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The dry deposition of sulfur dioxide on a loblolly pine plantation∗
oMw-69l?l/86S3.OO+O.M Perymon Journ& Ltd.
I Vol. 20. No. 6. pp. 13114312. 1986 &moqhu trlkmmm Printedin GIUI Britin.
DISCUSSION REFERENCE
THE DRY DEPOSITION OF SULFUR DIOXIDE ON A LOBLOCLY PINE PLACATION*
Hicks 8. B, Wesky M. Land Durham J. L (1980) Critique of metbods to measure dry deposition, Worksbop summary. U.S. Environmental Protection Agency Report EPA600/9-80X60.
The authors of tbis paper report measurements of dry deposition on a pine plantation using the fhnt gradient approach The relatively low SO1 concentrations at the experimental site and the use of an SOs analyxer with bigb temporaI rcsoIm.ion make this study potentirdly interesting Very few similar studii Iuive been pub&shed so far. However, a closer examination of the paper has raised a number of fundamental qnestions that need to be clari6ed before the conclusions can be accepted. For brevity we pass over several details including errors in equations, presumably typographic&, wrong numbering of the references to the equations etc. 1. Fit of ail, it is very bard to judge the quality of the data if no original profiis arc presented. The paper gives only monthly average Buxes and resistances. Tbe averages were formed over a limited number of “clear to partly sunny days”; ‘other periods yielded fluxes too small to measure”. It is also very difficult to assess the usefubtess of the averages, given the subjective nature of the data selection and that no information is provided about tbe diurnal distribution and total length of the mcasurcment periods. It is therefore most misleadmg to conclude that “average deposition velocity to the pine forest was 0.72cms-I”. It is also difficult to understand how one can claim that the deposition velocity did not vary greatly through the year when no information is given ahout the seasonal changes of the number of”ck.ar and partly sunny days”. 2 A very high precision of the SOx mcasuremtnts is required for an evaluation of the gux using the gradient method. Hicks et al. (1980) mention values of the required precision of the order of 1%. It hasnot heen demonstrated that the analyzer used in this study can actually measure differences of 0.1 ppbv. It would also he very interesting to know how the instrument was calibrated and how possible interferences from other gases were handled. 3. For a surface, such as a forest, with large roughness elements the question of fetch requirements is critical. No discussion is provided about this problem. How do the authors know that the borixontal homogeneity is sufficient? 4. Tbe internal resistance ‘ir is derived as the difference between the total resistance and the sum of the aerodynamic and canopy resistances As noted by the authors, such a method is bound to be very uncertain. Thus, it is hard to believe that the average internal resistance can be determined to two significant digits, as given in conclusion no. 3. In addition, the expression for rir derived in the paper contains a solubility parameter, s, that is not defined nor given a numerical value. WC are looking forward to clarifying comments on these matters from the authors and hope that a further discussion will contribute to a better undc~tanding of the important process of dry deposition of SO*.
AUTHORS’ REPLY
WC fully understand tbe diIficulty of making the measurements presented in this paper and welcome the opportunity to discuss the methods and meaning of the results in more detail. Our comments will be made in the order presented in the discussion of Johansson et al. 1. Tbedata on which the results presented in the paper are based on were taken over two hundred periods during which profiles of SO1, CO,, water vapor density, temperature and wind speed were measured. Because of the amount of data it was not practical to present the profiles themselves. These periods were a subset of a much larger data set. The profiles used for analysis were selected on the basis of visual inspection where linear relationships between temperature, SOs, CO2 and water vapor density over at least three lowest measurement heights were used as the criterion for selection. This excluded all-night data and most of the data from the early morning and late afternoon. Because measurements could he made under a limited number of conditions, the comment that the average value of deposition velocity of 0.73+/-0.65cm s-i is not a valid annual average is correct. We did not mean to imply that this value should be used for calculation of SO, exchange for long periods but only that it was the average for the periods during which wn could make a good estimate of the exchange. In addition, we did not mean to imply that the deposition velocity did not change over the period of our measurements. The large standard deviation of the measurements indicate that there was a great deal ofvariation even during the limited periods in which WCcould make measurements. However, what we meant to convey was that all efforts to relate this variation to such as temperature, humidity, season, and al1 other factors we measured simultaneously failed to show any relationships. 2. Measurements of the type reported in our paper do press the requirements of measurement instruments to the limit. The noise inherent in the analyxer WC used was accounted for by taking the average of many readings, The computer controlled data ~uisition and control system we used would switch between levels at every two minutes. After allowing a minute ‘settling’ time, measurements were recorded as fast as the instrument could make a cycle between all the instruments being serviced. The cycle time was just under 2 s. thus an average of approximately 30 individual measurements were made at each level in the 8 min it took to aytlc through all the kvcls. Seven cycles of the sampling levels were made in the i h in which profiles were computed. Thus. the measurements at each levelwere hased on approximately 210 individual readings of the analyzer. C. JOHANSSON Deporrmenl of Meteorology. During laboratory-calibration of the analyzer. it was A. RICHTER University of Stockholm, Sweden determined that the greatest limitation to accuracy was not H. RODHE the response of the analyzer but the effects of absorption of H. RG%Z SO, on the walls of the sampling equipment. Even the elimination of all metal left a considerable lag in responx. from absorption on plasticlinesand fitting. We found that the *I&rem R. and Murphy C. E., Jr (1985} ~~~spberi~ most practical way to overcome this problem was to move Encironmenr 19, 797-802. 1311
I Vol. 20. No. 6. pp. 13114312. 1986 &moqhu trlkmmm Printedin GIUI Britin.
DISCUSSION REFERENCE
THE DRY DEPOSITION OF SULFUR DIOXIDE ON A LOBLOCLY PINE PLACATION*
Hicks 8. B, Wesky M. Land Durham J. L (1980) Critique of metbods to measure dry deposition, Worksbop summary. U.S. Environmental Protection Agency Report EPA600/9-80X60.
The authors of tbis paper report measurements of dry deposition on a pine plantation using the fhnt gradient approach The relatively low SO1 concentrations at the experimental site and the use of an SOs analyxer with bigb temporaI rcsoIm.ion make this study potentirdly interesting Very few similar studii Iuive been pub&shed so far. However, a closer examination of the paper has raised a number of fundamental qnestions that need to be clari6ed before the conclusions can be accepted. For brevity we pass over several details including errors in equations, presumably typographic&, wrong numbering of the references to the equations etc. 1. Fit of ail, it is very bard to judge the quality of the data if no original profiis arc presented. The paper gives only monthly average Buxes and resistances. Tbe averages were formed over a limited number of “clear to partly sunny days”; ‘other periods yielded fluxes too small to measure”. It is also very difficult to assess the usefubtess of the averages, given the subjective nature of the data selection and that no information is provided about tbe diurnal distribution and total length of the mcasurcment periods. It is therefore most misleadmg to conclude that “average deposition velocity to the pine forest was 0.72cms-I”. It is also difficult to understand how one can claim that the deposition velocity did not vary greatly through the year when no information is given ahout the seasonal changes of the number of”ck.ar and partly sunny days”. 2 A very high precision of the SOx mcasuremtnts is required for an evaluation of the gux using the gradient method. Hicks et al. (1980) mention values of the required precision of the order of 1%. It hasnot heen demonstrated that the analyzer used in this study can actually measure differences of 0.1 ppbv. It would also he very interesting to know how the instrument was calibrated and how possible interferences from other gases were handled. 3. For a surface, such as a forest, with large roughness elements the question of fetch requirements is critical. No discussion is provided about this problem. How do the authors know that the borixontal homogeneity is sufficient? 4. Tbe internal resistance ‘ir is derived as the difference between the total resistance and the sum of the aerodynamic and canopy resistances As noted by the authors, such a method is bound to be very uncertain. Thus, it is hard to believe that the average internal resistance can be determined to two significant digits, as given in conclusion no. 3. In addition, the expression for rir derived in the paper contains a solubility parameter, s, that is not defined nor given a numerical value. WC are looking forward to clarifying comments on these matters from the authors and hope that a further discussion will contribute to a better undc~tanding of the important process of dry deposition of SO*.
AUTHORS’ REPLY
WC fully understand tbe diIficulty of making the measurements presented in this paper and welcome the opportunity to discuss the methods and meaning of the results in more detail. Our comments will be made in the order presented in the discussion of Johansson et al. 1. Tbedata on which the results presented in the paper are based on were taken over two hundred periods during which profiles of SO1, CO,, water vapor density, temperature and wind speed were measured. Because of the amount of data it was not practical to present the profiles themselves. These periods were a subset of a much larger data set. The profiles used for analysis were selected on the basis of visual inspection where linear relationships between temperature, SOs, CO2 and water vapor density over at least three lowest measurement heights were used as the criterion for selection. This excluded all-night data and most of the data from the early morning and late afternoon. Because measurements could he made under a limited number of conditions, the comment that the average value of deposition velocity of 0.73+/-0.65cm s-i is not a valid annual average is correct. We did not mean to imply that this value should be used for calculation of SO, exchange for long periods but only that it was the average for the periods during which wn could make a good estimate of the exchange. In addition, we did not mean to imply that the deposition velocity did not change over the period of our measurements. The large standard deviation of the measurements indicate that there was a great deal ofvariation even during the limited periods in which WCcould make measurements. However, what we meant to convey was that all efforts to relate this variation to such as temperature, humidity, season, and al1 other factors we measured simultaneously failed to show any relationships. 2. Measurements of the type reported in our paper do press the requirements of measurement instruments to the limit. The noise inherent in the analyxer WC used was accounted for by taking the average of many readings, The computer controlled data ~uisition and control system we used would switch between levels at every two minutes. After allowing a minute ‘settling’ time, measurements were recorded as fast as the instrument could make a cycle between all the instruments being serviced. The cycle time was just under 2 s. thus an average of approximately 30 individual measurements were made at each level in the 8 min it took to aytlc through all the kvcls. Seven cycles of the sampling levels were made in the i h in which profiles were computed. Thus. the measurements at each levelwere hased on approximately 210 individual readings of the analyzer. C. JOHANSSON Deporrmenl of Meteorology. During laboratory-calibration of the analyzer. it was A. RICHTER University of Stockholm, Sweden determined that the greatest limitation to accuracy was not H. RODHE the response of the analyzer but the effects of absorption of H. RG%Z SO, on the walls of the sampling equipment. Even the elimination of all metal left a considerable lag in responx. from absorption on plasticlinesand fitting. We found that the *I&rem R. and Murphy C. E., Jr (1985} ~~~spberi~ most practical way to overcome this problem was to move Encironmenr 19, 797-802. 1311