Precipitation scavenging of sulphur dioxide in an industrial area

Atmospheric

Enuironment

Vol.

10, pp. X79-890.

Pergamon Press 1976. Printedin Great Britain

PRECIPITATION SCAVENGING OF SULPHUR DIOXIDE IN AN INDUSTRIAL AREA T. D.

DAVIES

School of Environmental Sciences, University of East An&, Norwich, Norfolk, U.K. (First received 21 January 1975 and in jnal form 19 March 1976) Abstract-An attempt was made to determine the local removal of sulphur dioxide by precipitation in an industrial area (of about 12 km2 in Sheffield, U.K.). The basic sampling period was one hour and the project continued for one year. Since the washout measured depended only on a local process, it could be related in a meaningful way to local atmospheric SO2 concentrations and meteorological parameters. In absolute terms, precipitation removes only a small fraction of the SO2 in the industrial airshed. There appears to be marked differences between winter and summer washout. A detailed statistical analysis indicates some of the factors affecting washout. It proved difficult to determine any effect of precipitation on atmospheric SO2 concentrations by examining concentrations before and after the event. A determination of the relative importance of precipitation was attempted by assessing the effect of various meteorological parameters on SO2 concentrations during the rainfall. Even during precipitation episodes, wind speed appeared to explain more of the variation in atmospheric SO2 concentration than precipitation. Wind direction has a strong influence which is difficult to quantify. The smaller degree of explanation provided by the recorded variables illustrates the complexity of processes in an industrial atmosphere, although the sampling methodology might be usefully employed in less polluted areas to examine further the life-cycle of sulphur.

1. INTRODUCTION

Georgii (1968) believe that SO, contributes about 75% to precipitation sulphur, although it is to be expected that this figure will fluctuate widely. On Further knowledge of the rate and processes of occasions, significant proportions of atmospheric sulatmospheric sulphur removal is desirable for a fuller phur may consist of sulphate or sulphuric acid understanding of the life-cycle of pollutant sulphur. (Wailer, 1963), whereas other workers have found Precipitation scavenging is an important removal small proportions of SO2 transformed into sulphate mechanism, and widespread observation of sulphate concentrations in rainwater has been conducted for aerosols near the source (Rodhe, 1970, 1972; Rodhe many years. Attempts have been made to assess the et al., 1972; HGgstrem, 1973). Attempts at separating role of precipitation in influencing atmospheric SO1 the rainout and washout proportions have involved concentrations over relatively short time-scales (Georlaboratory experimentation (Georgii and Beilke, gii, 1960; Newall and Eaves, 1962). Various labora1966; Beilke and Georgii, 1968), high- and low-altitory experiments have provided basic information on tude ground-level precipitation sampling (Georgii and scavenging mechanisms and removal rates (Beilke and Weber, 1960, 1961) or analysis of cloud or fog dropGeorgii, 1968). More recent studies have examined lets (Oddie, 1962; Mrose, 1966). Most of the previous the spatial variation of sulphate removal by preceipifield investigations have measured sulphate in precipitation over relatively short time-periods (HijgstrGm, tation, although Lavrinenko (1968) determined sul1973) and precipitation scavenging of SO2 from a phite and sulphate after certain intervals of time, and power station plume (Hales et al., 1971). Hales et al. (1971) conducted the contemporaneous Identification of the components of the total sulstudy on sulphur dioxide washout from a power phur scavenged by precipitation, and the determinastation plume to which reference has already been tion of the proportions removed by rainout (incorpormade. ation into a droplet within the cloud) and washout The objectives of the present study were to assess (removal below the cloud-base) are necessary for a the role of precipitation as a scavenger of SOz in full understanding of the processes involved. Studies an industrialised urban area and to relate the rehave indicated that the rainout of SO*, when commoval, on a small time-scale, to ground-level atmospared with washout, is relatively unimportant (Georpheric SO2 concentrations and to various meteorogii and Beilke, 1966; Beilke and Georgii, 1968), but logical parameters. Consequently, it was necessary to the proportions will depend on locality and changing determine the local removal of SOz by precipitation rather than estimate the total sulphur resulting from environmental conditions. It is difficult to determine the rainout and washout processes. A more meaningthe proportions of precipitation sulphur which derive ful study could then be made of the relationships from the gas phase and from aerosols. Beilke and 879

T. D. DAVIES

880

between precipitation scavenging and local environmental factors than has been possible hitherto. A study of the effect of precipitation on atmospheric SO, concentrations was also undertaken. 2. INSTRUMENTS,

METHOD

AND

LOCATION

The sampler designed for this study (Fig. 1) incorporated an all-PTFE coated PVC or polypropylene rainwater pathway which reduced raindrop adherence and facilitated automatic hourly sluicing by distilled water. The dustcover protects the samples from sediment contamination and reduces evaporation. The funnels were used alternately, the non-collecting one being washed with distilled water and allowed to drip-dry. The turntable carried a new amberglass bottle into position under the collecting funnel every hour. The instrument housing was lined with polystryrene and was fitted with heaters in order to sample snowfall in real time. The sampler was constructed on a limited budget, and the necessarily bulky design disturbed the local airflow. But the sampler served its purpose of collecting a representative chemical identity of rainfall, and an adjacent automatic raingauge recorded true rainfall catch (in practice, the two catches were almost identical). The sheltered environment of the collecting bottles ensured that they all experienced the same exposure to atmospheric SOz, meaning that the contribution by direct impingement of atmospheric sulphur dioxide onto the collected rainwater sample could be estimated. Tests were conducted on all the sample bottles after twenty-four hour periods during which no precipitation occurred. Sulphite determinations (see below) then represented the direct atmospheric impingement of SO2 onto the absorbent. The tests were conducted under a variety of atmospheric conditions and the scatter of sulphite values during each test was found to be very small. Laboratory tests confirmed that the sulphite content of the absorbent was of atmospheric origin and not from glassware contamination. Thus, the determination of sulphite concentration in three or four bottles not exposed to rainfall during the 24 hr sampling period provided a suitable reference value from which to determine sulphite contribution to the absorbent from precipitation. Further details are to be found in Davies (1974). The direct impingement contribution was found to be considerable: over the study period approximately half of the apparent SO, removal by rainfall was, in reality, due to this process. Twenty-four hours was the maximum period for which the bottles were exposed. Previous studies have neglected the contribution of direct impingement onto the collected sample, although it may be considerable, even with a sheltered environment for the sample. Sodium tetrachloromercurate solution, with sulphamic acid and EDTA (West and Gaeke, 1956; OECD, 1964; Scaringelli et al., 1970) was placed in the sampling bottles -15cmsplash and bird guard insulated housing

x 0

rotating shaft -

containing TCM solution approx Kim

I)

Fig. 1. Diagrammatic sketch of the sampler (not to scale).

to stabilize the SO* removed by rainfall immediately on collection. The method is specific for SO2 and sulphite in solution. Analysis was conducted within 48 h of exposure. It has been suggested that the lowest 300-400 m of the atmosphere is the important layer for SOz accumulation in industrial areas (Meetham, 1950; Georgii and Jost, 1964). Georgii and Beilke (1966) proposed that the groundlayer of the atmosphere is the important zone for SO, washout. The time taken for a droplet to fall through the polluted layer is of the order of a few minutes only (approx. 1 min for a typical raindrop). Non-vertical fall-paths may increase the residence time of the raindrops in the atmosphere, but nevertheless, the transit time in the polluted layer will be small. The fall-time of snowflakes will be considerably greater. Oxidation rates of SO2 in solution determined by other workers (Johnstone and Conghanowr, 1958; Van Den Heuvel and Mason, 1963; Scott and Hobbs, 1967) suggest that, although initial oxidation rates are relatively rapid, over the time-scale of a few minutes, oxidation may be regarded as comparatively negligible. Under certain circumstances however, in the presence of ozone, the rate may be faster (Penkett and Garland, 1974). Simple laboratory experiments, which attempted to simulate local environmental conditions, confirmed that oxidation in solution may be ignored over the first few minutes (Davies, 1974). Hales et al. (1971) neglected oxidation in the raindrops in their study of SO, washout from a power station plume. Rainout is relatively unimportant for SO*, and it seems reasonable to assume that any sulphur dioxide taken up by a cloud droplet will be oxidised during the droplet’s lifetime (Beilke and Georgii, 1968), and that quasi-equilibrium conditions will prevail. Making these assumptions, and considering the facts of the previous two paragraphs, the disulphitomercurate formed as soon as the rainfall enters the collecting bottles is seen to represent SO1 removed by precipitation in its passage through the polluted layer (washout). Lavrinenko (1968), in an examination of sulphite and sulphate ions in rainwater, concluded that sulphite determination is meaningful only immediately after the precipitation event. The small fall-velocities of snowflakes was not considered a serious disadvantage because of the relatively infrequent occurence of snow. In addition, it was thought that the lower air temperatures would inhibit oxidation; an assumption also made by Hales et al. (1971) although McKay (1971) has suggested that the temperature coefficient is negative for the oxidation of SO2 in water droplets in the presence of ammonia. Because of rapid exchange rates between air and raindrops, Hales et al. (1971), found considerable desorption as the raindrops fell through the Keystone power station plume into zones of much lower atmospheric SO2 concentrations. Atmospheric SO1 concentration gradients in the airshed of a heavily industrialised area will be much less than those encountered in the Keystone study, except perhaps directly under a power station plume. In the Keystone case, the very small transit time through the narrow plume before the droplet encountered “background” concentrations again gave little opportunity for vigorous mixing in the droplet to achieve equilibrium concentration within the droplet, and so the subsequent surface desorption was very rapid. Except for the specific case of a single, high concentration plume, significant desorption should not be encountered in a heterogeneous industrial area. Consequently SO2 washout should be closely related to the amount of SO, in the air above the precipitation sampler. Mean hourly atmospheric SO, concentrations were recorded at representative ground-level (3 m) locations by Nash-pattern recorders manufactured by Gas Chromato-

Precipitation

scavenging of sulphur dioxide

881

ing the hypothesis that the collection and analytical technique measures SOz washout.

During the period mid-September 1969 to the end of SO2 at the sampling station was 1.0 x 10m4 g crnm2 (y-l), with an overall concentration of 2.1 mg I- I. Chamberlain (1960), assuming an a~osphe~c SOz concentmtion of 20 pg rnm3, has calculated that the deposition rate of SO, in precipitation in this country is 1.1 x 10e4 g cmv2 y-l. On the evidence of the Sheffield data, the nationwide wet deposition rate might be less. Sulphur dioxide washout in the industrial area accounts for only 0.03% of emitted SO*. Even during rainfallhours* only, less than 0.3% of the emitted SO2 was removed as SO* washout (Davies, 1974) (although more than that proportion of emitted SOz will be removed ultimately by rainfall, depending on transformation rates in the atmosphere, as total sulphur). The proportions of SO, washout (as sulphate equivalent weight) as a percentage of total sulphate, for those individual rainfalls? which were analysed for sulphate, ranged from 14 to 82% with a mean of 47% (Davies, 1974).

ofAugust 1970 the wet deposition

---

limitsof hwvii main sampling 1 other stations ring solphur dioxide P poww station T thermograph station l

Fig. 2. Location of sampling sites in the industrial Don valley (Sheffield). graphy Ltd. Some precipitation samples were analysed for sulphate, but unfortunately this could not be undertaken on a regular basis (Davies, 1974). The pH value of weekly rainfall amounts was recorded because operational difficul-

ties precluded determainations on a time-basis compatible with the other samples. The usefutness of such data is in doubt, since the pH of rainwater at ground-level is a function of (amongst others) pH of the raindrops. It is difficult to resolve the meaning of pH determinations at the surface. Figure 2 shows theiocaiion of the study in the relatively wide. flat-bottomed industrialised Don valley, The iron and steel industry dominates the area, although no large buildings lie within 350 m of the precipitation sampler site. The power station is 700 m distant. There are many sources of heavy metals. Ammonia was measured under a variety of synoptic situations, but not on a regular basis, and concentrations of greater than 10 pg mW3 were unusual. Two thermograph stations sited on the valley-side at heights of 50 and 130 m gave an index of vertical atmospheric stability. The temperature difference between the two stations does not represent a true lapse rate, but it does exhibit a strong correlation with ground-level atmospheric SO2 concentrations (Behilak, 1967). The factors affecting atmospheric SO, levels are examined in section 4. 3. LOCAL REMOVAL OF SO, BY PRECIPITATION

The SO* and sulphite removed by precipitation, as measured by the West-Gaeke method in this study, will be called washout. Washout of S02, at the main site and at a number of less polluted and country sites (collected manually), correlated well with local atmospheric SO2 levels (Davies, 1974). Washout at the outlying stations increased when polluted air was imported from the heavily-polluted area. These results are as im~rtant as the earlier arguments for support*A rainfall-hour is defined as any whole hour within which precipitation was recorded for some length of time: 9% of the total number of hours in a year. t An individual rainfall is defined as any period of consecutive hours during which rainfall was recorded. The precipitation may not be continuous. The definition is adopted because the basic sampling unit was one hour.

3.2 Mean monthly SOz washout Mean monthly SO, con~ntrations in pr~ipi~tion are shown in Fig. 3. The values represent the equivalent SOz concentration in a monthly sample, and not the mean concentration of the sum of concentrations in individual rainfalls. There is a general inverse relationship between monthly washout and rainfall intensity, although there are exceptions (notably October). The curve representing SO2 deposition at the ground by precipitation is very strongly related to the total rainfall throughout the year. Such a strong relationship is not surprising in view of the relatively small range in the washout concentrations. The monthly atmosphe~c SO2 concentration curve is similar to the washout curve, although the maxima do not coincide. The monthly air temperature curve exhibits close agreement with the washout curve. Air temperature is a good indicator of atmospheric SO, concentrations, but the similarity of temperature and washout levels in October and the summer months is of interest. Sulphur dioxide solubility varies inversely with temperature and this could well partially account for the very low October washout value. February shows the lowest air temperatures and the highest washout value. The most important factor, however, may be that air temperature indicates SOz ~ncentrations in the air above the sampler more satisfactorily than non-representative ground-level determinations (Davies, 1973). A fundamental objection to attempts to isolate causal relationships is the fact that only 9% of the total number of hours in the study-period were rainfall hours. Mon~ly means are not representative of the conditions prevailing during rainfall. Many pertinent

T. D.

882

3 7_

2

P

IO-

I

f----d Sulphur dioxide washout

1

xx)E

soRaintall total

‘: E TOOg O-

&phur

dioxide deposition by rainfall

: 1.5= tog ISO-

Rainfall intensity r----Ji

3

3I.-

lp

;_

300 S 250 ZOO sl50 ml 50

Y

15 l0 so-

1

-1

J

pli value of rainfdl

Atmospheric pi

Sul@‘w dioxide concentmfi

Air temperature /xi ONDJFMAMJJA

I

month

Fig, 3. Monthly means of SQz washout and other variables.

variables exhibit diurnal cycles; even rainfall frequency unde~ent marked diurnal variations, with a mi~mum at midday and maxima in the early moming and evening. The pH value of rainwater curve is similar to the washout curve. The meaning of the pH values is uncertain (Section 2), whereas pH determination of raindrops in the atmosphere or of rainwater immediately and continuously at collection would have been of more use.

DAVIES

the Frankfurt study probabfy represent the additional sulphate aerosol contribution to precipitation sulphate, although atmospheric SO2 concentrations in Frankfurt during the period of investigation appear to be slightly higher than atmospheric SO2 concentrations during the more recent Sheffield study (Georgii, 1960). A similar feature is illustrated during Sheffield rainfalls when precipitation sulphate and SO2 concentrations were determined (Davies 1974), although correlation coefficients are very small. The sulphate curve is represented by the power law h- ‘J 2 and the SO2 curve by the power law Jz+‘.‘~. The positive gradient of the SO2 curve is due to the very small sample (19 rainfalls). For the same reason, little cot& dence can be ascribed to the power laws. For expedience, the average washout values were computed for various classes of rainfall amount (Fig. 5). The whole-period observations illustrate the general decrease in trace gas content with increasing quantity of rainfall. However, the observations can be more clearly seen not to obey a power law than they can in Fig. 4. The decrease in washout concentration is greater when the quantity of rain is small, a finding similar to that of Georgii and Weber (1960) in Frankfurt. The point of gradient charge depends to some extent on the rainfall amount classes adopted, but the zone of gradient charge appears to be at a lower rainfall amount value than in the Frankfurt study. Georgii and Weber (1960) suggested that the gradient change for Frankfurt represented the initial removal of locally-emitted trace-substance. This does not appear to be the reason in Sheffield if the postulation of only washout being measured is accepted. An alternative reason is that initially, the accumulated SO2 in the urban airshed is available for removal, and then ~ontem~raneousIy-emitted gas is sub~uentIy removed. The difference in gradientchange position may also be ascribed to the greater

65

3.3 Individual rainfalls and SO2 removal The relationship between SO, washout and individual rainfall amount is illustrated in Fig. 4. The range of washout concentration values decreases with increasing rainfalf amount, a feature which agrees with the findings of Georgii and We&r (1960). The curve represents c = const. h-O.““; other workers have found the power law h -o.3 shaws good agreement with the removal of various atmospheric constituents, including sulphate in Frankfurt (Georgii and Weber, 1960). The curve in Fig. 4 and the curve of the Frankfurt rainfalls converge towards higher precipitation amounts. The convergence suggests that sulphate aerosol is being removed in the first stages of the precipitation event. The higher c (sulphate) values in

Fig. 4. Washout concentrations in individuaf rainfalls.

Precipitation

scavenging of sulphur dioxide

883

October -August

l

A October-March 0 April - August D snowfatl only

,2".35 .u 09

v2 111 %

010

on

romfol,cIo**es CJhptmi-mm

012

coo2 0021-0.06 O-07 -0.n 0.M - 0 39 OLO -060 0.61 - 1.70 1.11 - 6 50 l 6.51

p.sdum cm Gtack*a

11

0.01

.2

tv$mdnat* I

I

DOI

x3

x6

0.1

*2

1

x3

.6

1.0

repiesent*upper x2

limit o‘clclor I

III

-3

"6

,o

I

III

.2

13

100

.6

amount in individual rainfall - mm

Fig. 5. Washout concentrations by rainfall amount class.

amount of trace-substance (SO2 plus sulphate aerosol) to be removed in the Frankfurt airshed, whereas SO2 only removal is monitored in Sheffield. The two studies are of course not strictly comparable; one of the reasons being the adopted definition of individual rainfall in this paper. Figure 6 shows the relationship between intensity* of precipitation and washout of SO,. The whole-period observation pattern is a reflection, in part, of the short-duration, small-quantity falls which have yielded very high washout concentrations. But it also reflects the fact that continuous, less intensive rains are more efficient scavengers than short-duration high intensity showers. The winter months’ observations exhibit a similar pattern to the whole-period observations, except that the washout concentrations are slightly higher. The summer observation pattern is quite different; a very weak positive relationship between washout and rainfall quantity is detectable (Figs. 5 and 6). Part of the explanation may lie with the relatively small number of summer observations, but other workers have found inverse washout-rainfall quantity relationships in areas with low environmental SO2 concentrations. From the data of Georgii and Weber (1960) it is apparent that, on occasions, much lower trace-substance concentrations occur in light rainfall than with heavy rainfall; this is evident in Frankfurt but is more obvious in the mountains outside the city where atmospheric concentrations are lower. In the rainfall hour class of GO.05 mm h-‘, in the 18 observations during April-August there were ten occurrences of zero washout.

One of the reasons for the low washout values with low summer rainfall intensities could be the fewer and larger drops in summer light rainfall (e.g. tails of local convective showers) compared with winter cyclonic drizzle. The efficiency of removal would then be much impaired, although washout in high-intensity summer rainfall is not depressed so noticeably. Another plausible reason is enhanced oxidation rates in the falling raindrops in summer which will apparently depress SO2 washout, but will increase the sulphate content of rainfall at ground-level. Lavrinenko (1968) found that sulphite ion concentration in rainwater fell off sharply with time in summer because of the increased oxidation rate with higher temperatures. Oxidation is greater in smaller droplets

0

*Because of the use of the discrete unit of 1 h for sampling, the intensity referred to here will, on occasions, be smaller than the actual intensity of the rainfall.

i

.32

016 ' QO.06

0.06- 0.S

0.17-04‘

intensity of indiitid

Fig. 6. Washout concentrations

0 Es-O.63

LO.64

rainfalls - mm h -1

by rainfall intensity class.

884

T. D. DAVIES

because of the large surface area: volume ratio (Johnstone and Conghanowr, 1958). Ammonia concentrations might be greater in the summer {although irregular monitoring reveaIed few occasions with concentrations of greater than 10 lug m- ‘). Heavy metals might be suspended for a longer time in the more turbulent summer atmosphere. Higher pH values of summer rainfall would enhance oxidation rates. Oxidation has the chance to progress further in light rainfall because of the slow fall-velocity of the drops. Ozone has an important role in the oxidation of sulphite in solution (Penkett and Garland, 1974) and the rate may be faster than other mechanisms. Observed ozone concentrations in the U.K. are sufficient to suggest that this might be a realistic m~han~sm (Atkins et at., 1972). However, the full answer to the question of whether the low SO2 washout in light summer rainfall is a function of the method of measurement, or whether it reflects a real process can be determined only by the simultaneous regular measurement of sufphate in precipitation, and other important parameters, such as ozone. 3.4 SO* remova! by snow Between 17 November and 9 April 27 individual ~owfa~ls occurred. The average amount of precipitation per fall was i.7 mm. The average SO2 washout concentration was 2.21 mg l-l, with a range from 0.0 to 5.67. During the same period 101 other individual rainfall events occurred. The average amount of precipitation in these falls was 1.3 mm. The average SOZ washout concentration was 5.99 mg I- I, with a range from 0.0 to 139.0. However, relatively few snowfaIls were of very small quantities, and so the results are not directly comparable. A more vafid comparison is found in Figs. 4 and 5, Snowfall appears to be a marginally more efficient scavenger of SOZ than rainfall, but there is no evidence to suggest an increase of 35% (for sulphate) found by Georgii and Weber (1960). Snowflakes have a slow fallvelocity and large surface area which should increase washout efficiency, but the first factor could lead to more advanced oxidation of sulphite in solution. Hales et at. (1971) indicated that snow removed less SO*, at least when the temperature is below 0°C because snow is dry and crystalline below 0°C and wet and amo~hous above freezing point. No conciusions in this respect can be made from the Sheffield study, since the air temperature (ground-level) in half the cases of snowfall was less than o”C, and above 0°C for the other half. The average atmospheric SOZ concentration during snowfall hours was 144 pg mV3 (112 h), and 116 pg me3 during rainfall only hours (489 h). On this basis alone, the washout conce‘ntrations in snowfall might be expected to be higher. Results obtained by Lindberg in Oslo (personal communication) show the sulphate content of snow to be less than that of rain.

3.5 Wushuoutofatmospheric SO2 on an hourly basis Some of the difficulties inherent in using monthly averages discussed in Section 3.2 can be overcome by using averages computed from observations taken only when precipitation occurs. These means are shown in Fig. 7. May and June observations are combined because of the small number of rainfall hours in the months. Mean washout concentration attains a marked maximum in December and minima in October and the summer months. The December value is strongly influen~d by a few ~x~eptionaIly high values. There is not an obvious relationship between mean washout concentrations and mean hourly rainfall. The correlation between mean atmospheric SO, concentration during rainfall hours and mean washout appears reasonable. Mean hourly SO2 concentrations during rainfall hours are lower than the mean monthly SO2 concentrations (Fig. 3) although the curves are similar. The depressed values might be partially explained by precipitation scavenging. Mean air temperature during rainfall hours is almost a mirror image of mean monthly SO2 washout (Fig. 3), including the reversals of gradient between July and August. On an hourly time basis, air temperature seems to be a good indicator of mean SOZ

Fig. 7. Hourly means of SO2 washout and other variabIes during rainfall only.

Precipitation scavenging of sulphur dioxide washout. There are probably two main reasons: temperature is a good indicator of SO, emission and SO2 solubility is inversely related to temperature. Wind direction frequencies during rainfall hours vary considerably from month to month. However, a separate analysis by wind direction class failed to identify many meaningful relationships, except the obvious dependance of atmospheric SOz on wind direction. Calm conditions were also shown to exhibit low washout values in spite of low rainfall intensity and high atmospheric SO2 concentrations. Contributory reasons may be little air movement (strong air flow may enhance washout by effectively increasing mixing) and high relative humidity. October, which showed very low washout values, has a far greater proportion of calm conditions (26%) than any other month (next highest is January with about 7%). 3.6 Statistical analysis of hourly observations The multivariate nature of the atmospheric processes means that the recognitiion of causal relationships is fraught with difficulties. The use of mean values is of limited relevance in this case. An analysis which examines rather more than the gross characteristics of the collected series of data is needed. Partial correlation analysis allows the examination of causal pathways since it measures the correlation between the dependent variable and each of the several independent variables while eliminating any (linear) tendency of the remaining independent variables to obscure the relation (Ezekiel and Fox, 1959). Coefficients of partial correlation measure the importance of each of the several variables by determining how much it reduces the residual variance after all other specified variables except it are taken into account. A significant correlation may not be exhibited unless allowance has been made for any relation between the dependent variable and other independent variables. The recorded variables are listed below: 1. Precipitation amount 2. SO2 washout concentration (mg 1-i) 3. Ground-level atmospheric SO2 concentration at the main observation station. 4. Mean of the ground-level atmospheric SO2 concentrations at four sites in the industrial valley (Fig. 2). Originally, the mean concentration of a number of stations was thought to provide a more “representative” SO, value for the Don valley. However, it became apparent that the sites suffered large SO, concentration fluctuations because of purely local factors. The multivariate analysis results confirmed that washout was more closely related to SO, concentration at the same site than to a mean value from four sites.

*If there is a 1% or less probability that the correlation coefficient could have occurred by chance, the coefficient is statistically significant at the 1% level.

885

5. Wind speed 6. Wind direction 7. Air temperature 8. Relative humidity 9. Lapse-rate index. 10. Atmospheric pressure. The variables listed above represented some of the most important factors influencing washout of atmospheric S02. Some of the variables measure similar (but not exactly the same) effects: wind speed observations at one height do not strictly describe vertical stability. Atmospheric pressure was recorded as a rough index of weather type. It was not expected that pressure would be correlated with washout within individual rainfalls, but that a relationship might become apparent between rainfalls. Other parameters are better indicators of weather type, but atmospheric pressure is a type of quantitative assessment. It is likely that other variables besides the recorded ones have stronger effects on SO1 washout, or apparent washout (e.g. atmospheric heavy metal concentration). Unfortunately, the regular monitoring of other variables was impracticable. The employment of linear correlation methods requires the variables to be distributed normally. Precipitation amount, SO* washout concentration and atmospheric SO2 concentration are closely log-normally distributed. The remaining variables, except for wind direction, are closely normally distributed. Combinations of the variables are closely bivariate normally distributed. Because of the polar (assymetrical) distribution of wind direction, the variable could not be included in a straightforward multivariate analysis. It should be remembered that any further mention of variables 1, 2 and 3 (above) in this section is a reference to the log-transform. Table 1 shows the correlation statistics with washout (log) as the dependent variable. Only significant* partial correlation coefficients have been included. The analysis was conducted for the whole period (October-August) and also for monthly subsets in order to examine any changing nature of the relationships throughout the year. In addition, the division into monthly sub-sets considerably reduced the degree of serial correlation in the discontinuous time series (tested with the Durbin-Watson statistic, Yamane, 1967). Allowances for serial correlation cannot be made because a meaningful correlation will not exist between the individual time series which comprise the total discontinuous series. The reduction in serial correlation because of the data subdivision therefore greatly enhances the validity of the results. All variables were forced into the analysis so that any overall change in the nature of the relationships could be assessed more easily. The inclusion of nonsignificant variables caused very small changes in the statistics. September was excluded from the analysis because of the small number of observations and the general unreliability in that month of the meteorological data.

T. D. DAVIES

886

Table 1. Partial correlation coefficients with flog) washout concentration Rainfall

amount

(1%)

Oct.

*

Atmospheric

SO,

cw 0.3*

-0 32,‘. _*54.** *

Relative humidity

Temperature

Lapse-rate index

-.Q&!z 0.24”

0.27* 0.23” 0.39** 0.*1***

Aw O&-AU&

_a.% -0.15’”

0.21***

Pmssure

0.29. _a 32’“’

-0.28’**

NOV. Dec. San. Feb. MU. Apr. May & June July

0.21** 03****

0.4P”

Windspeed

as dependent variable

-0.21“ 0.34’8

0.11***

0.27” 0.23**

0.34

_

*$5***

0.27”

0.06*

*** = Significant at 1% Level; ** = Significant at 5% Level; * = Significant at 10% Level; Underlining highest partial correlation coefficient.

The obvious complexity of the relationships between washout and the other variables makes any brief summary difficult. In many cases it is possible to suggest physically sensible explanations, but often they are supported by very low levels of statistical explanation. Over the October-August period temperature is seen to exhibit the strongest relationship with washout. The partial correlation coefficient (r) is highly sign&ant aithough it explains only 6.8% of the variation in washout left unexpiained by the other variables. The I value for washout and atmospheric SO2 concentration is highly significant although atmospheric SO, explains only 4.4% of the variation left unexplained by the other variables. The regression coefficient is 0.28 whereas Georgii and Beilke (1966) found a value of 0.7 for sulphate concentration in droplets and SOz from laboratory experiments. Figure 8 shows the relationship determined by Georgii and BeiIke (1966) for a rainfall intensity of 1.0 mm h-1, which is near the mean value of rainfall intensities in this study. Also shown is the Sheffield curve, and the curves are seen to diverge with increasing values. With higher concentrations of atmospheric SOz, more sulphate will be available for washout which will depress the pH value of rainfall which will, in turn, reduce washout of SOz because of the strong dependence of solubility on PH. Over the whole period (October-August) all the recorded variables, excluding wind direction and excepting relative humidity, exhibit a statistically significant relationship wi&h washout (but at varying levels). Temperature, atmospheric SO2 concentrations and rainfall amount {in that order) are the most important. Some of the problems of a study of this nature are ilfustrated by the significant relationships which arise out of the subset analysis. The changing nature of the relationship between washout and rainfall amount throughout the year has already been examined, and the data in Table 1 illustrate the relationship. A strong inverse relationship is apparent in the early winter months. In December, precipitation amount explains 29% of the unexplained variation in washout. The relationship in February and March, although not statistically sign&ant, is positive. The

0.28” 0.32”*

-0.13”’

denotes

remaining months have significant (varying levels) and values which are all positive. It is unfortunate that more data are not available to see whether this apparent cyclical pattern is maintained. Atmospheric SO, concentration has an important effect on washout (this relationship is rather complex and will be dealt with further in Section 4). The relationship is stronger in summer, so that, for some reason, lower concentrations of, atmospheric SOz are more closely related to washout. Relative humidity is seen to exhibit significant relationships with washout in some months, although over the period as a whole this relationship is not significant. This is an example of the annual pattern over-riding the shorterterm variations. In an effort to unravel some of the complications of a determination of causality, the data were allocated into wind direction classes and subjected to the same multivariate analysis as the monthly data. It was hoped to establish whether or not the nature of the relationship between washout and the other variables varied in some systematic way with wind direction. However the analysis did not reveal any identifiable pattern which would help to determine any effects of changing wind direction. The effects of wind direction are possibly represented by an analysis of the other variables which are themselves partially functions of wind direction. The design of the project

Fig. 8. Sulphate removal by artificial rainfall in a laboratory and SO2 remova in Sheffield against atmospheric SO2 concentration.

Precipitation scavenging of sulphur dioxide possibly precludes any meaningful study of the effect of wind direction since it is postulated that local processes are under examination. It would then seem unlikely that exotics will affect washout; except perhaps for heavy metals or other catalysts which were not monitored, but such an effect is not apparent from the results. However, some of the apparent relationships between washout and atmospheric pressure, relative humi~ty, wind direction prompted a brief study of weather types. 3.7 Washout and weather type Lamb’s classification of weather types (Lamb, 1950) was adopted. Individual rainfalls were then grouped according to the weather type prevailing, and the mean washout concentrations and rainfall intensities computed. The results are shown in Table 2. All weather types with five or less individual rainfalls were omitted. On occasions it was impossible to specifically classify the weather, so combined classibcations were used. The results obtained are a function of the distribution of rainfall intensities. The cyclonic weather type has a greater proportion of light rainfalls occur which lead to the high intensity mean value. The high washout concentrations associated with low rainfall in winter, and the low concentrations in summer light rainfall explain and washout values in the cyclonic class. The problem of causality is again apparent. In the absence of evidence to the contrary, it is reasonable to assume that local washout may be related rather more to local parameters than to a broad generalization of the parameters into a weather type.

887

than the gradient of similar CUN~S constructued by other workers for sulphate washout. It is suggested that the difference reflects the initial removal of sulphate aerosol, or sulphuric acid droplets, in the early stages of the precipitation event. The curve does not strictly obey a power-law; the gradient-change may be due to the initial removal of accumulated SOz in the urban airshed. Washout concentrations in light summer rainfalls are very low, so much so that a positive relationship exists between summer washout and rainfall intensity. It is suggested that this could be due to different summer precipitation characteristics, or to increased summer oxidation rates which would apparently reduce the SO, washout. Regular sulphate determinations would go some way towards providing the answer. It would appear that snowfall does not increase the washout rate of S02, although this may be an apparent result because of the specific nature of the analysis. The shortcomings of using monthly mean data soon become apparent, although some broad relationships are obvious. The nature of the discontinuous time series data raised serious problems for detailed analysis. However, the division of the data into subsets reduced the effects of serial correlation, and enabled any change in the nature of the relationships throughout the year to be examined. The multivariate analysis confirmed the complexity of processes occurring in the atmosphere; the levels of explanation produced were very low.

4. EFFECT 3.8 Conclusions

OF PRECIPITA~ON

ON ATMOSPHERIC

The sampling methodology enabled estimates to be made of SO2 washout, and of the post-collection impingement of atmospheric SO2 onto the rainwater. The washout values could then be related to local environmen~l parameters on a short time-scale. In absolute terms, precipitation removes only a very small fraction of the emitted SOz in the industrial Don valley as SO, washout. Even during the hours of rainfall only, washout accounts for only a very small proportion of atmospheric SO,. When individua1 rainfalls are examined, the gradient of the washout-rainfall amount curve is smaller

SO, CONCENTRATION

Only the SO2 data from the recorder at the precipitation sampling station are used. 4.1 Apparent e&c% of i~iuid~~

ruinful~s on atmm-

pheric SO2 concentrution

During the observation period (15 September 1969-31 August 1970) 206 individual rainfalls were recorded. Local ground-level (3 m) hourly mean atmospheric SO* concentrations before each rainfall were compared with the post-rainfall con~entmtions. For this purpose the atmospheric concentrations in

Table 2. SO2 washout and precipitation amount in individual rainfalls by weather type MCin Numberof r*infafls Oct.-Mar.

Apr.-Aug.

Cychic Westerly Cyclonic/westerly North-Westerly Northerly Cyclonic/north cycionic Westerly Cyclon~/n#~heriy

westerly

36 31 14 11 10 8 31 13 8

Mean washout k-l@. I- ‘) 10.5 3.5 4.4 7.0 1.8 3.4 0.9 1.8 0.7

precipitation am*un* (mm) 2.6 1.0 1.4 1.4 1.9 1.7 3.5 1.1 0.5

888

T. D. DAVIES

the last hour before rainfall and in the last hour of the rainfall were used. Concentrations in the hour after the rainfall were not used because quite large increases in atmospheric SO1 concentrations occurred in many cases in the hour following cessation of precipitation. As it is, the use of the discrete unit of one hour rather than a continuous time-scale poses many problems. The number of cases when the atmospheric SO* concentration in the hour before rainfall (x) exceeded the concentration in the last hour of rainfall (y) was 111,or 54% of the total number of cases. In 60 cases y exceeded x (29% of total) and in 35 cases x equalled y (17%). Atmospheric SO2 concentrations in the last hour of rainfall (y) were also compared with concentrations one hour later (z). In 52 cases (25%) y exceeded z, in 100 cases (49%) z exceeded y, and in 54 cases (26%) y equalled z. It is evident that, with a majority of the rainfalls, atmospheric SO2 concentrations were greater before the rain than after it. An increase occurred in only 29% of the cases. Similarly, the termination of rainfall was associated with an atmospheric SOz increase in half of all the cases, whereas a decrease occurred in only 250/, of the cases. Taking into account the highly variable nature of SO2 emission in the area, the results suggest that rainfall, or associated factors, exerted a depressing effect on atmospheric SO2 concentrations. The mean of the 206 x observations (Z) was 140 pg me3, the y mean 6) was 122 pg rne3, and the z mean (2) was 130 pg rnm3. A student’s test between the x-and y-values yields a t value of 1.28. The t value between y and z is 0.9. These t values are not statistically significant. The value of y represents a 13% decrease on X, whilst Z represents a 7% increase on 7. The 3 - x decrease is not so large as that found by Georgii (1960) possibly because he examined SO* concentrations immediately before and after rainfall, and not on an hourly mean basis. In this case, the magnitude of the decrease is reduced by a small number of very large increases which are probably due to local emission characteristics. However, some workers have found that rainfall can induce large influxes of SO2 (Georgii, 1963). If rainfall does have a measurable depressive effect on atmospheric SO2 concentrations on an hourly basis, then this effect might be expected to be a function of rainfall intensity. The individual rainfalls were grouped into intensity classes and the jc-, y- and ?-values found for each class. The magnitude of the apparent effect of rainfall on atmospheric SO, concentrations appeared to bear no noticeable relationship to intensity of rainfall, although there was a slight tendency for the percentage of cases where x > y to increase with intensity of rainfall. However, a x2 test on x > y, x < y, and x = y frequencies indicates no relationship with rainfall intensity. Similarly the test shows no relationship between y > z, y < z and y = z frequencies and rainfall intensity.

These results do go some way towards confirming expectation, but they are sufficiently inconclusive to suggest that the role precipitation plays in the fluctuations of atmospheric SO2 concentrations can only be examined by establishing removal rates by direct means and relating these and other relevant factors to atmospheric concentrations. For this reason, the multivariate technique which was described in Section 3.6 was adopted, this time with (log) atmospheric SO* concentrations as the dependent variable. It is even more obvious in this case that the identification of a statistically significant correlation does not imply causality. However, all the variables were included in the analysis for similar reasons to those given in Section 3. The results of the analysis are shown in Table 3. Care needs to be exercised when considering the physical meaning of the atmospheric SOz-washout relationships. High washout values cannot be a cause of high atmospheric SO2 concentrations, but only a result. They were treated as such in Section 3. Nevertheless, washout does contribute to the removal of SO*. With controlled laboratory experimentation washout is proportional to the amount of SO2 remaining in the chamber. A time lag was introduced into the analysis in an attempt to relate washout at time (t) with atmospheric SO1 at time (t + 1). However, the computed relationships were very weak because of the large, variable and mostly unknown emission of SOr in the area. Washout was retained as an ‘independent’ variable but only because a separate analysis showed that the relationships between atmospheric SO* concentrations and the other variables differed only slightly with the exclusion of washout from the regression set. The strength of the relationship may give some indication of the effect of washout on atmospheric SO2 concentrations since it obviously does contribute to removal of the gas. Even if this is questionable, the other results of Table 3 are valid because of the small change caused by the inclusion of washout. 4.2 Conclusions The relationships computed from the multivariate analysis are complex and, in some cases, very difficult to interpret. However, it is possible to arrive at some general conclusions. Throughout the whole period of study, even during rainfall hours only, wind speed appears to have a marginally greater effect on atmospheric SO2 than does any other variable. The effect of washout is difficult to assess because of the problems of causality. The relatively strong relationship between atmospheric SO2 concentration and wind speed found for the whole period is not evident in the individual months. The correlation is strong in August, which appears to strongly influence the whole period relationships. The atmospheric SOz-washout significant r values are more numerous than any other r values throughout the individual months.

Precipitation

scavenging of sulphur dioxide

889

Table 3. Partial correlation coefficients with (log) atmospheric SO2 concentration Washout (log)

Oct. NOV. Dec. IaIl Feb. Mar. Apr. May & June July A@. Ott -Aug.

Rainfall

amount

(log)

u

Windspeed

Relative humidtty

-0.28’

Temperature u 0.35***

-0.21** 0.21”

as dependent variable

0.21”’

0.16

0.25”

0.6” -0.17’

-0.25”’ 0.3s*** 0.27’ 0.23.’ 0.39.’ 0.51***

Lapse-rate index

-0.17’

0.24*** -0.28” -0.36”’

-0.3** 0.2’

- 0.25’ -0.15***

-0.39*** 0.21” 0.48***

-0.58’** -0.67”’ -0.23”’

- 0.07’

-0.13***

*** Significant at 1% Level; ** Significant at 5% Level; * Significant at 10% Level; Underlining partial correlation coefficient.

In the monthly sub-set analysis, air temperature has a relatively large number of significant, and relatively strong, correlations with atmospheric SO*. All the monthly correlations are positive, whereas the whole period relationship is a weak negative one. If, for some reason, the atmospheric SO,-washout correlation is not fully accounted for in the analysis, the atmospheric SO, -temperature relationships would be positive during rainfall because of the inverse relationship between solubility and temperature. Part of the explanation may also lie with the colder northerly component winds which are associated with lower atmospheric SO, concentrations. Overall, washout and rainfall show the greater number of significant correlations with atmospheric SOZ, although other variables also exhibit significant relationships, particularly wind speed, temperature and relative humidity. Surprisingly, some of the significant correlations between atmospheric SO2 and relative humidity are positive. In these cases, relative humidity may be an indicator of more widespread environmental conditions affecting pollutant levels. Wind direction has an obvious effect on atmospheric SO2 concentrations, but the influence is difficult to quantify because of the nature of the variable. Precipitation does appear to have a depressant effect on atmospheric SO, concentrations, but the true nature of the relationship is difficult to determine because of the character of the industrial area and the time-unit of sampling. In many cases, however, precipitation is not the most important factor affecting the concentration during the precipitation event. 5. GENERAL CONCLUSIONS AND IMPLICATIONS

The nature of the determined washout relationships changes with respect to the time interal examined. Annual variations are superimposed on shorter-term variations (e.g. temperature exhibits the strongest relationship with washout throughout the whole period of study, and yet the relationship is significant only in one monthly sub-set, and then it is of opposite sign). Generally, the variation in washout explained by the recorded variables is quite small. The situation is obviously very complex. A shortcoming is the utiliAX.lo/lo--o

denotes highest

sation of ground-level observations at one point to explain a three-dimensional process. The measurement of other suspected important variables, such as ozone, would also be advantageous. More information on the chemical structure of the atmosphere is needed. However, some pertinent conclusions can be made about the nature of the removal of SO, by washout. The study has attempted to relate SO, washout to local parameters in an effort to gain insight into the results of the washout process at ground-level. Simultaneous monitoring of the separate phases of sulphur in rainfall and the atmosphere would provide valuable further information. The development of a sampling network would provide more knowledge about transportation rates and the life-cycle of sulphur in the atmosphere. The data from the country sites in the Sheffield study, and from the work of Hales et al. (1971) suggest that the methodology might be employed usefully in less-polluted areas. By relating such measurements to meteorological parameters, transportation and deposition rates may be investigated. Much could be learned of the oxidation rates of SO, in the free atmosphere. This information could help clarify the sometimes circumstantial evidence used in discussions of long distance transport and deposition rates of sulphur.

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T. D. DAVES

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Lavrinenko R. F. (1968) The content of sufphur in atmospheric precipitations. Tardy glav. geofiz. Ohs. 207, 87-91. Lindberg W. (1969). Personal communication, Oslo. McKay H. A. C. (1971) The atmospheric oxidation of sulphur dioxide in water droplets in the presence of ammonia. Atmospheric Environment S, 7-14. Meetham A. R. (1950) Natural removal of pollution from the atmosphere. Q. J. R. met. Sot. 76, 359-371. Mrose H. (1566) Measurements of pH, and chemical analyses of rain. snow. and fog-water. Tellus 18. 264-270. Newali H. E: and E&es A.71962) The effect of wind speed and rainfall on the concentration of sulphur dioxide in the atmosphere, Int. J. Air Mt. Pollat. 6., 173-177. Oddie B. C. V. (1962) The chemicai com~sition of precipitation at cioud levels. Q. J. R. met. Sac. 88, 535-538. O.E.C.D. (1964) Measurement of sulphur dioxide, Paris. Penkett S. A. and Garland J. A. (1974) Oxidation of suiphur dioxide in artificiaf fogs by ozone. Tellus 26, 284-289. Rodhe H. (1970) On the residence time of anthropogenic sulphur in the atmosphere. Tellus 22, 137-139. Rodhe H. (1972) A study of the sulphur budget for the atmosphere over Northern Europe. Tellus 24, 128-138. Rodhe I-I., Persson C. and Akesson 0. (1972) An investigation into regional transport of soot and aerosols. Atmospheric Environment 6, 675-693. ScaringeIIi F. P., Elters L., Norris D. and Hochheiser S. (1970) Enhanced stability of sulphur dioxide in solution. ifnat~r. Chem. 42, 1818~1820. _ Scott %. D. and Hobbs P. V. (1967) Formation of s&hate in water droplets. J. Atmos. Sci.‘24, 54-57. * Van den Heuvel A. P. and Mason B. J. (1963) The formation of ammonium sulphate in water droplets exposed to gaseous sulphur dioxide and ammonia. Q. J. R. met. Sot. 89. 217-275. West P. W. and Gaeke G. C. (1956). Fixation of sulphur dioxide as disulphiomercurate (II) and subsequent colorimetric estimation. Analyt. Chem. 28, 1816-1819. Yamane T. (1967) Statistics: an Introductory Analysis. Harper & Row, New York.