection are quoted here to demonstrate the diversity of research developments. Advances in industrial engineering techniques to control emissions and improve workroom atmospheres are extensively reviewed elsewhere.

Over the past 25 years, a contrast is evident between earlier localized investigations of the atmosphere and present day research projects on a global scale not only in the troposphere but also extending to the stratosphere. Classification of papers submitted to ‘Atmospheric Environment’ from 1973-1977 has shown more emphasis on aerosol (physico-chemical) research, comprising 46% of papers, than on gaseous species (34%); atmospheric transport and modelling account for 20% and cover both particulate and gaseous forms. In fact the total number of articles has increased exponentially since 1960.

Research on inorganic particulates in the atmosphere, their transfer pathways and effects has now become more inter-disciplinary, demanding the efforts of meteorologists, oceanographers, agriculturists, and medical and veterinary specialists, in addition to atmospheric chemists and physicists.

Terminology. — An aerosol is a system of fine solid or liquid particles in gaseous suspension, collectively referred to as ‘particulates’. Dust refers to a relatively coarse range of solid particles (diameter, d > 1 µm), produced by disintegration of minerals or from resuspension by wind when sand-blasting of soil particles may often cause comminution. Fine particles formed from the gas phase by condensation are termed ‘smoke’ and ‘fume’. In the case of fume the particles are generally from 0.01 — 1 µm diameter, and are often observed as agglomerates of smaller particles. Suspended particulate matter <15 µm diameter is usually defined as smoke.

Liquid droplets are often referred to as mists (d > 40µm) and fogs (d = 5 — 40 µm). Small hygroscopic particles, or condensation nuclei, are classified into Aitken nuclei (d<0.2 µm), large (d = 0.2-2µm), and giant (d > 2µm) types. The term ‘haze aerosol’ is frequently encountered in optical studies and includes any airborne particles that affect visibility. The fundamental properties and behaviour of aerosols and their formation from gases are the subject of several texts, Research on aerosol electrical properties have been discussed by several authors at a recent conference.

2 Methods of Sampling and Analysis

Collection Systems. — Several collection systems are available for sampling ‘total’ particulates in the atmosphere by filtration, for determination of mass or detailed chemical analysis; the EPA High Volume sampler and the German ‘LIB’ unit are examples. However, errors in sampling associated with various systems and devices used out-of-doors are well recognized. Errors may result from particle impaction or diffusion to the sampling probe or inlet manifold, and from the influence of increasing wind speed on the inertia of larger particles and thus on the particle size spectrum that is collected at a given intake velocity. The intake efficiency of 17 sampling devices under outdoor conditions showed great variation for particles between 20-50 µm diameter. Tests on a sampler with inlet velocity of 70 cm s-1 established that at wind velocities >4 m s-1, particles over 10 µm diameter were collected with <15% efficiency: 26 sampling of this restricted range of particle size may be considered adequate if the objective is to assess respirable particles.

Other workers have recommended that high volume samplers are operated inside shelters to prevent particles being deposited on2′ or blown off 28 the filter during idle hours. Benarie concluded from laboratory tests of the EPA High Volume sampler under turbulent rather than laminar air-flow conditions, that outdoor sampling can be achieved without significant distortion of the size spectrum of urban particulate matter that is collected.

Special procedures are required for certain types of aerosol. Mercer describes a technique for collection of sulphuric acid mist and particles emitted by copper smelters. For Saharan dust an isokinetic sampler is proposed, which is battery operated and collects by electrostatic precipitation, with an efficiency > 90% for most particle. To separate fibrous material from spherical particulates, the aerosol can be electrically charged and separated in an electric mobility spectrometer, when fibres show high mobility. An impingement sampler has been designed to collect salt aerosols at maritime locations or from cooling tower drift near to power stations.

Stratospheric aerosols have been collected by balloon-borne equipment designed to impact particles on carbon or copper films for subsequent electron microscopy, or by direct photoelectric counting of particles in situ. Measurements by both techniques have been compared. Collection of large (d > 50 µm) cosmic dust particles was made by balloon-borne apparatus at an altitude of ~30 km.

Development of personal air samplers has continued and is reviewed by Wallace. General approaches to air sampling for occupational hygiene purposes are discussed by Molyneux. A miniature prototype personal dust sampler, the ‘CIP’, is based on the annular impaction principle within a rotating housing to achieve separation into respirable and non-respirable particle. Another prototype instrument relies on the particle charging principle and was originally designed for detection of fire hazard in mines by spontaneous combustion: it is adapted for continuous monitoring of submicrometre particles in ambient air and is portable. Two types of self-contained personal samplers are available to measure respirable and non-respirable fractions of dust in quarries.

Filters. — The selection of filters for collection of air particulates must take into account not only collection efficiencies and effects of loading on flow rates but also interferences from metals present in the filter medium and its hygroscopic nature. These properties must be considered in relation to the objectives of the sampling and the method of analysis. Cellulose acetate, glass fibre, and polytetrafluoroethylene filters showed higher efficiencies (> 99%) for ambient dust particles than nucleopore (0.8µm) and Whatman 41 cellulose filters, which were 72% and 64% efficient at a face velocity of 49 cm s-1. Although the collection efficiency of cellulose fibre filters increases markedly with loading, other types of filter are preferred for retention of Pb in urban aerosol if only light loadings are collected. In fact, Whatman 1 and 40 grade cellulose filters approached 100% efficiency after 24 hours collection of the ambient aerosol at Stevenage, UK, but this was dependent on particle size and face velocity. In the Cleveland USA environment, a side-by-side comparison of total suspended particulate retained by Whatman 41 cellulose filters with glass fibre filters showed no difference at 16 sampling sites operated for 13 months.

The collection of aerosols by nuclepore filters has been reviewed by Manton with respect to impaction, interception at the pore orifice, and diffusion by Brownian motion both to the filter face and to the walls of the pores. Problems of pore clogging have been investigated by both theoretical and experimental approaches. Two modes of clogging are recognized, namely uniform pore filling and cap formation, the latter being predominant.

Evaluation of four types of glass filters for high-volume air sampling revealed small differences in collection of TSP and Pb, but large variations in SO2-4 and NO-3 catalytic oxidation of SO2 and NO2 on the filter surface was suspected. This reaction may add 64% to true SO2-4 levels and 5% to TSP; because filter alkalinity is implicated, use of the neutral pH quartz fibre type is advised if glass fibre must be used.

Methods of generation of fine particle aerosols (d<3.5µm) for research and calibration purposes are the subject of a symposium-revie~.~~ The practice of collection and storage of aerosols in aluminized mylar bags prior to analysis may lead to loss of submicrometre particles by electrostatic precipitation, but this can be alleviated by the use of antistatic agents.

Methods for Specific Aerosols. — Techniques for collection of sulphate aerosols prior to determination of total SO2-4 or sulphate species are summarized by Blokker, with a review of the behaviour of sulphates in the atmosphere. The problem of SO2 conversion to SO2-4 during sampling is insignificant if filters of PTFE microfibre or matrix are used, or alternatively, acid-treated quartz filters. The use of glass fibre filters to sample particulate NO-3 is beset with interferences from gaseous N compounds, mainly HONO2 and NO2. This is more serious than the formation of artifact SO2-4. Of several types of filters tested for accumulation of artifact NO-3 only a teflon type (‘Fluoropore’) gave negligible error.

Determination of particulate and vapour-phase arsenic has been made by using a nuclepore pre-filter to retain particulate As and impregnated cellulose fibre filters to absorb As vapour; collection efficiencies were established with arsenic trioxide vapour. From measurements in the ambient atmosphere, most As was in the particulate form. A similar scheme was reported for determination of particulate and gaseous Br, C1, and I; low uptake of gaseous halogens by the nuclepore pre-filter is an advantage when sampling in remote continental regions where the gaseous halogens exceed particulate forms by two orders of magnitude. A sampling technique has been devised to permit collection of particulate (organic + inorganic) and volatile organic pollutants separately for analysis.

Methods for sampling and identification of asbestos and asbestiform minerals are given by several authors at a recent Workshop. The UK Health and Safety Commission65 have made recommendations for monitoring asbestos dust (chrysotile, amosite, and crocidolite) in both non-occupational and workplace environments.

Collection of Dry Deposition and Rainwater. — Steen has summarized current techniques for measuring dry deposition of particulates from the atmosphere by direct accumulation on flat filter plates, natural surfaces, vertical deposit gauges, or open buckets of various diameter. From a theoretical examination of the British Standard Directional Dust Gauge it was concluded that chemical analysis of the deposit would help to identify different sources and their strengths: the gauge is strongly directional. A method of short-term dustfall measurement was developed to study nuisance levels up to 1.2 km from a cement plant, to relate to complaints by residents: monthly operated gauges were misleading and inappropriate for this purpose.

Rainwater is most frequently collected together with the dry deposition to the funnel, and in many cases deliberately, to represent the total (wet + dry) deposition to the ground. To exclude the dry deposition, various mechanical devices are now available that incorporate a moisture sensor to trigger exposure of the rain collector only when precipitation ‘O Results from one such device, originally developed to separate wet and dry deposition of radioactive fallout have been obtained by operation for one year at Chilton, Oxon, with monthly sample changes (see also Section 6, Total Deposition). Special apparatus is described to collect rainwater in forests, as throughfall and stem-flow.

Total Suspended Particulates. — Standard methods for sampling and measuring total suspended particulate matter (TSP) are listed in a manual issued by the W.H.0. to encourage their use and thereby assist comparisons of data from worldwide networks of monitoring stations. Three methods are based on gravimetry, and employ pre-weighed glass fibre filters. They are suitable for standard 24 h sampling periods and are (i) a high-volume method with constant flow, (ii) an EPA high-volume method, constant flow not maintained, and (iii) an OECD method, modified by the British Standards Institute. Two photometric methods are described, using sampling periods from 1 — 24 h: the soiling of glass fibre or cellulose filters is measured in a reflectometer. Improved accuracy in weighing particulates collected on cellulose and other filters is obtained by using a constant temperature and humidity chamber for all handling operations.

Comparative studies with filter tape samplers, whereby the thickness of particulate deposit is measured by attenuation of beta-radiation, prove the value of this technique to measure short-term (3 h) concentrations of TSP. A semi-continuous beta-monitor (Philips 9790) has been successfully used by Benarie.

Infrared extinction has been applied to measurement of aerosol mass concentration, at wavelengths between 9 and 12 µm; this method requires knowledge of the average refractive index and mass density of particles, but extinction is independent of particle size distribution at the selected wavelengths.

(Continues…)Excerpted from Environmental Chemistry Volume 2 by H. J. M . Bowen. Copyright © 1982 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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