In order to ensure compliance with the current and future air quality regulatory requirements, and to co-exist with increasing numbers of neighbors, practical and cost effective air mitigation options should be available to producers. Vegetative environmental buffers (VEBs) are a potential cost-effective strategy for reducing multiple air pollutants from farms. The VEBs can function as a living bio-filter as well as a windbreak. However, effectiveness of VEBs in reducing air pollutants is highly variable and usually depends on site specific design. Lack of information on performance and technical guidelines are barriers to adoption of VEBs. In this study, the effectiveness of VEBs in reducing multiple air pollutants from a swine facility was tested. Red Cedars were planted to form a line of VEB which is 120-150 feet away from the ventilation fans of the swine barn. Gas concentrations at various downwind distances from the swine facility were monitored with and without the VEBs to determine the effectiveness of VEBs in reducing air pollutants. The following three designs of the VEBs were tested to investigate the effects of height and depth of the VEBs on the results: one row of trees at 8 feet height, one row of trees at 12 feet height, and paralleled three rows of trees at 8 feet height. The results showed that the VEBs were effective in reducing downwind concentrations of hydrogen sulfide (H2S), ammonia (NH3), nitrous oxide (N2O), and methane (CH4). And the 3-row-8′ VEB was most effective as comparing with the other two designs. When wind speeds were lower than 1.5m/s, the 3-row-8′ VEB was able to reduce downwind concentrations by up to 60%, 33%, 26%, and 51%, for H2S, NH3, N2O, and CH4, respectively. No reduction on volatile organic compounds (VOCs) and odor was observed. As expected, how much air pollutants can be reduced depends on the thickness of the VEBs, while the downwind distance from the VEBs within which the reduction is effective depends on the height of the VEBs (the barrier height). For H2S, the reduction was no longer effective when the downwind distance was beyond 160 feet or 20 times the barrier height from the 3-row-8′ VEB. Moreover, the reduction effectiveness for H2S was sensitive to wind speed. As wind speed increased, the reduction effectiveness decreased. When wind speeds were larger than 3.5 m/s, higher downwind H2S concentrations were observed with VEBs (especially, for the single-row VEBs) as compared with the control scenario (no VEB), which could be due to unwanted turbulence or downwash effect caused by the VEBs at high wind speeds. The effect of VEBs on downwind PM10 concentrations was more complex than expected. On the one hand, 23% PM10 reduction across the 3-row-8′ VEB was observed; on the other hand, higher downwind PM10 concentrations were observed with VEBs as compared with the control scenario (no VEB), which could be due to reduced air movement associated with the VEBs. Further investigation is needed to confirm this observation. It is anticipated that further increasing the thickness of the VEBs could result in further reduced downwind PM10 concentrations, and thus could ensure the reduction of downwind PM10 concentrations as compared with the control scenario (no VEB). In summary, (1) Adequate thickness of the VEBs is very important in order to secure the expected effectiveness of VEBs in reducing air pollutants; and multi-row VEBs are recommended. (2) The downwind distance from the VEBs within which the reduction is effective could be estimated by multiplying the height of VEBs by 20; and reversely, the required height of the VEBs could be estimated when there is a sensitive location downwind of the VEBs needs to be protected. (3) At high wind speed, a single row VEB could result in higher downwind concentrations due to turbulence and downwash effect induced by the VEB. A multi-row design is desired to increase the effectiveness of filtering mechanism and thus overcome this effect. (4) More sophisticated investigation is needed to quantify the effect of VEBs in reducing VOCs and odor.
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