Ammonia and hydrogen sulfide emissions from swine production Ammonia and hydrogen sulfide emissions from swine production facilities in North America: a meta-analysis facilities in North America: a meta-analysis

Ammonia (NH 3 ) and hydrogen sulfide (H 2 S) emissions from swine production facilities receive considerable attention due to human health and environmental implications. Accurate quantification of farm emissions is essential to ensure compliance with regulatory requirements. The objectives of this study were to provide a review of the literature on NH 3 and H 2 S emissions from swine production facilities in North America with a meta-analysis that integrates results of independent studies, including measured emissions data from both swine houses and manure storage facilities as well as concentration data in the vicinity of swine production facilities. Results from more than 80 studies were identified through a thorough literature search, and the data were compiled together with results from the 11 swine sites in the National Air Emissions Monitoring Study (NAEMS). Data across studies were analyzed statistically using the MIXED procedures of SAS.


Introduction
Air emissions from swine production facilities receive considerable attention due to human health and environmental implications.Major farm emissions of interest include ammonia (NH 3 ) and hydrogen sulfide (H 2 S).The H 2 S is of interest mainly at the local level because of health concerns, whereas NH 3 has regional-scale impacts on ecosystems.Air emissions from industries are subject to permit requirements under the Clean Air Act (CAA) as well as reporting requirements under the Emergency Planning and Community Right-to-Know Act (EPCRA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) if emissions reach specified thresholds; for example, operations that exceed 100 lb/d NH 3 or H 2 S emissions are required to report under EPCRA.Accurate quantification of farm emissions is essential to ensure compliance with the regulatory requirements, but direct measurements of farm emissions are expensive and difficult.Fortunately, a large volume of published studies on NH 3 and H 2 S emissions from swine production facilities are available for a meta-analysis.Meta-analysis is a quantitative statistical analysis of a collection of results from individual previous studies for the purpose of integrating the findings.Results from meta-analyses are usually more robust and have less bias than individual studies because of improved statistical power.
The objectives of this study were to provide a review of the literature on NH 3 and H 2 S emissions from swine production facilities in North America, with a meta-analysis that integrates results of independent studies, including measured emissions data from both swine houses and manure storage facilities as well as concentration data in the vicinity of swine production facilities.

Literature search and data extraction
Multiple strategies were undertaken to identify potentially eligible studies to be included in the meta-analysis.The inclusion criteria were that studies must have been conducted in North America and must have reported measured NH 3 or H 2 S emissions data from swine production facilities, including manure storage systems, or concentration data in the vicinity of swine facilities.Data from reports of the 11 swine sites in the National Air Emissions Monitoring Study (NAEMS) were included in the database.Two individuals independently conducted the search processes and screened the studies by reading the title and abstract to select studies for full review according to the inclusion criteria.
The included studies were distributed to a group of reviewers for data extraction.Standard data extraction sheets were developed for consistency.Some studies provided emissions data from different sites or settings; in these cases, more than one data point was extracted from one study.Each study was reviewed in duplicate by two independent reviewers for quality control.After the data review and extraction processes, a metaanalysis database was created.Emissions data for NH 3 and H 2 S were compiled into the two emission sources (swine houses and manure storage facilities).Concentration data were compiled separately and included sampling locations and distances from emission sources.

Data analysis
Various units of emissions data have been used in the literature.To perform statistical analysis and compare emissions data between different studies, the units of measured emissions data were converted to kg/year per pig and kg/year per AU (AU is an animal unit corresponding to 500 kg of body mass) for emissions from swine houses and to kg/ year per pig and kg/year per m 2 for emissions from manure storage facilities.When unit conversion was not possible due to lack of key information, the original emissions data were excluded from statistical analysis.A full list of included studies and completed data extraction spreadsheets are available to allow for independent scrutiny of the process.
Data across studies were analyzed statistically using the MIXED procedures of SAS (SAS for Windows, Version 9.3, SAS Institute, Inc., Cary, NC).Study (or each publication) was treated as a random variable because some studies contain multiple data points.The ratios of emissions rate over SD were used as a weighting variable such that data points with relatively small SDs were given more weight in the analysis.Effects of production stage and manure handling/storage system on emissions rates were examined using Tukey's test.Significant effects were declared at P < 0.05.Multi-linear regression models were developed for certain emission sources to reflect the effects of indoor or ambient air temperature, average pig weight, size of operation (number of pigs), area of manure storage, recharging interval of manure pits, etc.A backward-elimination process was used to remove the confounded terms and to reduce non-significant terms one by one.When a regression model failed to pass normality tests, a natural log transformation was applied to the response variable (emission rate).

Statistics of NH 3 and H 2 S emissions from swine houses and manure storage facilities
The ranges, means, and medians of NH 3 and H 2 S emission rates for swine houses and manure storage facilities are presented in Table 1.Large variations in emission rates were observed.Histograms of NH 3 and H 2 S emission rates for swine houses and manure storage facilities all showed a positively skewed distribution.The median emission rates were believed more robust, and the means were all larger than the medians due to a few large values.For swine houses, the median NH 3 emission rate was 2.78 kg/ year per pig, whereas the highest emission rate was 11 times higher; the median H 2 S emission rate was only 0.09 kg/year per pig, but the highest emission rate was 35 times higher.For swine manure storage facilities, the median NH 3 emission rate was 2.08 kg/ year per pig, whereas the highest emission rate was 11 times higher; the median H 2 S emission rate was only 0.20 kg/year per pig, but the highest emission rate was 7 times higher.

Emission rates from swine houses: Effects of production stage and manurehandling system
Means and least squares means of NH 3 and H 2 S emission rates from swine houses for various production stages and manure-handling systems are presented in Table 2. Swine hoop houses had significantly higher NH 3 emission rates than other manure handling systems (P < 0.01 for NH 3 emission rates in both kg/year per pig and kg/year per AU).Effects of production stages (gestation, farrowing, nursery, or finishing) were not significant for NH 3 emission rates from swine houses (P = 0.23 and 0.15 for NH 3 emission

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rates in kg/year per pig and kg/year per AU, respectively).Deep-pit houses had higher H 2 S emission rates than other manure-handling systems (P = 0.03 and <0.01 for H 2 S emission rates in kg/year per pig and kg/year per AU, respectively).Farrowing houses had the highest H 2 S emission rates, followed by gestation houses, and finishing houses had lowest H 2 S emission rates, regardless of whether emission rates were expressed in kg/year per pig or kg/year per AU (P < 0.01 in both cases).

Emission rates from manure storage facilities: Effects of production stage and storage type
Means and least squares means of NH 3 and H 2 S emission rates from manure storage facilities for various production stages and storage types are presented in Table 3.No storage type or production stage effects were observed for NH 3 emission rates (in kg/ year per pig, P = 0.45 and 0.24, respectively; or in kg/year per m 2 , P = 0.75 and 0.30, respectively), or H 2 S emission rates (in kg/year per pig, P = 0.47 and 0.13, respectively; or in kg/year per m 2 , P = 0.06 and 0.60, respectively).

Regression models for NH 3 and H 2 S emission rates
Regression models for NH 3 and H 2 S emission rates were developed for deep-pit finishing houses, finishing houses with recharge pits, and lagoons for finishing operations (Table 4) to reflect the effects of indoor or ambient air temperature, average pig weight, size of operation (number of pigs), area of manure storage, recharging interval of manure pits, etc.The indoor air temperatures ranged from 8 to 28 o C; average pig weights ranged from 21 to 249 kg; number of pigs ranged from 6 to 13,680; recharge interval of manure pits ranged from 1 to 42 d; ambient air temperatures ranged from 2 to 32 o C; and areas of lagoons ranged from 1,131 to 97,600 m 2 .
For finishing houses with deep pits or recharge pits, NH 3 emission rates were positively related to indoor air temperature.Finishing operation lagoons had NH 3 emission rates that were positively related to ambient air temperature (P < 0.01).Effects of temperature on H 2 S emission rates were not significant.The recharge interval of manure pits in finishing houses significantly affected H 2 S but not NH 3 emission rates.Swine houses with pits that had longer recharge intervals emitted more H 2 S (P < 0.01).The NH 3 and H 2 S emission rates from swine houses in kg/year per pig increased with increasing pig weights.When expressed in kg/year per AU, NH 3 emission rates were no longer influenced by pig weight, but for finishing houses with recharge pits, H 2 S emission rates in kg/year per AU remained positively related with pig weight (P = 0.01).The H 2 S emission rates were also influenced by size of operation.Deep-pit finishing houses with larger pig numbers tend to have higher H 2 S emission rates in kg/year per AU (P = 0.02).

Swine farm sizes that may trigger the need to report NH 3 or H 2 S emissions
The EPCRA and CERCLA require reporting of NH 3 and H 2 S emissions that exceed 100 lb/d.Swine farm sizes that may trigger the need to report NH 3 and H 2 S emissions under EPCRA and CERCLA were calculated and are presented in Table 5.

SWINE DAY 2013 NH 3 concentrations in the vicinity of swine facilities
The average NH 3 concentration at the edge of the emission sources (swine houses or lagoons) was 5.5 ± 5.2 ppm (ranging from 0.3 to16 ppm), which is higher than the acute minimum risk levels (MRL) for NH 3 (1700 ppb4 ).The ambient NH 3 concentrations in the vicinity of swine facilities decreased quickly to be less than the chronic MRL (100 ppb) as distances from emission source increased (Figure 1).At distances of 30 to 1,185 m from emissions sources, the average ambient NH 3 concentration was 66 ± 66 ppb (ranging from 10 to 280 ppb).In comparison, the average background ambient NH 3 concentration outside swine production areas was 7.7 ± 3.5 ppb, whereas Godbout et al. (2009 5 ) and Donham et al. (2006 6 ) reported the average ambient NH 3 concentration within swine production areas was 11.8 ± 5.5 ppb.The average ambient NH 3 concentration in the vicinity of swine facilities (66 ± 66 ppb at distances from 30 to 1,185 m) was about 8 times higher than the average background ambient NH 3 concentration in areas not influenced by swine production facilities (7.7 ± 3.5 ppb).

H 2 S concentrations in the vicinity of swine facilities
The average H 2 S concentration at the edge of the emission sources (swine houses or lagoons) was 40 ± 48 ppb (ranging from 0.9 to146 ppb), which is less than the acute MRL (100 ppb) but higher than the intermediate MRL (20 ppb) for H 2 S7 .The ambient H 2 S concentrations in the vicinity of swine facilities decrease quickly to be less than 20 ppb as distances from emission sources increase (Figure 2).The average ambient H 2 S concentration was 3.1 ± 6.2 ppb at the distances of 30 to 1,185 m from emission sources.In comparison, Godbout et al. (2009 8 ) and Donham et al. (2006 9 ) reported average ambient H 2 S concentrations of 1.9 ± 1.1 ppb in areas not influenced by swine production facilities.

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07017T a ) NH 3 emission rates in kg/year per m 2 = EXP (-1.3843+0.07373T a )Note: AU = animal unit corresponding to 500 kg body mass; T i = indoor air temperature in swine houses, o C; T a = ambient air temperature, o C; W = average weight of pigs, kg; N = number of pigs in the farm; R = recharge interval of manure pits, in days.

Table 1 .
Statistics of NH 3 and H 2 S emissions from swine houses and manure storage facilities

Table 4 .
Regression models for NH 3 and H 2 S emission rates from various emission sources

Table 5 .
Sizes of swine farm that may trigger the need to report NH 3 or H 2 S emissions