A methodology for the measurement of distributed agricultural sources of ammonia outdoors Part 2: field validation and farm measurements

Transcription

1 Atmospheric Environment 39 (2005) A methodology for the measurement of distributed agricultural sources of ammonia outdoors Part 2: field validation and farm measurements Denise C. Welch a,, Jeremy J. Colls b, Theo G.M. Demmers a, Christopher M. Wathes a a Silsoe Research Institute, Wrest Park, Silsoe, Beds MK45 4DP, UK b School of Biosciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 17 May 2004; received in revised form 10 October 2004; accepted 22 October 2004 Abstract Recurved passive ammonia flux (PAF) samplers based on absorption by oxalic acid were evaluated in two contrasting field environments. First, a flux frame 12 m high and 55.2 m long was used to support a vertical array of up to 156 flux samplers downwind of a controlled point or line source of ammonia (NH 3 ). The upwind distance of the NH 3 source was chosen so that the whole NH 3 plume was captured by the flux frame. These measurements were used to calibrate the flux frame s collection efficiency (CE). The plume dispersion was modelled with ADMS. Second, individual columns of seven flux samplers at heights between 0.3 and 10.4 m were used to measure the NH 3 emissions from a free-range sow paddock. The controlled release measurements gave measured overall CEs of over 80% and strong agreement between the measured and modelled flux densities (R 2 40:9). The farm measurements showed that this method is insufficiently sensitive for use in a climate for which the wind direction and other critical meteorological parameters may vary widely throughout the several days required for a measurement. r 2004 Elsevier Ltd. All rights reserved. Keyword: Passive flux sampler; Free-range pig; ADMS; Point source; Line source 1. Introduction Agriculture is the major source of anthropogenic emissions of NH 3 in the UK (Sutton et al., 1995). In this paper, we describe a technique based on flux frames that may be suitable for various gaseous and particulate aerial pollutants with applications within and outwith agriculture. Currently 25 30% of the UK pig breeding herd is kept outdoors on an estimated land area of 8000 ha, principally based in eastern, central, southern Corresponding author. Tel.: address: denise.welch@bbsrc.ac.uk (D.C. Welch). and southwestern England (Chambers, 1998; Seabrook, 2004). However, there is very little literature on the contribution of this source to ammonia (NH 3 ) emissions or to other forms of pollution such as nitrate leaching. One study (Chambers, 1998) suggests that nitrogen inputs to outdoor pig systems can be considerable. Chambers (1998) calculated that annual nitrogen inputs in feed for systems stocked at 14 dry sows ha 1 were 625 kg N ha 1, compared to estimated outputs in pig meat of 119 kg N ha 1. The surplus N is largely returned to the soil via faeces and urine, which are then potentially available for volatilization or leaching. Other micrometeorological methods for measuring /$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi: /j.atmosenv

2 674 D.C. Welch et al. / Atmospheric Environment 39 (2005) NH 3 emissions from diffuse sources have been described by Denmead (1983) and Klarenbeek et al. (1993). Whilst the flux frame method requires more measurements than other micrometeorological methods, capture of the entire plume of emissions is assumed to increase the accuracy of the emission estimate. There are few NH 3 emission measurements for freerange pigs. NH 3 volatilisation, along with nitrate leaching and nitrous oxide emissions, were measured by Williams et al. (2000) during a2-year field study on outdoor pig farming systems. Ferm tubes coated with a solution of 3% oxalic acid in acetone were exposed for periods of 4 weeks at heights of 0.2, 0.4, 0.8, 1.1, 2.2, 3.3, 4 and 5 m above ground level, 40 m from the southwestern edge of each paddock. Wind speed and direction measurements were taken throughout the sampling period. Losses of NH 3 from outdoor dry sows were found to be in the region of 11 g NH 3 sow 1 day 1 (equivalent to 4.0 kg NH 3 sow 1 yr 1 ). These losses were reported to be similar to those measured by Misselbrook et al. (2000) for grazing dairy and beef cattle. Sommer et al. (2001) used passive ammonia flux (PAF) samplers in a mass balance technique, together with measurements of spatial variability using dynamic chambers, to estimate the annual NH 3 volatilisation from sows with piglets on organic farms. They found that NH 3 volatilisation was spatially very variable; the greatest volatilisation was nearest the feeding area and pig huts, corresponding with the pigs urination behaviour. From their measurements, and a model which took account of the parameters affecting NH 3 volatilisation, they estimated an NH 3 volatilisation rate of 4.8 kg NH 3 (sow with piglets) 1 yr 1. In the flux frame method, the source strength is calculated by intercepting the plume of NH 3 with a vertical array of flux samplers (Michorius et al., 1997). In Part 1 of this paper, Welch et al. (2004) (referred to from now on as Part 1) describe extensive validation measurements using recurved passive NH 3 flux samplers in the controlled environments of the wind tunnel and Atmospheric Flow Laboratory at Silsoe Research Institute (SRI), UK. They showed that the flux frame method was 87.4% efficient in its capture of NH 3 emissions from a controllable point source. In Part 2, we report the results of field trials in which the flux frame method was used to measure NH 3 emissions both from outdoor controlled point and line sources and from a field of free-range sows. 2. Methods and materials 2.1. Controlled point and line sources All the measurements from controlled point and line sources were conducted at the SRI wind engineering site (Silsoe, UK), which has been well documented from previous field measurements (eg. Hoxey et al., 2002). It is an area of flat land with an upwind fetch of several hundred metres over a homogeneous cut-grass sward which is clear of trees and other obstructions. The closest downwind obstructions were over 50 m from the flux frame. Point and line sources were constructed using FEP tubing (o.d. 7 mm, i.d. 5 mm) mounted 0.3 m above ground level and positioned either 15, 25, or 50 m from the frame. To simulate the line source, a uniform flow rate was delivered over an 80 m (maximum) length of tubing by releasing NH 3 under pressure from 21 stainless-steel orifices (0.85 mm diameter) spaced at 4 m intervals using specially adapted compression T-joints. The length of the line source could be varied between 80, 40 and 20 m by positioning the T-joints at appropriate intervals. NH 3 was delivered at a controlled rate via a flow meter (Model GTF2BHS, Roxspur Platon, Tadley, UK) from a cylinder wrapped with heat trace cable to prevent it freezing due to the evaporative heat loss at high NH 3 release rates. The cylinder was placed on a balance so that the weight loss during each experiment, and hence the mean NH 3 release rate, could be measured directly. A 2 m high mast was erected upwind of the source, on which three measurements of background NH 3 flux were taken. A vertical flux frame was erected, in a plane rotated 241 anti-clockwise from the north-south direction. Hence the flux frame was approximately perpendicular to the prevailing south westerly wind direction to maximise the probable number of measurement days. The permanent supports for the frame consisted of nine, 12 m high masts at 6.9 m spacing (total length 55.2 m), giving a flux frame area of m 2. On ameasurement day, NH 3 flux samplers were mounted on eight aluminium sub-frames which were hoisted between adjacent masts by ropes and pulleys. Each sub-frame supported three columns of samplers, with four samplers per column on each end frame and six per column on the six remaining frames. This arrangement resulted in 132 sampling positions at horizontal separations of 2.3 m and heights of 1.47, 4.37, 7.27 and m (end frames) or 0.99, 2.92, 4.85, 6.78, 8.71 and m (remaining frames). After the first five measurement runs described below, an extra row of samplers was added to each column 0.3 m above the ground, to give a total of 156. The NH 3 flux samplers used in all measurements were recurved PAF samplers, as described in detail in Part 1. The NH 3 was captured on a coating of 4% oxalic acid in acetone solution, and analysis was by NEN (Nederlandse Eenheids Norm Dutch Standard Method) Method 6472 and spectrophotometry at 655 nm. The net horizontal flux density F h at each sample height was

3 D.C. Welch et al. / Atmospheric Environment 39 (2005) calculated from the equation F h ¼ M s M b Kpr 2 ; t where M s, M b are the masses of NH 3 captured on the tubes facing source and background, respectively; r ( ¼ 0.5 mm) is the radius of the orifice separating the source and background tubes; t is the exposure time; and K is an empirically determined non-dimensional sampler constant, determined in Part 1 to be In aseries of experiments, the influence of various factors was investigated, including the distance of the frame from the source, the source geometry and meteorological conditions (particularly wind speed and direction). The distance of the frame from the source is critical, as the height of the masts and frame was limited to 12 m, and the plume must be sampled over its whole height (Michorius et al., 1997); if the distance from the source increases, it is more likely that the upper part of the plume will pass over the top of the frame. It was shown in Part 1 that the maximum air speed at which the samplers can be used with the existing orifice size and without breakthrough is 7 m s 1. A minimum speed was not determined due to limitations of the wind tunnel, although clearly an air speed greater than zero is required to drive the NH 3 through the sampler for collection. At extremely low air speeds, NH 3 would be expected to diffuse equally into both the source and background sampler tubes. It was also shown in Part 1 that incident wind directions up to 801 from the axis of the sampler were acceptable with little loss of collection efficiency (CE). Both these conclusions limited the range of conditions under which experiments in the field could take place. Therefore, it was planned to run experiments on days when a wind speed of 2 5 m s 1 from the southwest (ie. perpendicular to the flux frame) was forecast, along with dry conditions. Forecasts were obtained from Field runs with exposure periods of between 4.5 and 8 h took place between August 2000 and November The duration of the exposure period was limited by practical factors: the time it took to set up and retrieve afull set of samplers; the need to only expose tubes during daylight hours to avoid condensation in the NH 3 release pipe; and not starting a run until early morning dew, which would increase NH 3 deposition, had evaporated. A total of 12 runs was performed which satisfied the requirements for suitable wind conditions and the absence of rain both shortly before and during the trial. Each run was also modelled with the Advanced Dispersion Modelling System (ADMS 3.0, CERC, Cambridge, UK), which is a widely used dispersion model based on Gaussian methodology. Appropriate run-specific values for the parameters required by ADMS were used (Welch, 2003), determined by the meteorological conditions measured throughout the experimental period at a weather station located 50 m to the northwest of the flux frame. Measurements of wind speed and direction were made with a cup anemometer (Type A100 Porton anemometer, Vector Instruments, Rhyl, UK) and wind vane (Type W200P potentiometer wind vane, Vector Instruments, Rhyl, UK). Rainfall measurements were made using a tipping bucket rain gauge (Model ARG 100/EC, Campbell Scientific, Shepshed, UK). One-minute averages of these variables were stored on a data logger (21X Micrologger, Campbell Scientific, Shepshed, UK). The ADMS model is designed to predict concentrations on a horizontal rather than vertical grid. However, multiple individual receptor locations may be specified by their (x,y,z) coordinates. Hence the flux frame was represented by a two-dimensional array of co-planar points on a rectangular grid of appropriate dimensions. After the concentrations at each point had been determined with ADMS, they were multiplied by the average wind speed at that height to convert them into fluxes, with U H ¼ u k ln H þ H 0 ; H 0 where U H is the wind speed at height H, H 0 is the aerodynamic roughness of the site, u * is the friction velocity and k is the von Karman constant. When H H 0 ; we have U REF ¼ u k ln H REF ; H 0 where REF refers to the height of the wind speed sensor (5.4 m in this case). Rearranging the above equations to eliminate u * and k gives U REF ln½h=h0š U H ¼ ln½h : REF=H 0Š The wind engineering site at SRI, which had been well characterised from previous field programmes, was known to have a roughness length of 0.01 m (Hoxey and Richards, 1992; Hoxey et al., 1998). Both the measured and modelled grids of flux values were plotted as contours using Surfer (Golden Software, Golden, TX, USA), with kriged gridding and medium smoothing. The resulting contours were then integrated over the area of the flux frame to find the measured and modelled CEs expressed as a percentage of the total NH 3 released during the course of each run Free-range pig farm Following the trials described above with a defined controlled source, the flux frame method for

4 676 D.C. Welch et al. / Atmospheric Environment 39 (2005) m M m m measuring NH 3 emissions was evaluated on a spatially heterogeneous source at a free-range sow farm in Bedfordshire, UK. The site had been used for pig production since 1996, before which it had been under cereals. The pigs were Landrace Duroc Large White hybrids, with a ratio of approximately one boar per six sows. The latter were typically years old (four litters), and scatter-fed daily. The paddock was almost rectangular, with side lengths of 435, 330, 525 and 363 m (Fig. 1) and an area of approximately km 2 (15.8 ha). Farrowing sows were housed on adjacent plots to the south and east of the paddock. A simplified version of the flux frame was used at the farm. The size of the paddock meant that there was no possibility of capturing the whole width of the plume as had been achieved with the line source. Instead, six of the 12 m masts were erected at separate locations around the outside of the paddock, to sample the vertical profile of the paddock NH 3 plume. The six locations were chosen to allow sampling to take place for all wind directions during the measurement period, with two masts on each of the longest sides of the paddock (Fig. 1). A pulley system allowed seven flux samplers on each mast to be raised to heights of 0.3, 0.99, 2.92, 4.85, 6.78, 8.71 and m above ground level. Metal guide wires that ran from the top to the bottom of the sampling mast were pulled taut, thereby maintaining the correct orientation of the samplers. The pulley arrangement on individual masts avoided having to lower the large and cumbersome frames (which had been used for the controlled release measurements) into the pig paddock and reduced the risk of contamination due to the close proximity of the source whilst the samplers were being attached. Flux sampler preparation, deployment and analysis were carried out according to the protocols described in Part 1. Relevant environmental parameters were measured throughout the 5 N m 4 A507 Fig. 1. Positions of the six sampling masts and the meteorological station around the paddock perimeter. experimental period at a transportable meteorological station adjacent to the paddock, labelled M on Fig. 1. The instrumentation was the same as that used for the point and line source measurements described previously. Soil moisture content and ph are important mediators of NH 3 volatilisation (Freney et al., 1983), and measurements of these properties were therefore taken during the field campaign. Twenty locations were chosen at random across the paddock to determine the soil properties and their spatial variability; three soil cores were taken at each location. The three cores were taken by driving a stainless steel cylinder (80 mm internal diameter 50 mm long) into the soil. After digging it out, the core was transferred to a plastic bag. Sampling, storage and analysis of the soil cores took place according to British and European Standards (EN 13,040:2000; BS 7755: Section 3.1: 1994; ISO 11,465:1993; BS EN 13,037:2000). In later runs, the number of sampling positions was reduced to six because of the lack of variability shown by the earlier surveys. Five runs, ranging in length from 2 to 4 days, were completed during the period May July Of these, three were successful and two were rendered useless by rain, which washed the acid coating out of the samplers. With the natural variability of wind direction in the UK, it was important to anticipate the most clear-cut combination of weather conditions wind direction steady, wind speed sufficient to drive sampler flow but less than 7 m s 1 (to avoid breakthrough see Part 1), and no precipitation. There were other potential NH 3 sources in the paddock area, so it was expected that with a predominant wind direction from any one sector during arun, the difference between downwind and upwind masts would identify the paddock contribution. 3. Results and discussion 3.1. Controlled point and line sources Table 1 shows a summary of all the results during successful field experiments with five point sources and six line sources. Although Run 9 was completed successfully, no meaningful results were obtained because the sample bottles into which the samples were extracted became contaminated. Two further runs are not included in the table because the coating was washed off the tubes by rain. Dew was visible on the grass at the beginning of Runs 10 12, but evaporated within the first hour of NH 3 release in all cases (by around am). For trials with the point source, the source-frame separation was increased from 15 to 25 and then to 50 m.

5 D.C. Welch et al. / Atmospheric Environment 39 (2005) Table 1 Summary of results for all experimental runs successfully performed and analysed Modelled CE (% b ) Modelled CE (%) Expected sampling error (%) Measured CE (%) Lateral spread (deg a ) Mean wind direction (deg) Max. wind speed (m s 1 ) Mean wind speed (m s 1 ) Release rate (g s 1 or gm 1 s 1 ) Exposure period (h min) Line source length (m) Source distance (m) Date Source type Run no. 1 17/8/00 Point /9/00 Point /9/00 Point /10/00 Point /10/00 Point /8/01 Line /9/01 Line /10/01 Line /10/01 Line /10/01 Line /11/01 Line a Lateral spread: the standard deviation of the wind direction throughout the exposure period. b CE modelled with an extra row of samplers at a height of 0.3 m m Wind direction 258 ± 12.9 degrees 24.0 At the latter distance the top row of PAF samplers gave significant values, which confirmed that the height of the plume exceeded the height of the flux frame. For trials with the line source, the separation was fixed at 25 m, and the source length was varied between 20 and 80 m. For alength of 20 m, the whole of the flux cross-section was captured by the flux frame. Runs 6 and 7 were attempts to simulate an infinite line source by using a line source that was longer than the flux frame. The CE was corrected by the ratio of the source lengths (80/55.2) to derive an effective CE. The efficiencies for Runs 6 and 7 then increased to 23.0% and 49.6%, respectively. Representative results have been given below for one example each of a point source (Run 3) and a line source (Run 10). For the point source used in Run 3, 25 m from the flux frame, Fig. 2 shows a scale diagram of the source, flux frame and wind direction geometry. Hourly values of the meteorological parameters that applied during Run 3 and which were used in ADMS are given in Table 2. For the same run, Fig. 3 shows the measured and modelled plots of NH 3 flux density. Fig. 2 shows that, for the wind direction prevailing during Run 3, the flux frame would be expected to capture the whole of the width of the plume, with the plume centreline somewhat to the south of the frame centreline. Fig. 3a shows this to have been the case, with the plume height well below the top of the frame. The modelled flux contours of Fig. 3b confirm the location, shape, extent and quantitative fluxes over the plume cross-section. Integration of the flux values over the contours gave measured and modelled CEs of 60.6% and 59.7%, respectively. Fig. 4 gives the scatter plot of measured and modelled fluxes, with an R 2 value of (n ¼ 132; po0:01) Fig. 5 is the corresponding scale diagram of the source and flux frame for Run 10, in which a 20 m line source N m 12.9 Fig. 2. Scale diagram showing point source, flux frame and wind direction geometry for Run 3.

6 678 D.C. Welch et al. / Atmospheric Environment 39 (2005) Table 2 Meteorological parameters for run 3 Time of start of hour Wind speed (m s 1 ) Wind direction (deg) Wind direction std deviation (deg) Cloud cover (okta) Height / m (a) Width m Height / m Flux density / mg NH 3 m -2 s -1 (b) Width m Fig. 3. Run 3: Point source 25 m from the flux frame. Flux frame contours viewed from downwind of the flux frame. (a) Measured flux densities at 132 sampling locations (CE ¼ 60.6%); (b) Modelled flux densities for the same sampling locations (CE ¼ 59.7%). Modelled flux / mg NH 3 m -2 s -1 Run y = x Measured flux / mg NH 3 m -2 s -1 Wind direction 246 O ± 9.5 O 55.2 m N 0. 0 m Fig. 4. Run 3: Scatter plot of measured and modelled flux densities. Point source 25 m from the flux frame. Fig. 5. Run 10: Scale diagram showing the geometry of the line source (2) and wind conditions relative to the flux frame. was positioned 25 m from the flux frame. The positions of the flux frame and line source are shown relative to each other and to north. The double-ended arrow is the line source, and grey lines indicate the lateral spread of the plume based on the standard deviation of the wind direction during the run. It can be seen that, taking into account the length of the source and the wind direction standard deviation, the whole width of the plume should

7 D.C. Welch et al. / Atmospheric Environment 39 (2005) Table 3 Meteorological parameters for run 10 Time of start of hour Wind speed at 5.4 m (m s 1 ) Wind direction (deg) Wind direction std deviation (deg) Cloud cover (okta) Height /m Height /m (a) Width (m) Flux density / mg NH 3 m -2 s -1 (b) Width (m) Fig. 6. Run 10: 20 m line source, 25 m from the flux frame: (a) flux densities measured by 156 samplers (CE ¼ 81.2%); (b) flux densities modelled for the same sampling locations (CE ¼ 82.6%). be captured by the flux frame. Hourly averages of the meteorological parameters used by ADMS are given in Table 3. Figs. 6aand b show the measured and modelled contour plots for Run 10, and confirm that the whole plume of NH 3 was being captured. There is strong agreement between the shape and position of the measured and modelled plots. However, the fluxes at the centre of the modelled plume are not as high as those at the centre of the measured plume. The runs in this configuration had measured CEs above 80% (range %), while the modelled plots had CEs in the range %. Fig. 7 shows ascatter plot of the measured and modelled flux densities for Run 10, with an R 2 value of 0.9 (n ¼ 156; po0:01) Free-range pig farm It proved to be hard to meet the requirements outlined above for consistent meteorological conditions, and none of the three valid runs was as clear cut as had been hoped. Table 4 shows that the wind direction was generally variable; although Paddock Run 2 had the Modelled flux / mg NH 3 m -2 s Run 10 y = x Measured flux / mg NH 3 m -2 s -1 Fig. 7. Run 10: Scatter plot of measured and modelled flux densities. 20 m line source, 25 m from the flux frame. greatest proportion from a single sector, there were significant proportions from all sectors. The measurements of soil characteristics that were taken across the area of the paddock showed no significant spatial variation of moisture content or ph (data not shown). The stocking densities (Table 5) show

8 680 D.C. Welch et al. / Atmospheric Environment 39 (2005) Table 4 Proportions of time that the wind blew from four sectors during the three paddock runs Paddock run no. Table 5 Paddock pig population during the three paddock runs Run Date Paddock pig population May July July a small variation in population. The pigs were not weighed specifically for these experiments, but the farm management estimated that the stock would have been of similar weights for the three runs Paddock run 2 Proportion (%) of wind from sector North East South West Nominal live weight (kg) A detailed analysis is presented of Paddock Run 2, because it had the most consistent wind direction and hence the greatest probability of success. Figs. 8aand b show the wind direction and speed, respectively, during the run. During the 4 days for which the flux tubes were exposed, the wind direction backed from southwest through south and east to north. There was a significant excursion back to southwest during the night of 16/17 July. Hence, although the wind direction was from the east sector for the greatest proportion of time, all sectors were sampled. Fig. 8b shows that the wind speed was generally within the required range, with typical wind speeds of afew m s 1 and two short periods when the wind speed dropped below 1 m s 1. The values of F h at the seven heights on six masts for Paddock Run 2 are shown in Table 6. Overall, the fluxes were very small, typically a few mg NH 3 m 2 s 1. Fig. 9 shows the variation in net horizontal flux density with height for each of the six sampling masts. There was no clear trend in NH 3 flux density with height for any of the masts, and no similar trend between different sampling masts. Table 7 shows the measured fluxes on each individual tube of the flux sampler. There was no consistent difference between source (S) and background (B) tubes and both have similar flux densities, which accounts for the positive and negative values in Table 6. These results, and especially Fig. 9, do not indicate any systematic change of flux density with height. Hence, no meaningful interpretation could be made of the NH 3 release rate from the paddock. Williams et al. (2000) exposed their NH 3 flux samplers for periods of 4 weeks, which would allow enough NH 3 to be captured from such a diffuse source. However, wind direction and speed data are not presented, and rainfall is presented as a total over a whole season (1 October 31 March). It is unlikely that any four week period would be dry throughout, and as no wind direction data are presented the length of time the wind blew from each direction can only be assumed. Sommer et al. (2001) report that 10% of the measurements using PAF samplers were rejected because of wind direction difficulties (i.e. wind not passing over the site before reaching the sampling mast) and one whole run was excluded because background fluxes were so high that no NH 3 enrichment occurred on the source tubes. This was seen to occur during Paddock Run 1, where large negative fluxes were measured (data not shown), possibly due to large emissions from the farrowing sows and piglets to the East of the measurement site. Sommer et al. (2001) report how NH 3 volatilization increases with the amount of feed given to the pigs, especially to sows suckling piglets. Although there appeared to be no trend of net horizontal NH 3 flux density with height for any of the sampling masts, and no consistency between different Runs, a crude estimate of the maximum NH 3 emission rate from the paddock can be made by using the largest flux measured during the three Runs performed. Assuming that the background net flux was zero; that the maximum measured flux density applied to the total height of the sampling mast (10.44 m to the highest sampling position); and that this flux density applied to the whole length of the paddock (525 m). The greatest net flux density (measured during Run 1 on mast 4 at a height of 0.99 m) was 28.4 mg NH 3 m 2 s 1. Hence the total flux leaving the paddock would be g s 1. This figure is equivalent to a source strength of 11.1 kg NH 3 (sow) 1 year 1. If asow is taken to be 190 kg in weight, acomparison can be made between the four emission rates presented in Table 8. Whilst the emission rates reported by Williams et al. (2000) and Sommer et al. (2001) for outdoor sows are within the range reported by Groot Koerkamp et al. (1998) for housed pigs, the emission rate for housed pigs reported by Demmers et al. (1999) is four times those measured for outdoor sows. This supports Jarvis (1991), who found that NH 3 losses associated with housed animals can be between 3.5 and 11.4 times greater per animal than for grazing cattle and sheep. Of the two

9 D.C. Welch et al. / Atmospheric Environment 39 (2005) Direction (degrees) (a) 0 15 /07/02 15 /07/02 16 /07/02 16 /07/02 17 /07/02 17 /07/02 18 /07/02 18 /07/02 19 /07/02 19 /07/02 20 /07/02 00:00 12:00 00:00 12:00 00:00 12:00 00:00 12:00 00:00 12:00 00:00 Date /Time Wind speed m s (b) 0 15 /07/02 15 /07/02 16 /07/02 16 /07/02 17 /07/02 17 /07/02 18 /07/02 18 /07/02 19 /07/02 19 /07/02 20 /07/02 00:00 12:00 00:00 12:00 00:00 12:00 00:00 12:00 00:00 12:00 00:00 Date /Time Fig. 8. (a) Variation of wind direction during Paddock Run 2. (b) Variation of wind speed during Paddock Run 2. estimates of NH 3 emissions from free-range sows, the Williams et al. (2000) study lacks rainfall data, which makes it difficult to assess the reliability of the results, and the measurements by Sommer et al. (2001) were taken for a small number of sows (6 11). The maximum paddock emission rate estimated above, 11.1 kg NH 3

10 682 D.C. Welch et al. / Atmospheric Environment 39 (2005) Table 6 Paddock Run 2: Net horizontal ammonia flux densities (mgm 2 s 1 ) at seven sample tube heights on six masts Sample ht. (m) Mast no sow 1 year 1, compares well with those given in Table 8. It is known that higher ph values lead to greater NH 3 emissions (Freney et al., 1983; Sherlock and Goh, 1984), so it would be expected that greater NH 3 emissions would be measured for Run 1, where the highest mean soil ph of 6.95 was measured. Hoff et al. (1981) found that 65% of ammoniacal-n was emitted when the soil/ manure ph was 47, but was reduced to just 14% over the same time period when the ph was 6.4. For Runs 2 and 3 the mean soil phs were 5.38 and 5.52, respectively, which may explain the difficulties in measuring NH 3 emissions from the farm throughout the measurement runs. Net horizontal flux density, µg NH 3 m - 2 s Sample height, m Mast 1 Mast 2 Mast 3 Mast 4 Mast 5 Mast 6 Fig. 9. Variation of NH 3 flux density with height on six masts for Paddock Run Limitations and conclusions Recurved PAF samplers in a flux frame are suitable for the measurement of large flux densities from strong sources of NH 3 to which the samplers respond within hours while weather conditions remain reasonably steady. The low flux densities measured at the pig farm (mgm 2 s 1 as compared to mg m 2 s 1 determined for the point and line sources) require an exposure period of several days to achieve sufficient NH 3 deposition. Hence dry periods of several days are needed, with a steady wind direction, medium wind speeds and no calm Table 7 Paddock Run 2: NH 3 flux densities (mg NH 3 m 2 s 1 ) measured on individual tubes of each sampler Sample ht (m) Mast no., Tube facing source (S) or background (B) S B S B S B S B S B S B Table 8 Comparison of reported NH 3 emission rates Reported emission rate Emission rate (kg NH 3 sow 1 yr 1 ) Reference 11 g NH 3 sow 1 day Williams et al. (2000) 4.8 kg NH 3 sow 1 yr Sommer et al. (2001) 46.9 kg NH 3 lu 1 yr Demmers et al. (1999) mg NH 3 animal 1 h Groot Koerkamp et al. (1998)

11 D.C. Welch et al. / Atmospheric Environment 39 (2005) periods. Although such periods can be identified retrospectively from meteorological data, the acquisition of field measurements requires that personnel and other resources are available for setting up, exposing, retrieving and analysing flux tubes when suitable weather periods are forecast. These activities will frequently be wasted if the weather changes. For the limited number of field measurements carried out at the paddock, there was no evidence of systematic change of flux density with height, and hence no robust basis for estimating the areal emission strength. These findings suggest that the recurved PAF sampler is not suitable for measurement of the low NH 3 flux densities evident at the pig farm, particularly when the wind direction is variable. Small emissions may be successfully measured if a constant wind direction and a wind speed greater than, say, 1.5 m s 1 are prevalent. Alternatively, a tunable diode laser open-path monitor (GasFinder 2.0 Portable System, Boreal Laser Inc., Spruce Grove, Albt., Canada) could be used to measure NH 3 concentration. Concentration is integrated over the length of the beam, typically between 30 m to 1 km (Richardson and Phillips, 2001) and concentrations as low as 1 ppb may be detected, with a sensitivity of 1 ppm m along a 1 km beam, making them suitable for measuring low flux emissions. Whilst the initial outlay for the equipment is high, the results are available in real-time eliminating the laboratory costs associated with the analysis of large numbers of recurved PAF samplers (Richardson and Phillips, 2001). Acknowledgements This work was partly supported by the Department for the Environment, Food and Rural Affairs under project WA References Chambers, B.J., Sustainable Systems of Outdoor Pig Production. Ministry of Agriculture, Fisheries and Food, London, UK. 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