Rain scavenging from tall stack plumes: a new experimental approach

Armospheric Enmronmenr Vol. 16. No. Printed

in Great

1I, pp.2709-2714.

1982

03%6981/82/l 0

Britain.

RAIN

SCAVENGING FROM TALL STACK PLUMES: EXPERIMENTAL APPROACH

1270’06

1982 Pergamon

$03.00/O Press Ltd.

A NEW

MILLAN M. MILLAN* Barringer Research Ltd., 304 Carlingview Drive, Rexdale, Ontario, Canada M9W 5G2

SYDNEY C. BARTON and N. DOUGLAS JOHNSON Ontario Research Foundation Sheridan Park Research Community Mississauga, Ontario, Canada L5K lB3 BORIS WEISMAN The MEP Company, 850 Magnetic Drive Downsview, Ontario, Canada M3J 2C4 MARIS LUSIS, WALTER CFAN and ROBERT VET? Ontario Ministry of the Environment Air Resources Branch, 880 Bay Street Toronto, Ontario, Canada (First received 15 October 1981 and infinal form

8 February 1982)

Abstract-A method which can be used to locate a point source plume during a period of rain, and collect water precipitated through it for detailed chemical analysis has been developed and tested. A correlation spectrometer remote sensor is used to locate the SO2 plume and to define the boundaries and lateral motion of the plume during the entire rain sampling period. Precipitation samplers are positioned at selected locations under and outside of the plume, thus allowing a comparison to be made of rainwater which has been influenced by the plume with “background” rain. Studies conducted on a smelter plume in the Sudbury, Ontario, area show well-defined profiles of acidic constituents which correlate well with the overhead plume position as defined by the SO1 burden. Sulphate levels in rainwater collected directly under the plume were higher than background levels by approximately 60 %, whereas nitrate levels were essentially identical to background levels, consistent with the known characteristics of this source. Trace metal levels were elevated by factors of 5 to 20 under the plume.

1.

INTRODUCHON

The eventual fate of sulphur emitted into the atmosphere is of widespread concern because of the ecological damage caused by acidic precipitation (Beamish and Harvey, 1972; Likens, 1976; Oden, 1976; Hutchinson and Havas, 1980) and the need exists to quantify the relative contribution to wet deposition of acids and other substances made by major point sources both locally and in potentially affected downwind areas. Past plume washout studies, in some instances, have shown considerable differences regarding the estimated plume washout efficiency. Minor contributions of sulphur from plumes of power plants and smelters have been reported to occur within 50 km from the source (Granat and Rodhe, 1973; Hutcheson and Hall, 1974; Wiebe and Whelpdale, 1974; Larsen et al., 1975; Maul, 1978; Chan et al., 1982), while other workers estimated that a major part of the emitted sulphur was deposited by washout within SO-120 km from the source (Enger and Hogstrom, 1979). Lack of definition of the plume, in both space and time, must be ranked as one of the major problems which has plagued previous plume washout studies, Current addresses: *Laboratorios L. J. Torrontegui, P. 0. Box 1234, Olaveaga Bilbao 13, Spain; tconcord Scientific Corporation, 2 Tippett Road, Downsview, Ontario, Canada M3H 2V2.

and may have contributed to the reported differences in washout coefficients. Two principal causes of uncertainty as to the actual significance of collected rain samples are: (a) Lack of spatial resolution. Often tall stack plumes are mixed with clouds during precipitation events, and are not visible from the ground. In this situation the definition of plume boundaries during precipitation, and plume position relative to the samplers, is not well known. Wind direction shear, for example, can account for large displacement between surface and upper level winds. Thus, estimates of plume location based upon surface wind observations can be seriously in error. (b) Lack of timeresolution. This occurs because of plume meandering and its overall change of direction during the frontal passage. The concentrations in water collected by a sampler while under the plume can be considerably diluted by rain collected after the plume has moved aside. The possibility of locating the position and lateral extent of an SOP plume during a rain period with a remote sensor was intially advanced and subsequently tested by one of the authors during earlier plume studies in Sudbury, Ontario, by Environment Canada. The plume from the INCO smelter was successfully

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tracked in rain on several occasions proximately 50km from the 381 m stack. 2. EXPERIMENTAL

up

M. MILLANet

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METHOD

The experimental method is based on the use of remote sensingofS0, to locate theplumeinrain,inconjunction with conventional and we&tested rain coltection and analyticaI methods. Field staff and equipment were deployed in respdnse to an advisory forecast provided by a 24-h weather office. Deployment to the study area was made on the basis of expected frontal characteristics. Each field campaign included remote sensing of SO2 overburden, ground levei SOL determinations and precipitation collection along two predefined arcs located approximately 10 and 2.5km from the source. Steady-state cyclonic storms which are anticipated to last for several hours or more are required to ensure successful sample caliectiction. Plume

location

Two COSPEC remote SO2 sensors (Millan and Hoff, 1978) were utilized to locate the overhead position and horizontal extent of the plume before and during precipitation events by performing consecutive traverses along the arc(s). Along with the remote sensor signal, ground level SOz concentrations were measured with a SIGN-X monitor that was equipped in each vehicle to indicate plume impingement. This constant awareness of the plume position was used to determine the mcsi appropriate kxation of precipitation collectors along the predefined arcs and subsequent evaluation of the plume proSles permitted the positive identification of underplume and background rainwater samples for analysis. During each traverse, the coded number of each rainwater collector and time was noted on the recorder in order to correlate the plume position with the collector.

al.

Precipitation

collection

The spacings of collectors along the inner and outer arcs were 0.5 and 1 km, respectively. At the outset of the study, each potential sampling point was selected in accordance with establish& site selection criteria (Vet and Chan, 1980) and designated with coded marker stakes near the road and at the actual position in the field. Linear polyethylene funnel/bottle rainwater collectors and stands were assembled prior to field trips and the configuration is shown in Fig. I. AlI containers were rigorousiy cleaned before use by scouring with a brush and water, soaked in distilled water and extensively rinsed with deionized water. The bottles and funnels were stored in individual plastic bags which were aiso used to cover the coifector until just before the onset of precipitation. Sampling For dissolved SO, in rainwater was done using duplicate collectors at every third site which contained sodium tetrachioromercurate (TCM) as a preservative. The method used was based upon the work of_Hafes and Dana (1979) and Davies (f9?4), but further evahiation and refinement was required.“Scavenger” blank collectors identical in configuration to actual samples, but fitted with an inverted “u” tube to prevent rainwater collection and allow only gaseous SO2 diffusion also were installed alongside sampIe collectors in order to compensate for the potential uptake of atmospheric SO, by the fixing reagent. Meteorological

forecasts

The forecast program consisted of daily 36 to 48 h forecasts of pre&~p~tationand wind direction in the Sudbury ares. Two to five-day outlooks also were provided. Specific limitations on candidate rainfall were: (i) the necessity for precipitation to occur during daylight hours (to permit adequate light for remote sensing), (ii) the requirement for appropriate wind directions (as defined by existing roadway access) in conjunction with persistent

-RING

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Fig. I. Schematic diagram of field rain collectors, showing double collector arrangement used to collect and stabilize dissolved SO, in TCM solution (not to scale).

Rain scavenging from tall stack plumes: a new experimental approach rainfall and (iii) the necessity of limiting responses to those cases involving continuous precipitation rather than showers (convective storms) to ensure adequate sample volume. Actual meteorological measurements were obtained from the Sudbury Airport tower to assist in data interpretation. The quantity and rate of precipitation was determined in the field with a tipping-bucket rain gauge located on one arc. A standard rain gauge and the rainwater collectors also were used to define the spatial resolution of rainfall along the arcs. Field deployment

Upon receiving confirmation to proceed with the study, a vehicle fitted with COSPEC and SIGN-X instruments was deployed a short time before the predicted onset of the event to continously monitor the location and behaviour of the plume at each sampling arc. After receiving information as to the plume location by radio contact, other team members distributed covered rain collectors along each arc. Collectors were also placed well beyond the observed plume boundaries and additional collectors below the plume centreline. Dissolved SOI collectors were positioned at every third site. Upon completing the installation, the team returned to a standby position at the plume centreline on their respective arcs and awaited further instructions for uncovering of the collectors.

At the onset or during precipitation, personnel began to open the collectors with each team working outwards from their standby position at the plume centreline. In general, an effort was made to commence opening just after the onset of rain in order to avoid potential entrainment of wind-blown dust. The time of opening the collector was recorded. After uncovering all collectors on their arc, the teams then returned to the plume centreline for further contact with COSPEC units. Termination of sampling was determined in one of two ways: (i) the extent of plume directional change as defined by the COSPEC measurements or (ii) by the collection of adequate sample volume when a persistent rain under stable plume conditions occurred, Sample analysis

After collection and immediate return to the laboratory, the volume of each sample was recorded, and a portion removed for pH determination. The samples were then stored at 4°C to minimize potential degradation and subsequent analyses for all components were completed within 34 days of sample collection. Sulphate, nitrate, fluoride and chloride concentrations were determined by ion chromatography, calcium and mag-

0

DISTANCE I

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BACKGROUND SAMPLERS I AND 2

SCALE

:

Fig. 2. Map of the Sudbury experimental area showing the location of rain collectors used in the 17 October rain collection event.

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et al.

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nesium were determined by atomic absorption, and graphite furnace flameless atomic absorption was used for iron, copper, nickel, lead and zinc analysis. The methods employed permitted complete analysis using approximately 3@40 m/ of sample. Dissolved sulphur dioxide in rainwater was analyzed colourimetrically using a modified WestClaeke method. Because the concentration of the TCM stabilizing reagent is reported to significantly alter the results (Dana, 1980, personal communication), calibration of the method was done using several diluted concentrations over a range of 0.01-0.1 M TCM (i.e. to cover the range normally obtained in the field samples having an adequate preservative strength).

START

3. RESULTS Field

study conditions

The results of a study undertaken in the Sudbury area on 17 October 1980 will be used to illustrate the capabilities of the method. The meteorological conditions which prevailed during the study period can be summarized as follows: (a) rain occurred overnight and light, persistent rain and showers continued throughout the day until approximately 1500h on 17 October; (b) ground level winds at 0900 h were SE and remained from this quadrant until approximately noon at which time the wind shifted to S and (c) upper level winds, as determined from the location of the SO, plume by the COSPEC, were SSW unttl approximately 1130 h (i.e. the plume was centred over collecting sites 13-14 as shown in Fig. 2) and shifted to a more southerly direction by the end of the rain period (i.e. the plume was centred over site 20). The wind direction, in conjunction with accessible roads in the area, permitted measurements over one arc only on this occasion. Samplers were installed at an average arc distance of 12 km north of the source and two remote background samplers were installed at 10 km east of the source. Installation and opening of the samplers was completed by 1400 h. The rain period ended at approximately 1500 h at which time the samples were collected and returned to the laboratory for analysis. Obsewations

Plume profiles (as seen by an observer facing downwind) were determined at regular intervals throughout the precipitation sampling period and the SO, profiles are shown in Fig. 3. It can be seen that the plume meandered somewhat during the sampling period with a general drift of the centreline to the west, resulting in an overall backing of the plume trajectory during the sampling period. It is also apparent from the profiles that precipitation passing through the plume would be collected in samplers ranging from approximately sites 12 to 20. In contrast with some other occasions, no ground impingement of SO, was observed with the SIGN-X monitor during this event. The average SO, profile for the sampling period is

I& Fig. 3. Family of COSPEC during

the rain collection

SAMPLER

SITE

SO, plume profiles obtained period of 17 October.

shown in Fig. 4 along with the other parameters measured during the study. Relevant features to note are: (a) the average SO2 profile, determined by averaging the SO2 burdens corresponding only to periods when collectors were opened, indicates a well-defined plume centred over sampler 16B; (b) the rainfall rate (+ 2.9 mm h- ‘) was quite uniform across the sampling arc; (c) there is excellent correlation between several of the chemical constituents of the rain samples and the average SO, profile, indicating significant washout of these species from the plume and (d) the paired samplers located at the remote background site showed good precision in the methodology and agreed well with the background samples collected on the arc on both sides of the plume.

DISCUSSION

The monitoring and analytical results summarized in Fig. 4 provide a detailed definition of the washout and deposition processes occurring in the vicinity of the INCO plume during this precipitation event. Because of the persistent light rainfall, a large number

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Fig. 4. Profiles of chemical constituents and COSPEC vs collector position for the Sudbury 17 October rainfall event.

of COSPEC traverses were completed, resulting in a clear definition and delineation of the plume location and behaviour during the sampling period. Sulphate concentrations in the rainwater samples correlate very well with pH depression under the plume, and both the pH and sulphate profiles correspond closely with the average SO2 overburden as AE16:11 _L

defined by COSPEC measurements. Sulphate concentrations in the under-plume samples were approximately 60% higher than background levels and account for virtually all of the observed pH depression. However, the percentage of total emitted sulphur that was deposited by precipitation within approximately 15 km of the INCO stack was estimated to be relatively

small (approximately 0.2;Y;). This agrees with the results of recent cumulative deposition network studies conducted by the Untario ~~~~~ir~ of the ~~~i~~~rn~nt (Ghan ef c&, 19gt), and artier event studies by Atmospheric Environment %&ice (Wiebe and Whelpdale, 1974). Most of the sulphate in the plume at this distance from the source will be primary in origin and can be expected to reorient approximat& I :< of the totaf sutphnr emissions, Thus it is estimated that r#u~hly 20 7; of the primary sulphate is removed within 15 km from the source. The ~strjbution of dissolved SO, in min is significa&y skewed towards the left side of the plume. It is possible that the plume was sheared in $p&ze,with its left-most edge closer (but not yet touching) the ground. In this situation, the SOa dissoIv& by tHt: rain travelling through the upper and right-most side of the! plume had sufficient time to be desorbed on its longer way through the atmosphere to the collector. The rain ~r$~i~~tatjng throtrgh the left-m& and lower edge of the plume, on the other hard, woufd have bad mu& less time to desorb before being collected atld fixed by the TCM solution, This, however, is only a tentative explanation which must await ~an~~rn~tio~ by the results of further events. Low ievef SBr emission, with expected plumes on the ieft side of the major plume based on ground level wind data, could also have contributed to the observed profile. Same ~~vestjg~to~ have adv~t~ the measurement af dissolved SO2 in rainwater (Hales and Dana, 1979; Davies, 1979; Hutcheson, 1974) and have disa cussed the possibility of oxidation of SOa within ~~stab~l~~~ sampIes p&r to anaIyses which FO&% account For a portion of o&ed suIph&e IeveIs in some ~recipi~tinn sampling network data* It should be noted that the low level of dissolved SO2 below the major plume in conjunction with the high level of sulphate at station 16A suggest that in-sample oxidation has not accounted for the underplume sulphate levels observed in this study. Nitrate and ammonium ion profiIes show ~fosign%cant correlation with the iNC0 plume Iocation, because these substances are not emitted in significant amounts from this source. The gradual increase in ievefs ofthese constituents from west to E& afong the arc can be att~but~ te cheat from plume and possibly some low-level smeftcr, with the highest levels being more urbanized sector of the arc. As for sutphate and acidity, trace

the toEal axrbas sources at the observed in the m&al concen-

trations in the precipitation samples show a marked corr&tion with the plume and indicate an efficient washout mechanism for these species. It sboufd be noted that imn, copper aizd nickel are ch~acteristic of smelter emissions and considerable elevation above background was observed far these spo;cip?s, Efficient

removal of these particulate constituents by precipitation (acids, ~~pb~t~ and several trace metals) also has been Found in previous studies in the Sudbvry area (Wiebe and Wel~Ie, 1974 and Ghan et & 1982).

5. CONCLUSIONS The method d~v~l~~~ provides detailed information on the washout and deposition processes occurring in the vicinity of a large point-source plume. Preliminary results showed that local sources can make significant contributions to focal concentrations of bydrogea and snl~~~te ion, as welf as trace metafs, aad the method catl be used to a~urateIy d&me the geograp~~~l extent of this contribution. Studies are &ont~uuin~ and the resulting data also will be used to more accurate& define atmospheric modefing parameters.

PEFIBIENCES Beamish R. f. and Hawey H. H, fl9?Zj A~~~~~~~ ofthe La Clothe Mountain Lakes, Ontario, and R~u~tjug Fish Mortalities. J. ~~~~~~~~~~ Res. Bd Con, 59, 1131-.lf43.

Ghan W. H, Ro C., &u& I& A. and Vet R. J. (1982).Impact of the INCO nickel smelter emissians on pre+&ation qua&y in the Sudburyarea, sltmosphericEnvhmnent 16,8OI-814. Davies T. D. (1974) Determination of the local remaval of suipbur dioxide by ~~~~pi~tio~ Speciai Env~ronmeu~~ Report No. 3, ~~~~~~5~ and ~~surern~~t of Atmospheric PO&&~, WEAO-No. 368. Enger L. and Hogstrom U. (1979) Dispersion and wet deposition of sulphur from a power plant plume. Atmospheric Environwnl: 13, 797410. Granat L and Rodbe N. j1913] A study

of &Bout by ~~~~~o~ around aa oiI&red power p&et. ~~~s~~~ ~~~~~~~~~ 7* X%-792. HaIs f. M. and Dana M, 2’.(1979) Regio~l-ale de~sition af

sulpbr

pr~~itation s~venging. 1121-E 132. Hlatcbeson M. R. and Haft F. F. (19741 Sufphate washout from a coal-fired power plant pXume. Armospheric Environment 8.23-28 and 773. Hutchinson T. C. and Havas M. (l~S0~ Effects of acid ~r~i~i~t~o~ on terrestriat ecosystems. NATO Conference %er&, Series 1: ~~~~~~, Vol. 4: Plenum Press, New York. Larson T. V.. Char&n R. .I.. Knudson E. J.. Christian G. D. and Harrison H. (1975) The influence of~a sulfur dioxide point source on the rain chemistry of a single storm in the Puget Sound region. Wur. A& Soil Pdua. 4* 319-328. Likens G. f f976)The abuts prub&m: aE outfine ofconcepts, ArmospheFic

dioxide

~~v~r~~~~~~

by

13,

coefficient ior s;lphur

dioxi;fe using data from an East Midlands ground level monitoring network. ~r~u~~~e~~~ ~~~jro~~~i 12,251~2517. Mill&n M. M.. and Ho@ R. M. (1978) Remote sensing of air pollutants by cnrrdation spectrametry-instrumental

response character~s~~e~~ Atmospheric ~~vjp~~rn~~~~12, 853-864.

Oden S. (1476) The. a&&y problems an outErie of concepts. War. Air So3 #V&r, 6, X37-l&% Vet R. .I. and Chan W, H, (1980) The acidic precipitation in Ontario study--cumulative precipitation sampling network. Air Resources Branch, Ontario Ministry of tbe