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In Situ Transmissiometer Measurements for Real-Time Monitoring of Dust Discharge during Orchard Nut Harvesting

Biological and Agricultural Engineering, One Shields Ave, UC Davis, Davis, CA 95616

^* Corresponding author (ddowney{at}ucdavis.edu).

Received for publication August 20, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 
Rapid assessments of operating conditions and field preparation on dust discharge from nut harvesters are needed to guide improved equipment design and grower practices for dust reduction. An industrial opacity sensor, typically used for industrial stack monitoring, was adapted for use on a nut harvester to measure relative dust intensity during nut pick-up operations in almond orchards. Due to the high volume of discharge air and the presence of large debris such as leaves, additional components were coupled with the sensor to enable subsampling of the air. Pre-harvest windrow preparation conditions were evaluated. Results indicated that relative dust intensity decreased by 32% during harvest activities after windrow preparation with proper nut sweeper adjustment. Conventional harvesting results indicated that under typical operating conditions, reducing the separation fan speed could reduce relative dust intensity by 54%. Ground speed also had a strong effect; reducing speed from 4.8 to 2.4 km h^–1 reduced opacity of discharged air by 50%. The measurement system was also mounted on a separate vehicle and used as a tool for comparing modifications in harvest machine designs where direct measurement of discharge may not be feasible due to mechanical constraints. A comparison between a conventional harvester and one modification in the harvester design found that the machine modification decreased relative dust intensity by 73%. The measurement tools described in this work can be used to provide rapid feedback on harvester operating conditions, orchard cultural practices, and machine design modifications.

Abbreviations: PM, particulate matter • PM2.5, particulate matter with aerodynamic diameter of 2.5 µm or less • PM10, particulate matter with aerodynamic diameter of 10 µm or less • PTO, power take-off • SJV, San Joaquin Valley

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 
DUST generation during agricultural operations is a concern within many areas because soil preparation, cultivation, and harvesting activities can generate visible and respirable dust, especially in the arid areas of California. Agricultural operations for livestock and poultry are increasingly subject to discharge guidelines for ambient particulate matter (PM), and there is growing emphasis on field and orchard crop operations. Additionally, the San Joaquin Valley (SJV) basin in central California is considered a serious air quality nonattainment area for PM matter with aerodynamic diameter of 10 µm or less (PM10) in ambient air (USEPA, 2004). Particulate concentrations in the SJV exhibit seasonal variations; peak levels occur in the fall and winter months (Alexis et al., 2003). These peak levels are coincident with the peak agricultural harvest season, especially for nuts such as almonds and walnuts that are harvested from August through November.

Air quality in certain areas of California is a public issue with respect to visible and suspended ambient PM and human respiratory concerns. Increasingly, concern has focused on almond harvesting as the single highest producing dust emission activities from agricultural production systems. Emission factors (estimated ambient dust loads as PM mass per unit land area) generated from these practices are frequently a parameter for regulatory activity. These factors are often uncertain and greatly generalized and have been reported as high as 45 kg PM10 ha^–1 for nut harvesting operations, whereas the next highest agricultural harvesting estimate is for cotton at 4 kg PM10 ha^–1 (Flocchini et al., 2001). Although almonds are often considered a minor crop on a national basis, the almond industry is significant in the Central Valley of California. Almond production in California is over 220000 ha, and the 2005 almond crop was valued at 2.7 billion U.S. dollars (Almond Board of California, 2006).

Because cultivation and harvesting are essential operations, concern over their contribution to existing ambient dust levels has motivated research and development into mitigation strategies, especially those potentially implemented in the near future and with little disruption of existing practices. Changes in cultural practices, in addition to design and adoption of improved equipment, are strategies potentially compatible with agricultural production and address the need for reduced dust discharge. However, growers and the equipment manufacturers cannot develop and adopt such improvements without fundamental, quantitative information on the effects of design or operational changes on emitted dust.

Regulations for ambient air quality and research into emission factors are based on active air samplers and sensitive gravimetric analysis. These measurements are potentially limited in scope because results are typically averaged over long periods and large geographic regions. Ambient gravimetric PM measurements using PM10 and PM2.5 instruments are labor intensive. Filters can become overloaded if dust plumes become too dense, filters are required to be equilibrated under specific conditions before monitoring, and care must be taken when transporting filters back to the laboratory to minimize particle deposition loss. This methodology requires meticulous attention to details and a climate-controlled room for filter equilibration and analysis. Gravimetric analysis limits the number of experiments that can be executed. Also, because results tend to be averaged over long periods, it becomes difficult to determine if dust emission varies across the field, is relatively constant, or is primarily due to intense but transient events.

For the equipment industry to develop improved nut harvesting equipment and for growers to understand how cultural practices they can control would help mitigate dust generation, a more rapid and focused—in time and space—measurement would be valuable. Using a light measurement instrument can reduce laboratory work associated with gravimetric analyses; however, it is unlikely to replace the accuracy and precision of absolute data associated with precise gravimetric methodologies. An in-line sensor could provide real-time information and allow a fast, lower-cost method for evaluating changes in machine design, operation, or orchard cultural factors. One advantage of an in-field, harvester-mounted, optical system over a full-scale emission factor measurement is that many machine and harvest conditions can be tested in a short time and without the complications of waiting for the weather to develop and stabilize in a narrow, acceptable set of conditions. Promising technologies identified from optical measurements can be advanced to the more rigorous gravimetric testing, allowing research and development resources to be more efficiently and strategically invested.

Published data on the basic machine design parameters and operation of nut harvesting equipment are sparse. Southard et al. (1997) reported that altering the harvester design by lengthening the cleaning chain could significantly reduce dust generation. This established that machine modifications are a method of reducing dust generation during harvest. The harvester industry has historically designed equipment with the primary goals of increasing harvest speed and producing the cleanest possible product in the field; dust mitigation has only recently become an additional design consideration. The most desirable, and likely to be adopted, mitigation techniques are those that fit into existing orchards, cultural practices, industry practices, and processing facility capabilities. The premise of one of our project goals was that dust generation from tree nut harvesting, specifically the "pick-up" operation, could be reduced through changes in harvester design and operation. The project included a strong outreach and technology transfer component to provide a continuum from research to industry education to adoption of mitigation techniques. Emphasis was placed on equipment designs that can retrofit to existing harvesting machines and reduce the economic barrier for grower access to the technology.

Preliminary project work established that a real-time monitoring system could be mounted and operated on harvesters to evaluate relative dust intensity during nut pick-up operations. Initial field runs with the monitoring system suggested that correlations existed between emitted dust and harvest speed, machine settings, soil type, and soil conditions. The goal of this project was to establish that a commercially available sensor, mounted on an almond harvester during pick-up operations, could provide real-time estimates of relative dust intensity during different harvesting operations. Additionally, evaluation of machine operating conditions and other cultural practices could be measured within time frames for immediate assessments. The project approach has been to work closely with the harvester industry to facilitate testing of designs and improvements to conventional designs.

The primary goal of this project was to adapt an established, robust particulate monitoring technique for measuring dust intensity to an on-vehicle, in situ system, mounted on the harvester during commercial harvest operations and to instantaneously determine the relative dust intensity in air discharged by harvesting equipment. Concurrent with this goal was to use the system to evaluate the effects of different machine operating conditions on relative dust intensity. A secondary goal of this project, based on the success with the primary goal, was to adapt the monitoring system to a separate ground vehicle for relative dust intensity measurements off the harvester (e.g, in adjacent tree rows during harvest or from nut-sweeping operations where dust is generated from the operation and not just from a point source of air discharge).

The specific objectives of this project were as follows:

Adapt an industrial laser transmissiometer used for opacity measurement to an in-field, agricultural vehicle-mounted dust measurement system for nut harvesting;

Determine the effects of easily adjusted machine operating conditions of ground speed, pick-up chain speed, and nut/debris separation fan speed on the relative dust intensity, as measured by opacity of the emitted air stream during nut pick-up operations;

Determine the effect of a simple modification to a conventional harvester on reducing dust intensity, as indicated by opacity, in the orchard during harvest operations.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 
Nut Harvester
Harvesting nuts involves shaking the tree, allowing nuts to fall to the orchard floor, sweeping the nuts into windrows for solar drying, harvesting the nuts by picking up the windrows, and mechanically and aerodynamically separating the nuts from debris. The field product is then transported to the huller for further processing before entering the market. Figure 1 shows a representative schematic of the nut harvester. The harvester is towed by a tractor (power-take-off [PTO] models) or driven (self-propelled, engine-driven models) over the windrows where nuts are picked up and conveyed onto a cleaning chain. Openings in the cleaning chain allow soil and debris to fall back to the orchard floor while transporting nuts up to a separation fan that removes the remaining soil and debris from harvested nuts before transferring the nuts to a shuttle cart for transport off-site.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Schematic of nut pick-up machine showing nut harvest with in-field soil and debris removal (Flory Industries, 2006).

Laser Transmissiometer
The measurement device used in this study is typically used for industrial stack monitoring (Model FW300; Sick Maihak GmbH, Germany). The device is a two-step direct transmissiometer. Light (an LED emitting visible light at 650 nm) is transmitted across the measuring path to a reflector and back to a photo-diode detector. The light source and photo-diode detector were housed in a connection unit where an electrical signal due to light attenuation from particulate laden air was transformed by a microprocessor into percent opacity or transmission, where 0% opacity (or 100% transmission) implies clean air. Figure 2 shows a conceptual representation of the FW300 monitor and attachment configuration.

View larger version (32K): [in this window] [in a new window]   Fig. 2. Conceptual configuration for measuring dust intensity by optical transmissiometer (from Sick Maihak, 2004).

The transmissiometer unit was interfaced to a laptop computer (Dell Inspiron 4800; Dell Computers, Inc., Austin, TX) based on the manufacturer software (MEPA-FW; Sick Maihak) with menu-driven parameterization for the FW300. Connection between the computer and FW300 connection unit was made with an RS-232 (DB-9) cable. The software allowed instantaneous viewing of opacity data during system measurement. The system response time was set to 1 s (adjustable from 0.1 to 600 s); opacity data were saved at 5-s intervals for later retrieval. System alignment and signal response were normalized based on manufacturer recommendations and software before field deployment.

Harvester-Mounted Measurement System
The FW300 was mounted on the towed harvester; system configuration is shown in Fig. 3 , 4 , and 5 . The primary mounting platform was fabricated from steel and fastened to the harvester with 1.9-cm bolts. This platform was used to stage two 2-kW electrical generators (Model EU2000i; American Honda Motors, Co., Alpharetta, GA), an external purge air blower, and a suction fan for subsampling the separation fan exhaust air/dust from the harvester during nut pick-up activities. The generator (the additional generator was a back-up) supplied power to the optics connection unit and external purge air blower; primary power demand was for the vane blower providing suction for subsampled air/dust transport. The secondary platform mounted on top of the primary platform provided a vibration buffer/dampener between the transmissiometer unit and the harvester. Four vibration dampeners (Model 1S3-013; Goodyear Tire & Rubber Co., Akron, OH) were used to separate the platforms and fasten the secondary platform to the primary base. Nominal inflation pressure for the vibration dampeners was 300 kPa. The secondary platform included an alignment saddle (16-gauge steel, laser cut) for the flange coupling (connection unit side) and device attachment (reflector side) to ensure optical alignment of the system. The flange couplings were faced with 8.9-cm square plates and bolted to the air plenum. Once the optical alignment was set, the flange coupling face plates were welded to the air plenum.

View larger version (111K): [in this window] [in a new window]   Fig. 3. Laser transmissiometer measurement system mounted on nut harvester.

View larger version (90K): [in this window] [in a new window]   Fig. 4. Side view of harvester with sub-sampling tubes placed in air discharge.

View larger version (102K): [in this window] [in a new window]   Fig. 5. Close-up of sub-sampling tubes entry into air-plenum for measuring relative dust intensity.

Dimensions for the air plenum were 8.9 cm width (flange couplings), 25.4 cm thick (subsampling tube entry), and 76.2 cm length (air-flow direction). The air plenum transitioned to a circular outlet 25.4 cm in diameter. This outlet was attached to a transfer box with a transitional hose (25.4 cm diameter, 30.5 cm transitional length). The transfer box enabled the air plenum to be attached to the suction side of the fan. The fan (Model 9LS, 22.9 cm diameter, 39.7 cm wheel, with outlet area of 0.131 m²; Chicago Blowers, Inc., Glendale Heights, IL) provided the suction air flow through the subsampling tubes at the harvester separation fan outlet and air plenum for opacity measurements. Power for driving the suction fan was provided by the towing tractor hydraulic system. Velocities within the subsampling tubes, measured approximately half way between the harvester separation fan outlet and air plenum entry, averaged 21.3 m s^–1. Previous velocity measurements at the harvester separation fan outlet averaged 22.9 m s^–1, indicating that the subsampling tubes and suction fan provided an approximate iso-kinetic flow sample through the air plenum.

Truck Bed-Mounted Measurement System
Based on the experience from mounting the transmissiometer measurement system on the harvester, a similar system was fabricated for a small truck bed. The truck was a circa 1980 five-speed Datsun, nearing or perhaps past its design life. The mounted measurement system is shown in Fig. 6 . This measurement system was independent of the one mounted on the harvester. The primary and secondary platforms and alignment saddle were fabricated in a similar fashion as for the harvester-mounted system. The primary platform was welded to the truck bed. The secondary platform was fastened to the primary base in the same manner as discussed previously. A flat steel plate (20 gauge) branch guard was placed on the top of the truck bed and welded to 2.54-cm square steel supports that were also welded to the truck. All component connections and external purge air and generator configurations were similar to the system mounted on the harvester. The air plenum configuration was the only difference between the systems.

View larger version (85K): [in this window] [in a new window]   Fig. 6. Transmissiometer measurement system mounted on small truck-bed (A) and close-up of measurement system (B).

Field Opacity Measurements and Dust Concentration
The measurements provided by the transmissiometer are not an absolute measurement of the actual dust concentration (mass dust per unit of air volume). However, the relationship between transmissiometer data and dust concentration has been theoretically established through the Lambert-Beer Law (e.g., Sick Maihak, 2004). With the optical value of opacity (as used in this report) being 1 – transmission (in percent), the relationship between dust concentration and ln (1/transmission) is linear. However, this relationship holds only for a constant particle size, particle density, and spatial distribution of dust within the measurement pathway. So, although opacity can be rigorously related mathematically to absolute concentration under ideal and laboratory conditions, the field conditions within the orchard environment and the harvester discharge and the physical properties of the dust entrained within the emitted harvester air are so heterogeneous that direct concentration estimates cannot be made within acceptable levels of confidence. However, opacity does provide, by definition, a reliable measurement of visual appearance of the dust cloud and some insight into total volumetric loading of dust and other small debris within the air flow. These measurements are valuable in guiding stewardship practices by growers to mitigate visual effects from harvesting and to quantify the nominal effects of changes in harvester design and grower practices on total material loading within the emitted air plume.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 
Harvester Operating Conditions
Initial field studies during the 2004 harvest season resulted in several observations. The harvester-mounted monitoring system proved feasible as a measurement tool for establishing and evaluating changes in opacity, or relative dust intensity, based on cultural and mechanical factors. Dry, loose, soils were found to generally create high opacities during harvest at ground speeds of 4.8 km h^–1; increasing ground speed increased the relative dust intensity during harvest, whereas lowering ground speed had an adverse effect. Additionally, hard compacted soils were found to have relatively low dust intensity measurements during harvest operations for all ground speeds.

A representative response of opacity measurements for one row during nut pick-up operations is shown in Fig. 7 . This general type of opacity measurement signature was observed for all harvesting conditions. Initially, as the harvester engaged the fan and began forward movement within the row, picking up nuts, and separating out dust and debris, opacity increased. As the harvester moved along the row, a relatively constant opacity level was observed (when field conditions were unchanged along the row). At the end of the row, the harvester slowed down and came to rest, disengaging the fan. Opacity returned to that of clean air.

View larger version (15K): [in this window] [in a new window]   Fig. 7. Typical in-row opacity during nut pick-up operations showing harvester start-up, harvesting, and disengaged fan.

Results from the windrow preparations based on sweeper head depth and resultant ground speed of the harvester during nut pick-up operations are shown in Fig. 8 . At a harvest ground speed of 2.4 km h^–1, relative dust intensity decreased by 55% when the sweeper head depth was set at the orchard floor surface. Harvesting nuts at 4.8 km h^–1 ground speed resulted in a 32% reduction in relative dust intensity when setting the sweeper head height at the orchard floor surface. A comparison of the two ground speeds during harvest, after sweeping with the conventional setting, resulted in a 52% decrease in relative dust intensity for the slower harvesting ground speed.

View larger version (14K): [in this window] [in a new window]   Fig. 8. In-row opacity during harvesting at 2.4 (A) and 4.8 (B) km h–1 after standard and deep sweeping pre-harvest operations (start-up and end row opacity effects removed from data results).

View larger version (13K): [in this window] [in a new window]   Fig. 9. Effect of fan speed and pick-up chain speed on in-row opacity during harvest at 4.8 km h–1 ground speed.

View larger version (120K): [in this window] [in a new window]   Fig. 10. Harvester modification attached to separation fan outlet on the harvester; nylon mesh bag was 3 m long and had 2-mm square openings with 20 openings cm–2.

View larger version (11K): [in this window] [in a new window]   Fig. 11. Comparison of in-row opacity for a conventional harvester and modified harvester (modification was nylon mesh bag fastened to harvester separation fan outlet).

	Discussion and Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

This project adapted current industrial technology to a mobile agricultural operation, integrating it into an in situ, on-vehicle system to quantify machine performance effects on opacity of an air plume emitted during nut harvesting. This will guide manufacturers and growers to assess the effects of changes in machine designs, soil types, or harvest practices. The advantage of the measurement system developed in this study is that tests can be conducted quickly, and data are available immediately and can be used to make decisions on experiments, often those to be run within minutes. The disadvantage is that the technique does not directly measure emission factors commonly used to regulate industrial emissions. However, the technique is intended as a screening tool to identify and aid refinement of alternative designs and practices that can be advanced to the more challenging and resource-intensive emission factor measurement field studies.

Testing conditions found that differences in orchard location, soil type, and harvest speeds resulted in varying air opacity values as determined by the measurement instrument. However, dust generated at the harvester on a per unit time basis must be corrected by the actual time required for harvest to develop conclusions on the effects of speed and potential dust generation at the machine source per unit land area or per unit of harvested crop.

The success of adapting industrial instrumentation to this agricultural field operation and the determination of simple, low-cost, immediate strategies to reduce visible and nuisance dust generation from dust-intensive agricultural operations is encouraging. Results from this project suggest that reduced environmental effects can be achieved with low investment and little disruption to time-critical operations like harvesting.

Future work using the monitoring systems developed in this study will continue to evaluate harvesting practices and other cultural factors that have an effect on relative dust intensity generated during nut pick-up operations. The results from this measurement tool can be used for immediate feedback on potential dust intensity based on soil conditions, soil types, and harvester operating conditions.

	ACKNOWLEDGMENTS

 
Project funding was provided by the California Agricultural Experiment Station and the Almond Board of California. This material is based on work supported by the National Research Initiative Air Quality Program of the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture under Agreement No. 05-35112-15329. The Research and Development Section of Flory Industries, Salida, CA collaborated on development, installation, and field testing of the laser transmissiometer; their dedicated involvement is gratefully acknowledged.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 
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	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion and Conclusions REFERENCES

 

• Alexis, A., M. Benjamin, V. Bhargava, P. Cox, M. Johnson, A. Lin, D. Look, C. Hguyen, and M. Nystrom. 2003. The 2003 California almanac of emissions and air quality. California Air Resources Board, Sacramento, CA.
• Almond Board of California. 2006. Available at http://www.almondboard.com (verified 6 Mar. 2007).
• Flocchini, R.G., R. Southard, and K. Scow. 2001. Sources and sinks of PM10 in the San Joaquin Valley (Interim Report), a study for United States Department of Agriculture Special Grants Program. Contract Nos. 94-33825-0383 and 93-38825-6063. Cooperative State Research, Education, and Extension Service, Washington, DC.
• Flory Industries. 2006. Available at http://www.floryindustries.com/prod_harvester_7488.htm (verified 6 Mar. 2007).
• Sick Maihak. 2004. FW300 dust concentration monitor operating instructions. Sick Maihak, GmbH, Reute, Germany.
• Southard, R.J., R.J. Lawson, H.E. Studer, and M. Brown. 1997. Modified almond harvester reduces orchard dust. Calif. Agric. 51 :10–13.
• USEPA. 2004. Fact Sheet: Proposed approval of San Joaquin's 2003 PM-10 Plan Elements. USEPA, Washington, DC. Available at http://www.epa.gov/region09/air/sjvalleypm/fact042904.pdf (verified 6 Mar. 2007).

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Downey, D. Articles by Thompson, J.F. Search for Related Content PubMed Articles by Downey, D. Articles by Thompson, J.F. Agricola Articles by Downey, D. Articles by Thompson, J.F. Related Collections Field evaluation techniques Agricultural Systems Air Pollution