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The use of conversion factors in air pollution studies: Sulphur dioxide ppm—μg m−3
THE USE OF CONVERSION FACTORS IN AIR POLLUTION STUDIES: SULPHUR DIOXIDE ppm-ic,o mv3 A. GARNFIT.
P. READ and D. FISCH
Air Pollution Research Unit, Dept. of Geography. Sheffield Universit>. U.K.
Abstract-Some recently developed and now widely used continuous sulphur anal\scrs express SO, sampling in ppm. which must be converted to pugm-’ where comparability arises of gaseous with particulate and aerosol sampling, or with other types of continuous SO2 analysers recording directlq in pg m-‘. The percentage deviations from the conversion factor now generally used (related to NTP values) that arise with the substitution of the actual climatic variables under which sampling takes place. are discussed. A method for the graphical calculations of these deviations is suggested. for application where there is need for closer precision in the conversion of ppm to ,~_nrn-~ than the factor presenti! used (2858) can provide.
I.
ISTRODGCTIOS
Units of measurement concerned with air pollutionsampling raise inherent difficulties when a comparison of gaseous and particulate or aerosol pollutants is attempted-as. for instance, in the case of sulphur dioxide and smoke. Concentrations of normal gases in the atmosphere are generally designated with reference to parts per million by colume of gas to colume of air (= ppm). On the other hand pollution from particulates and aerosols (e.g. smoke) is expressed in Mteighr,‘colnmeunits, i.e. in micrograms of particulate matter per cubic metre of air, (= pg mw3). With the increasing development and use of more sophisticated instrumentation for the measurement of sulphur dioxide some restatement of the problems involved in conversion from ppm to pg m- 3, and the degree of accuracy attained by methods currently used may be of valueparticularly in cases of research where a high degree of accuracy is required over short time-period sampling. At normal temperature and pressure (NTP) l.Oppm of sulphur dioxide equals 2858 pg m-‘, a derived figure which is the basis for the conversion factors most commonly quoted, and currently widely used. Since. this estimation involves the application of the Gas Law it might be argued that it accepts the erroneous assumption that air and SO2 necessarily behave as perfect gases. though SO2 is probably dilute enough to be so considered. Further complications arise in that the base layers of the atmospheric Boundary Layer (i.e. near surface levels) include varying amounts of both particulate matter and of water vapour. The effects of the latter have been assessed as rarely equivalent to more than cu. l”/d change in the Gas Constant; thus errors so induced can be considered to be negligible. Of greater importance however as a potential source of error is the use of the
NTP standard. L’;s. that normal temperature is defined as U-C and pressure as that of a standard atmosphere at 1013 mb. The estimation of a conversion factor on this basis implies acceptance of values for these two meteorological elements from bvhich the effects of any chance fluctuations have been eliminated. In fact. they do not necessarily bear a close relationship with the real values that are present over the period when. and at the locality wherein. sulphur dioxide levels of concentration have been sampled. The differences in the conversion factor that may arise when variable climatic parameters rather than NTP standards are substituted, must therefore be considered.
2. THE CONVERSION REFERENCE
FACTOR
ASSESSED
TO CLIMATIC
N’Il’H
MEASS
For Great Britain the mean annual sea level pressure is. overall. quite close to standard pressure as defined above. e.g. at Kew it is 1015 mb. but at stations near sea level in the British Isles the meail annual temperatures ranges from cu. 7’C in the Shetlands to more than 12°C in the southwest of England. The mean annual temperature cannot be regarded as more than a generalised climatic index of limited value, but since 1O’C is widely representative of such temperature means in the British Isles and is sometimes given as the mean surface temperature of the world? the substitution of IO’C for O’C could be made. This would give a lower conversion figure of 2757. On the basis that l.Oppm SO2 = 27571cgme3. conversion must then represent a deviation of -3.Yb from the NTP derived figure. The wider ranging effects of day to da- and seasonal weother variations on the estimated conversion factor also merit consideration.
325
A. GARSFTT, P. READ and
26
D. FINCH
Table I Temperature (‘CJ General or for Month of oc‘currencr
Pressure
specific regions
(mb)
in England
January January January
CU. 1032 97-M 1020
s-9 i
!&rch
966
bfay
990
June
cn. locKI
July
cff. 1016
3. THE COSX-ERSIOS
FACTOR IN RELATIO3
I -\TIO\S
TOT
_6p-_7’ North 0” Central 10’ North I’ South 10’ Southeast 33’ 29-32’
P \‘.4R-
b\ ITH \\‘ETHER
For a random szkction of specific days in England identified from daily weather maps, instances of varying T,P relationships are indicated in Table I. Abo included are the percentage deviations from the ‘WfP-derived” figure that the substitution of these climatic standards would provide for each of the days considered. These vary but are in excess of the deviation of 3.59; considered in Section 2 in all but two cases. It will be noted From the above table that though negative values are more frequent. percentage deviations can be both positive or negative according to the relationship of the weather elements present. Since the conversion factor-however derived-is proportional to pressure and inversely proportional to temperature. theoretically the largest negative discrepancies would be associated with summer conditions in a spell of very warm weather and moderate to low pressure. Such a case, in fact occurred under 16) in Table 1. giving rise to a negative value of nearly iz”/,. It could also occur at temperatures of 20°C but the pressure would then have to be as low as 960mb which would be unusual as low pressure systems in summer are generally shallow, as compared with those characteristic of the winter season. At the same time, for a variety of reasons the SO, levels also tend to be lower during the summer season. The question may arise as to how far the above range of T/P relationships actually occurs vvith respect to a prescribed period of time at any specified place.
4.
Percentage deviation from YTP-dcritcd conversion factor - I4”,, -6.5 -3.3 -4.7 -S.O - 2-7 -5-s - I I.7 -Y.-l-- 10.2
From the 730 evaluations thus obtained an overall range of 350 was identified, whose distribution and frequency are shown in Fig. 1. During that year the weather brought no outstanding periods of extremes; for the most part in fact the winter months were very mild. Temperatures ranged from a minimum of -3’C in association with pressure at 1024-1028 mb. to a maximum of -+-75X’ with ‘c
VARIATION 14 T’P REL.~TlONSHtF’S IN SHEFFIELD DURISG THE YEAR I!?74
From the study of daily weather charts and local meteorological data simultaneous T/P relationships at Sheffield were assessed twice daily throughout the year 1974. These were related to early morning and mid-afternoon periods of each day so as to include the likely times of both maximum and minimum daily temperatures together with the diurnal fluctuations of pressure, (where the latter have occurred). A single daily assessment would obviously have been inadequate for the identification of the extreme range of relationships that character&d the year as a whole.
Fig. 1. Simultaneous pressure:temperature relationships recorded in SheReId for the year 1971 (twice daily): (number Or occasions indicated in each case). AA’ = line defining NTP derived conversion factor (ZQjS); BB’ = line
defining mean climatic T/P-derived conversion factor (37573.Lines also drawn to show equal percentage deviations from the E\iP derived factor roof;. <&AI’).
Comewon
facrors m air pollution studies
pressure at 102&1014mb. The lowest pressure (968-9721 was related to temperatures of f I. 10 and 1I’C. and the highest pressure (1oJ~lOU mb) to f 1O’C. For other vears the vagaries of changing weather conditions could gi\-e rise to much wider ranges: the extremes for 197-I. listed above. give a percentage deviation of from +3”,0 to -yOO: Extreme T/P relationships 197-I O’C,‘(mb)
Percentage deviation NPT derived figure
f I I ‘/972 i lO’i972 + l’j972
for all occasions. but it must be remembered that the correction only gives a difference of ca. ?S”,. The potential range of daily weather variables is considerable. but provided that local records of temperature and pressure are available. the percentage deviation from the “NTPderived” factor can readily be ascertained by the graphical representation of data included in Table 2. This includes a range of values covering most extremes likely to be encountered under British weather conditions with possibility for further extrapolation and interpolation as needed.
cfl. -Y”, ca. -8””
+ 10’/1o-u
cl7. --PO CCI.- l”,
+25’~1020 -3;. 102s
ctl. -SO, cfl. ;Y,
I. coscLl_3loss
The seasonal ranges for the six winter months and three midsummer months are shown in Fig. I. and indicate that a wider range of T P relationships occurred during the winter months-a fact not without importance in view of the higher sampling levels of SO2 that generally characterise that season. A few of the relationships w?re distinctive only to the spring and autumn (months April. May and September) and referred to temperatures of from 19 to IYC. with pressure at 996-9SOmb. For these a percentage deviation of ~(1.7-S”,, is implied. In Fig. I the lines .-\‘A and B’B join points at which the actual weather-defined T P relationships for 1974 gave rise tither to the same conversion figure as that appropriate to the %TP-derived” standards (A’A = 2858). or fo the climatic annual mean standards using IO’C (B’B = 2757): thus along these lines no deviation existed with respect to the factor standards used in each case. From Fig. 1. hotvever, it will be noted that the line B’B (2757) approximates more closely to the overall mean range of T,P relationships for the year in question; furthermore, for the three midsummer months the line ;\‘A (2858) falls outside the entire range of T P relationships that were actually identified for that season, from which it follows that all data derived from the NTP factor must be inaccurate for that season. A case therefore might be made for the use of the climatically based conversion factor (2757) during the summer season. or even
The general acceptance of the NTP derived conversion factor (2858) is now so well established that the introduction of a new figure for general purposes would inevitably give rise to some confusion-logical though the use of annual clirrlatic means might seem to be. Long term means of pollution sampling (e.g. mean annual or seasonal) represent no more than somewhat generalised pollution indices that often conceal a wide range of extremes, and the climatically derived “r~rar~” error that could be applied to such data would often be too small to be of importance. It could be argued however. that with respect to rneall daily levels a better assessment would be achieved using mean annual climatic standards (i.e. 103C) leading to the adoption of a new conversion factor of 2757 for all general purposes. But though a small (3.5’3;)improvement would be achieved. the mean climatic-derived factor would still be imprecise: nor. necessarily. does it appear to offer a uniform degree of advantage at all seasons of the year: (see Fig. I). Where a higher standard of precision is sought. in keeping with the more sophisticated sampling techniques now used and the nature of some problems investigated (e.g. in micro-biotic fields). the use of graphical calculations (based on Table 2) presents a useful compromise whereby. without disturbing the widespread acceptance of the standardised “NTP-derived” conversion factor. account can be taken of the relationship to the more theoretically derived figure of short term acnral temperature and pressure fluctuations. The matters discussed in the present instance have been related basically to atmospheric environmental considerations but could be applied to problems related to laboratory and technological fields of
Table 2. Percentage deviations in the NTP-derived conversion factor for a range of T P climatic conditions Temp. (0
1040
1030
1020
1010
1000 990 Pressure (mb)
+2j‘ +20: +lj + 10: +j’ 0:
- 6.0 - 4.4 - 2.7 - 1.0 +o.s + 2.6
-6.9 - j.3 -3.6 - 2.0 -0.2 + 1.6
- 7-s - 6.2 -4.6 - 2.9 - 1.2 +0.7
-5.7 -7.1
-9.6 -8.0
-j.j
-6.j
-3.5 -2.1 -0.3 +I.5
-4.8 -3.1 - 1.3 +@j
-jl
++j
+ 3.j
+Zj
- IO.5 -9.0 - 7.1 - 5.S -.+.I -2.; -0.j
980
970
960
-11.1 -9.9 -8.3 - 6.7 - j.0 -3.3 - 1.5
- 12.3 - 10.8 -9.3 - 7.7 -6.0 -4.3
- 13.2 - 11.7 - 10.2 -8.6 - 7.0 -5.2 -3.5
-2.j
950
940
- 14.1 - 15.0 - 12.6 - 13.6 -11.1 - 12.0 -9.6 - IO.5 - 7.9 -s.9 -6.2 - 7.2 -4.5 _ j.j
31s
A. G.ARXETT.P. READ and
study. though the latter would ~II\O~VS the use of somewhat different ranges of temperature. pressure relationships. not necesssarily concerned with weather variables. Records of gaseous and particuiate pollutants at a given air sampling site do not continuously follow an identical or mirrored pattern of changing severity and it is important that accurate methods of conversion to a common system of measurement should be available where studies involving comparability are undertaken. For other purposes sulphur dioxide is obviously most satisfactorily measured only by units of volume (ppm) and not those which attempt to combine both volume and weight (/~g m-‘).
D.
FINCH
The errors discussed above obviousI! have taken no account of other sources that ma)- occur-not least in importance being those concerned with both human and instrumental operational faults, to which attention equally should be directed.
.4cknowIrdgemmr-The Air Pollution Research Unit. Department of Geography. Shetlield L’nitcrsity. is much indebted to the British Gas Corporation for financial assistance in support of its work. (A. Garnett. Ph.D.. E/wriMS Professor. P. Read. BSc.. Srtziur E.~prrimmral O$icrr. and D.. Finch. B.A.. M.Sc.)
P. READ and D. FISCH
Air Pollution Research Unit, Dept. of Geography. Sheffield Universit>. U.K.
Abstract-Some recently developed and now widely used continuous sulphur anal\scrs express SO, sampling in ppm. which must be converted to pugm-’ where comparability arises of gaseous with particulate and aerosol sampling, or with other types of continuous SO2 analysers recording directlq in pg m-‘. The percentage deviations from the conversion factor now generally used (related to NTP values) that arise with the substitution of the actual climatic variables under which sampling takes place. are discussed. A method for the graphical calculations of these deviations is suggested. for application where there is need for closer precision in the conversion of ppm to ,~_nrn-~ than the factor presenti! used (2858) can provide.
I.
ISTRODGCTIOS
Units of measurement concerned with air pollutionsampling raise inherent difficulties when a comparison of gaseous and particulate or aerosol pollutants is attempted-as. for instance, in the case of sulphur dioxide and smoke. Concentrations of normal gases in the atmosphere are generally designated with reference to parts per million by colume of gas to colume of air (= ppm). On the other hand pollution from particulates and aerosols (e.g. smoke) is expressed in Mteighr,‘colnmeunits, i.e. in micrograms of particulate matter per cubic metre of air, (= pg mw3). With the increasing development and use of more sophisticated instrumentation for the measurement of sulphur dioxide some restatement of the problems involved in conversion from ppm to pg m- 3, and the degree of accuracy attained by methods currently used may be of valueparticularly in cases of research where a high degree of accuracy is required over short time-period sampling. At normal temperature and pressure (NTP) l.Oppm of sulphur dioxide equals 2858 pg m-‘, a derived figure which is the basis for the conversion factors most commonly quoted, and currently widely used. Since. this estimation involves the application of the Gas Law it might be argued that it accepts the erroneous assumption that air and SO2 necessarily behave as perfect gases. though SO2 is probably dilute enough to be so considered. Further complications arise in that the base layers of the atmospheric Boundary Layer (i.e. near surface levels) include varying amounts of both particulate matter and of water vapour. The effects of the latter have been assessed as rarely equivalent to more than cu. l”/d change in the Gas Constant; thus errors so induced can be considered to be negligible. Of greater importance however as a potential source of error is the use of the
NTP standard. L’;s. that normal temperature is defined as U-C and pressure as that of a standard atmosphere at 1013 mb. The estimation of a conversion factor on this basis implies acceptance of values for these two meteorological elements from bvhich the effects of any chance fluctuations have been eliminated. In fact. they do not necessarily bear a close relationship with the real values that are present over the period when. and at the locality wherein. sulphur dioxide levels of concentration have been sampled. The differences in the conversion factor that may arise when variable climatic parameters rather than NTP standards are substituted, must therefore be considered.
2. THE CONVERSION REFERENCE
FACTOR
ASSESSED
TO CLIMATIC
N’Il’H
MEASS
For Great Britain the mean annual sea level pressure is. overall. quite close to standard pressure as defined above. e.g. at Kew it is 1015 mb. but at stations near sea level in the British Isles the meail annual temperatures ranges from cu. 7’C in the Shetlands to more than 12°C in the southwest of England. The mean annual temperature cannot be regarded as more than a generalised climatic index of limited value, but since 1O’C is widely representative of such temperature means in the British Isles and is sometimes given as the mean surface temperature of the world? the substitution of IO’C for O’C could be made. This would give a lower conversion figure of 2757. On the basis that l.Oppm SO2 = 27571cgme3. conversion must then represent a deviation of -3.Yb from the NTP derived figure. The wider ranging effects of day to da- and seasonal weother variations on the estimated conversion factor also merit consideration.
325
A. GARSFTT, P. READ and
26
D. FINCH
Table I Temperature (‘CJ General or for Month of oc‘currencr
Pressure
specific regions
(mb)
in England
January January January
CU. 1032 97-M 1020
s-9 i
!&rch
966
bfay
990
June
cn. locKI
July
cff. 1016
3. THE COSX-ERSIOS
FACTOR IN RELATIO3
I -\TIO\S
TOT
_6p-_7’ North 0” Central 10’ North I’ South 10’ Southeast 33’ 29-32’
P \‘.4R-
b\ ITH \\‘ETHER
For a random szkction of specific days in England identified from daily weather maps, instances of varying T,P relationships are indicated in Table I. Abo included are the percentage deviations from the ‘WfP-derived” figure that the substitution of these climatic standards would provide for each of the days considered. These vary but are in excess of the deviation of 3.59; considered in Section 2 in all but two cases. It will be noted From the above table that though negative values are more frequent. percentage deviations can be both positive or negative according to the relationship of the weather elements present. Since the conversion factor-however derived-is proportional to pressure and inversely proportional to temperature. theoretically the largest negative discrepancies would be associated with summer conditions in a spell of very warm weather and moderate to low pressure. Such a case, in fact occurred under 16) in Table 1. giving rise to a negative value of nearly iz”/,. It could also occur at temperatures of 20°C but the pressure would then have to be as low as 960mb which would be unusual as low pressure systems in summer are generally shallow, as compared with those characteristic of the winter season. At the same time, for a variety of reasons the SO, levels also tend to be lower during the summer season. The question may arise as to how far the above range of T/P relationships actually occurs vvith respect to a prescribed period of time at any specified place.
4.
Percentage deviation from YTP-dcritcd conversion factor - I4”,, -6.5 -3.3 -4.7 -S.O - 2-7 -5-s - I I.7 -Y.-l-- 10.2
From the 730 evaluations thus obtained an overall range of 350 was identified, whose distribution and frequency are shown in Fig. 1. During that year the weather brought no outstanding periods of extremes; for the most part in fact the winter months were very mild. Temperatures ranged from a minimum of -3’C in association with pressure at 1024-1028 mb. to a maximum of -+-75X’ with ‘c
VARIATION 14 T’P REL.~TlONSHtF’S IN SHEFFIELD DURISG THE YEAR I!?74
From the study of daily weather charts and local meteorological data simultaneous T/P relationships at Sheffield were assessed twice daily throughout the year 1974. These were related to early morning and mid-afternoon periods of each day so as to include the likely times of both maximum and minimum daily temperatures together with the diurnal fluctuations of pressure, (where the latter have occurred). A single daily assessment would obviously have been inadequate for the identification of the extreme range of relationships that character&d the year as a whole.
Fig. 1. Simultaneous pressure:temperature relationships recorded in SheReId for the year 1971 (twice daily): (number Or occasions indicated in each case). AA’ = line defining NTP derived conversion factor (ZQjS); BB’ = line
defining mean climatic T/P-derived conversion factor (37573.Lines also drawn to show equal percentage deviations from the E\iP derived factor roof;. <&AI’).
Comewon
facrors m air pollution studies
pressure at 102&1014mb. The lowest pressure (968-9721 was related to temperatures of f I. 10 and 1I’C. and the highest pressure (1oJ~lOU mb) to f 1O’C. For other vears the vagaries of changing weather conditions could gi\-e rise to much wider ranges: the extremes for 197-I. listed above. give a percentage deviation of from +3”,0 to -yOO: Extreme T/P relationships 197-I O’C,‘(mb)
Percentage deviation NPT derived figure
f I I ‘/972 i lO’i972 + l’j972
for all occasions. but it must be remembered that the correction only gives a difference of ca. ?S”,. The potential range of daily weather variables is considerable. but provided that local records of temperature and pressure are available. the percentage deviation from the “NTPderived” factor can readily be ascertained by the graphical representation of data included in Table 2. This includes a range of values covering most extremes likely to be encountered under British weather conditions with possibility for further extrapolation and interpolation as needed.
cfl. -Y”, ca. -8””
+ 10’/1o-u
cl7. --PO CCI.- l”,
+25’~1020 -3;. 102s
ctl. -SO, cfl. ;Y,
I. coscLl_3loss
The seasonal ranges for the six winter months and three midsummer months are shown in Fig. I. and indicate that a wider range of T P relationships occurred during the winter months-a fact not without importance in view of the higher sampling levels of SO2 that generally characterise that season. A few of the relationships w?re distinctive only to the spring and autumn (months April. May and September) and referred to temperatures of from 19 to IYC. with pressure at 996-9SOmb. For these a percentage deviation of ~(1.7-S”,, is implied. In Fig. I the lines .-\‘A and B’B join points at which the actual weather-defined T P relationships for 1974 gave rise tither to the same conversion figure as that appropriate to the %TP-derived” standards (A’A = 2858). or fo the climatic annual mean standards using IO’C (B’B = 2757): thus along these lines no deviation existed with respect to the factor standards used in each case. From Fig. 1. hotvever, it will be noted that the line B’B (2757) approximates more closely to the overall mean range of T,P relationships for the year in question; furthermore, for the three midsummer months the line ;\‘A (2858) falls outside the entire range of T P relationships that were actually identified for that season, from which it follows that all data derived from the NTP factor must be inaccurate for that season. A case therefore might be made for the use of the climatically based conversion factor (2757) during the summer season. or even
The general acceptance of the NTP derived conversion factor (2858) is now so well established that the introduction of a new figure for general purposes would inevitably give rise to some confusion-logical though the use of annual clirrlatic means might seem to be. Long term means of pollution sampling (e.g. mean annual or seasonal) represent no more than somewhat generalised pollution indices that often conceal a wide range of extremes, and the climatically derived “r~rar~” error that could be applied to such data would often be too small to be of importance. It could be argued however. that with respect to rneall daily levels a better assessment would be achieved using mean annual climatic standards (i.e. 103C) leading to the adoption of a new conversion factor of 2757 for all general purposes. But though a small (3.5’3;)improvement would be achieved. the mean climatic-derived factor would still be imprecise: nor. necessarily. does it appear to offer a uniform degree of advantage at all seasons of the year: (see Fig. I). Where a higher standard of precision is sought. in keeping with the more sophisticated sampling techniques now used and the nature of some problems investigated (e.g. in micro-biotic fields). the use of graphical calculations (based on Table 2) presents a useful compromise whereby. without disturbing the widespread acceptance of the standardised “NTP-derived” conversion factor. account can be taken of the relationship to the more theoretically derived figure of short term acnral temperature and pressure fluctuations. The matters discussed in the present instance have been related basically to atmospheric environmental considerations but could be applied to problems related to laboratory and technological fields of
Table 2. Percentage deviations in the NTP-derived conversion factor for a range of T P climatic conditions Temp. (0
1040
1030
1020
1010
1000 990 Pressure (mb)
+2j‘ +20: +lj + 10: +j’ 0:
- 6.0 - 4.4 - 2.7 - 1.0 +o.s + 2.6
-6.9 - j.3 -3.6 - 2.0 -0.2 + 1.6
- 7-s - 6.2 -4.6 - 2.9 - 1.2 +0.7
-5.7 -7.1
-9.6 -8.0
-j.j
-6.j
-3.5 -2.1 -0.3 +I.5
-4.8 -3.1 - 1.3 +@j
-jl
++j
+ 3.j
+Zj
- IO.5 -9.0 - 7.1 - 5.S -.+.I -2.; -0.j
980
970
960
-11.1 -9.9 -8.3 - 6.7 - j.0 -3.3 - 1.5
- 12.3 - 10.8 -9.3 - 7.7 -6.0 -4.3
- 13.2 - 11.7 - 10.2 -8.6 - 7.0 -5.2 -3.5
-2.j
950
940
- 14.1 - 15.0 - 12.6 - 13.6 -11.1 - 12.0 -9.6 - IO.5 - 7.9 -s.9 -6.2 - 7.2 -4.5 _ j.j
31s
A. G.ARXETT.P. READ and
study. though the latter would ~II\O~VS the use of somewhat different ranges of temperature. pressure relationships. not necesssarily concerned with weather variables. Records of gaseous and particuiate pollutants at a given air sampling site do not continuously follow an identical or mirrored pattern of changing severity and it is important that accurate methods of conversion to a common system of measurement should be available where studies involving comparability are undertaken. For other purposes sulphur dioxide is obviously most satisfactorily measured only by units of volume (ppm) and not those which attempt to combine both volume and weight (/~g m-‘).
D.
FINCH
The errors discussed above obviousI! have taken no account of other sources that ma)- occur-not least in importance being those concerned with both human and instrumental operational faults, to which attention equally should be directed.
.4cknowIrdgemmr-The Air Pollution Research Unit. Department of Geography. Shetlield L’nitcrsity. is much indebted to the British Gas Corporation for financial assistance in support of its work. (A. Garnett. Ph.D.. E/wriMS Professor. P. Read. BSc.. Srtziur E.~prrimmral O$icrr. and D.. Finch. B.A.. M.Sc.)