Role of Vegetation in Mitigating Air Emissions Across Industrial Sites

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Role of Vegetation in Mitigating Air Emissions Across Industrial Sites in the US Varsha Gopalakrishnan, Guy Ziv, and Bhavik R Bakshi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04360 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Sustainable Chemistry & Engineering

Role of Vegetation in Mitigating Air Emissions Across Industrial Sites in the US Varsha Gopalakrishnan1∗, Guy Ziv2 , Bhavik R. Bakshi1† 1

Lowrie Department of Chemical and Biomolecular Engineering The Ohio State University, Columbus, OH 43210, USA 2

School of Geography University of Leeds, Leeds LS2 9JT, UK December 12, 2018

Abstract Despite wide adoption of pollution control technologies, industrial facilities emit about half the criteria air pollutants in the US and contribute to poor air quality in many regions. Vegetation such as trees, shrubs, and grasses also have the capacity to directly remove air pollution. This work assesses the role of vegetation, particularly trees, in mitigating air emissions near point sources at nearly 20,000 sites across the US. Additional mitigation capacity due to ecological restoration to the average local vegetation is also determined. Comparing emissions with the average uptake capacity at each site indicates that currently most sites in the southeastern part of the US have enough vegetation cover to offset most emissions while industrial facilities, particularly those in the western part of the country can benefit the most from restoration. A relatively large fraction of sites in the Mining, Quarrying and Oil & Gas Extraction, Transportation and Warehousing, and Management of Companies and Enterprises sectors have enough current or restored capacity to mitigate their emissions. Land around facilities in the Finance and Insurance, Real Estate, and Retail and Leasing sectors lacks much capacity. Such results encourage further work toward sustainable engineering by seeking synergies between industrial and ecological systems.

Introduction Air pollution impacts people across the world, especially those living in cities with lowto middle- income. A recent report by the World Health Organization (WHO) estimated ∗ †

Currently at Ramboll, San Francisco, CA USA Corresponding author; Tel: +1 614 292 4904; E-mail: [email protected]

1

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that more than 92% of the global population live in regions where the air quality is worse than WHO ambient air quality limits, primarily for particulate matter (PM2.5 ). 1 In the US, extreme levels of air pollutants have also resulted in poor air quality and severe health issues mainly due to the presence of six Criteria Air Pollutants (CAP): Carbon monoxide (CO), Nitrogen dioxide (NO2 ), Lead (Pb), Ozone (O3 ), Particulate matter (PM10 , PM2.5 ) and Sulfur Dioxide (SO2 ). Industries are among the major sources of air emissions in the US, contributing to about 21% of GHG emissions and close to 50% of CAP resulting in a need to identify novel ways to mitigate these emissions. 49% of industrial air emissions are from the electricity generating sector, followed by the manufacturing sectors. Air emissions leaving the stack get dispersed into the atmosphere resulting in a reduction in ambient air quality mainly in the vicinity of industrial sites. Pollutants like NO2 , more generally NOx , and SO2 either get dry deposited on surfaces in the troposphere or react with water and oxygen to form nitric acid and sulfuric acid, the primary contributors to acid rain. NO2 emissions are also precursors of ozone that cause photochemical smog. PM10 and PM2.5 particles may either get washed out of the atmosphere along with acid rain by wet deposition on surface or oceans, or they undergo direct deposition on buildings, monuments, trees and any surface that may intercept these particles via dry deposition. Thus, effective control and direct removal of pollutants from the atmosphere is essential to maintain air quality and prevent excessive damage. Efforts to reduce air pollution have been ongoing since the enactment of the Clean Air Act of 1970. Since then, drastic changes in air quality due to the implementation of local and federal regulatory programs to meet set targets have been achieved. These include development of State Implementation Plans (SIPs) to attain and maintain the National Ambient Air Quality Standards (NAAQS) at local and regional scales, policies like the New Source Performance Standards (NSPS) for stationary sources and stringent tailpipe emission standards for vehicular emissions. For industrial sites, most efforts for reducing environmental impact have focused on improving system efficiency to directly reduce emissions and the use of end-of-pipe controls such as ‘Best Available Control Technology’ (BACT). Successful implementation of control technologies such as scrubbers, catalytic converters, particulate filters and flue gas desulphurization units have helped in maintaining air quality standards, enough to meet regulatory limits. However, despite such advances in technology, further reduction in air pollution is still needed in many areas. Excessive air pollution has resulted in acidification of lakes, eutrophication of local water bodies and coastal waters, and accumulation of mercury in aquatic food webs. 2 Nature-Based Solutions (NBS) including green infrastructure methods like trees, grasslands, green roofs and urban farms are being increasingly used for improving air quality through direct interception of particles from the atmosphere onto their leaf surface. 3,4 Several studies have highlighted the benefits of using urban forests in cities 5 and trees for the removal of significant amounts of pollutants from the atmosphere. 6,7 These solutions have also proven to be more cost-effective than conventional control measures for different pollutants and at different scales. 8,9 Studies have also pointed out the benefits of strategic tree planting and reforestation as an ozone abatement strategy especially in non-attainment areas. 10,11 Industries are also starting to acknowledge the economic and environmental benefits of using NBS. For example, The Nature Conservancy (TNC) and Dow Chemical Company 2


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have analyzed the economic and environmental benefits of land restoration to improve and regulate air quality in an industrial setting. 12 Industrial facilities often have land that is mostly left barren. Judicious restoration and protection methods of these sites can provide ecosystem services that are beneficial to both the company and society. The goal of this work is to assess the capacity of NBS such as restoration of open-space around industrial sites with locally appropriate canopy cover, to mitigate air emissions in the vicinity. Gaussian plume models are used to predict the diffusivity of particles from point sources within a 500 m boundary from the source, and land within this boundary is considered for restoration. To the best of our knowledge, this is the first work that assesses the role of vegetation to mitigate air emissions across a large fraction of industrial sites in the US. These benefits are assessed for industries belonging to different North American Industrial Classification System (NAICS) sectors on a regional basis. Under steady state conditions, results from the study indicate that several industrial sites in the South East already have enough land cover to offset emissions, while restoration is likely to be most beneficial for industries in the West and Southwest.

Materials and Methods To determine whether reforestation can be adopted as a method of reducing air pollution from industries, we considered facilities that emit NO2 , PM10 , and SO2 from point sources in the lower 48 states and quantified the capacity of current and restored vegetation around each site to take up pollutants.

Demand of Air Quality Regulation Ecosystem Service Demand for the air quality regulation ecosystem service from point sources in the conterminous US were obtained from the National Emissions Inventory (NEI) 2011 database 13 for NO2 , PM10 and SO2 . A total of 67,894 point sources from various NAICS sectors including Agriculture, Forestry, Fishing, Transportation and Warehousing, Mining etc. were included in the analysis. Some point sources in the Gulf of Mexico, Hawaii, and Alaska that outside the conterminous US were excluded from the analysis. Information on the latitude and longitudinal position of the stack and the total quantity of pollutants leaving the stack were obtained from the NEI database. Each of these facilities were classified as being rural or urban depending on their location and population in the locality. Facilities located in areas with a population of at least 2,500, with at least 1,500 residing outside institutional group quarters were classified as urban areas while the rest were classified as rural areas. 14 This classification was assigned to each point source to account for the variation in sequestration rates for air pollutants by vegetation. Emissions rate for each source in the database was converted from short tons per year to units of kg per year and all results presented in this paper are in units of kg of pollutant per year.

3



Agriculture, Forestry, Fishing, Hunting

Real Estate Rental & Leasing

Wholesale Trade

Health Care & Social Assistance

1.00

1.00

1.00

1.00

0.75

0.75

0.75

0.75

0.50

0.50

0.50

0.50

0.25

0.25

0.25

0.25

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Mining, Quarrying & Oil & Gas Extraction

Percentage of Facilities

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2

0.3

0.4

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Prof., Sci., & Tech. Services

Retail Trade 1.00

1.00

1.00

0.75

0.75

0.75

0.75

0.50

0.50

0.50

0.50

0.25

0.25

0.25

0.25

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Utilities

0.2

0.3

0.4

0.5

Transportation & Wareshousing

0.00 0.005 0.1

0.2

0.3

0.4

0.5

Mgmt. of Companies & Enterprises

0.00 0.005 0.1

1.00

0.75

0.75

0.75

0.75

0.50

0.50

0.50

0.50

0.25

0.25

0.25

0.25

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Construction

0.2

0.3

0.4

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

1.00

1.00

1.00

0.75

0.75

0.75

0.75

0.50

0.50

0.50

0.50

0.25

0.25

0.25

0.25

0.2

0.3

0.4

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Finance & Insurance

Manufacturing

0.2

0.3

0.4

0.5

0.00 0.005 0.1

Education Services

1.00

1.00

1.00

0.75

0.75

0.75

0.75

0.50

0.50

0.50

0.50

0.25

0.25

0.25

0.25

0.2

0.3

0.4

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.3

0.4

0.5

0.2

0.3

0.4

0.5

0.2

0.3

0.4

0.5

Public Administration

1.00

0.00 0.005 0.1

0.2

Other Services

1.00

0.00 0.005 0.1

0.5

1.00

Admin., Supp., Waste Mgmt. & Rem. Services

Information

0.4

Acco. & Food Services

1.00

1.00

0.3

Arts, Entertainment & Rec.

1.00

0.00 0.005 0.1

0.2

0.5

0.00 0.005 0.1

0.2

0.3

0.4

0.5

Footprint in Sq.km Figure 1: Percentage of facilities with a land footprint < 0.5 sq.km. Red lines represent fraction of facilities versus their minimum land footprint for NO2 , PM10 and SO2 while the blue lines represent fraction of facilities versus their maximum land footprint for NO2 , PM10 and SO2 . 4


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Supply of Air Quality Regulation Ecosystem Service Air quality regulation ecosystem service by canopy cover was estimated based on the capacity of trees to sequester pollutants directly from the atmosphere. Sequestration rate for each pollutant was obtained from the i-Tree canopy database for rural and urban regions in every county in the US for 2010. This database developed by The Davey Institute and the U.S Forest Service 15 contains comprehensive estimates of air pollution sequestration by canopy cover in rural and urban regions in the lower 48 states. The i-Tree tool estimates pollution removal by vegetation based on the the total tree cover and leaf area index. Percent tree cover for each county was based on evergreen, deciduous and mixed forest cover in each county. Hourly pollution removal is estimated as a function of the hourly pollutant concentration in the atmosphere and the deposition velocity of the pollutant on the leaf surface. 7 The hourly pollutant concentration is obtained from the closest monitoring station located in each county, and it accounts for diurnal variations of meteorological parameters and sequestration rates. This hourly concentration is assumed to be representative of the county-level average concentration and it represents the best available data. These hourly concentrations may be different from the air concentration around each specific site but estimating site specific concentrations and sequestration rate is beyond the scope of this study. Further details on air pollution removal by trees and different vegetation classes can be found in the following studies 7,16 and in the Supporting Information. Figures S1 - S3 show the spatial variation in pollutant sequestration across each county, and Figure S4 shows the spatial variation in canopy cover in the US. Pollutant sequestration around each site is calculated based on the sequestration rate and the total land footprint.

Restoration Area Around Industrial Sites To determine the land footprint or the restoration area around industrial facilities to offset emissions, air pollutant sequestration rate by canopy (Fi ) across each county in the US was used. As discussed previously, removal rates of pollutants were obtained from the i-Tree canopy database. 15,17 For each point source, a minimum and maximum land footprint for each pollutant type was determined based on the emissions rate and sequestration per unit of tree cover. These two parameters provide insight into the minimum and maximum land that needs to be restored around facilities to displace emissions leaving the stack. The minimum land footprint is the smallest area needed to offset one of the three pollutants. It is calculated as, Di,f i = NO2 , SO2 and PM10 (1) Λf,min = M ini Fi where Di,f represents the emissions demand of pollutant i in units of kg/year and Fi represents the canopy sequestration rate of air pollutant i in g/m2 /yr depending on the county in which each facility is located and the type of region as rural or urban. Minimum land footprint for 43 facilities were calculated to be larger than the county area. This is particularly true for facilities belonging to sectors 22 (Utilities) and 31-33 (Manufacturing) due to their large volume of emissions. These sectors were filtered from the analysis.

5



Similarly, the maximum land footprint is calculated as, Di,f i = NO2 , SO2 and PM10 Λf,max = M axi Fi

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(2)

The maximum land footprint is the largest area needed to offset all the three pollutants. Λf,max was observed to be larger than the county area for a total of 1,363 out of 67,894 facilities, and hence equation 2 also holds true only for facilities with footprint lower than the county area. Facilities with a minimum land footprint (Λf ) between 0.005 - 0.5 km2 i.e. between 1.23 to 123 acres for the three pollutants were considered for further analysis. This filtering was performed to keep the restoration scenario economically viable for the facilities since they would only need to change land cover on land that is likely to belong to them, as opposed to land than belongs to others. Figure 1 represents the percentage of facilities in each sector that have a land footprint between 0.005 - 0.5 km2 . For each sector, the red line represents the percentage of facilities versus their minimum land footprint and the blue line represents the percentage of facilities versus their maximum land footprint. This filtering scheme of selecting facilities with footprints between 0.005 and 0.5 km2 was used due to lack of availability of land ownership information for each facility. However, this may also lead to elimination of facilities that own large amounts of land around their point sources. Information on land ownership by different industrial sites will provide a better estimate of the availability of land for restoration. From this figure, we can see that a higher number of facilities with a smaller emissions demand (Management of companies and enterprises, Education Services, Health Care & Social Assistance, Accommodation, and Food services) have a footprint < 0.5 km2 compared to facilities that may be large emitters (Mining, Utilities, Manufacturing). A total of 18,500 point source facilities distributed across the lower 48 states were considered for further analysis. Table S1 in the Supporting Information lists the NAICS sectors and number of facilities in each sector. Figure S5 in the Appendix shows a map of all the facilities analyzed in this work. Emission from the 18,500 facilities analyzed in this work contribute to 7% of the total NO2 and PM10 , and < 1% of the SO2 emissions. However, sector specific emissions contribution from the 18,500 facilities were observed to be as high as 78% for some sectors. The transportation and warehousing sector (sector 48-49) contributed highest to the emissions inventory, followed by the mining sector (sector 21) and the manufacturing sector (sector 31-33). More details on the sector-level contribution to emissions are discussed in the Results Section.

Determination of buffer distances To determine a reasonable buffer distance for land restoration around facilities, the maximum pollutant concentration downwind of the source location for some sample facilities was calculated. Gaussian dispersion equations for point source emissions were used to model dispersion of pollutants from the sample facilities. Pollution concentration downwind of the source was estimated for NO2 , SO2 and PM10 . Section ’Determination of Buffer Distance’ in the Supporting Information provides more details on the buffer distance calculations. 6


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Figure 2: Facility in Nebraska with a 500 m buffer around point source (Facility ID 7705511). We chose a radius of 500 meter to be the region around the point source that can supply the ecosystem service of air quality regulation to the emitting site. This decision is considered reasonable since the peak of pollutant concentration occurs within this region and because this region is small enough to be owned by the emitting entity. Thus, decisions about changing the land cover are likely to be easier as they may not require changes on land owned by others. Furthermore, this work focuses on facilities with relatively small emissions, and the emissions leaving the stack are assumed to be well-mixed with the ambient air. In practice, the concentration in the 500 m radius is likely to be larger than what is assumed, so the estimated capacity of the vegetation to mitigate emissions is likely to be an underestimate.

Geographic Land Cover Analysis in Buffer Zone For each of these point sources, the Euclidean Allocation tool available in ArcGIS was used to identify non-overlapping areas within a 500 m buffer zone around each point. Figure 2 shows a sample point source with a 500 m buffer around the stack located in Nebraska. Yellow regions in the image represent the area that can be restored. The 2011 National Land Cover Database (NLCD) 18 was used to identify land area covered by canopy cover (Λf,curr ) based on the the fraction of area that belongs to the forest classifications evergreen, deciduous and mixed forests (land classes 41, 42 and 43, respectively). Land areas around the stack that can be potentially restored were identified based on the land classification as barren, shrub and grasslands (land classes 31,52 and 71, respectively). These land areas have very little to sparse vegetation or are dominated by shrubs, short trees, and grasslands that are not subject to any intense tilling or management practices. Each of these categories were quantified as a fraction of the total available land around the stack (≤ π(0.5)2 = 0.78 km2 ). Finally, as rain-fed canopy cover cannot be found in areas with very low precipitation, a precipitation cut-off criterion of 400 mm was applied for each location 19 to determine areas where restoration is feasible. This available land area (Λf,avail ) is assumed to be restored with canopy cover, specific to each region to maintain ecological integrity. This represents a hybrid-restoration scenario 20 with low-to-moderate degree of human intervention, with management intentions to preserve ecosystem function and maintain native-composition of canopy in that region. The capacity of these restored areas to provide air quality regulation service is then calculated based on the sequestration capacity of canopy in the county where 7



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each facility is located.

Techno-Ecological Synergy Once the fraction of land cover belonging to different land classes was identified, current and potential sequestration of pollutants were calculated based on the county level sequestration value and total land area. Fraction of emissions sequestered by vegetation cover around each point source was quantified using the Techno-Ecological Synergy metric 21 as, Vi,f =

Si,f − Di,f Di,f

(3)

where Di,f represents the ecosystem service demand for pollutant i from a point source ∗ ∗∗ facility f and Si,f and Si,f represent the current and potential supply of ecosystem service for pollutant i, respectively. Here, the current and the potential supply of ecosystem services for a pollutant is the removal of emissions from the atmosphere equivalent to the emissions from different industrial facilities. The fraction of emissions sequestered or the fraction of supply based on the demand translates to the sustainability index with the interpretation that, a negative value of Vi,f indicates an overshoot in demand for an ecosystem service over its supply. A negative value for an ecosystem service like air quality regulation would indicate the lack of availability of ecosystem services to sequester all the emissions at a particular scale. This implies that the current or potential removal of pollutants does not offset emissions entirely. A positive sustainability index would indicate a situation where the services demanded by industrial systems are within the capacity of ecological systems to supply them. This would lead to a situation of net zero or ’net-positive’ system, where the capacity of ecosystems to dissipate emissions is higher than the quantity of emissions from an industrial system, around the facility boundary. In such a situation, a net positive supply would indicate the presence of additional ecosystem services available to society. A situation of Vi,f > 0 need not mean that there is no impact of air pollution, and concentration of pollutants in the atmosphere need not be zero for Vi,f > 0. This metric is static in nature and does not consider dynamic or spatial aspects of emissions in the atmosphere. Even though local variation in concentration is accounted for on an hourly basis, the sequestration rates are aggregated to the county-scale. Similar to situations encountered in the calculation of carbon footprints, a positive value of V simply indicates that the restoration scenario goes beyond offsetting emissions from an individual facility to remove additional pollutants from the atmosphere. Detailed calculations of the TES metric or sustainability index for the facility in Nebraska (Figure 2) are shown in the Supporting Information. ∗ Thus for each facility, ecosystem services provided by the current vegetation (Si,f ) and ∗∗ by restoration of barren land around the stacks (Si,f ) were analyzed. These two parameters provide insight into the land area that has to be restored for facilities to stay within local ecological capacity, and the extent to which current operations overshoot the carrying capacity. One of the assumptions made while determining the potential sequestration and potential sustainability index is the same value of ecosystem service demand during and after 8


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West of Mississippi

East of Mississippi Mining, Quarrying, and Oil and Gas Extraction Transportation & Warehousing

n=17.3% n=37.2% n=0.02% n=6%

Mgmt. of Companies & Enterprises

n=0.3%

Utilities Arts, Entertainment & Rec.

n=0.7%

Prof., Sci., & Tech. Services Admin., Supp., Waste Mgmt. & Rem. Services

n=1.5% n=2.1% n=1.4%

Public Administration Acco. & Food Services Wholesale Trade Information Retail Trade Construction Other Services Manufacturing

n=1.4% n=1% n=0.7% n=0.4% n=2.3% n=21% n=1.6% n=3% n=0.2% n=0.2% n=1.8%

0%

n = 4.1% n=29.1% n=0.1% n=5.6% n=0.1% n=1.2% n=1.6% n=1.7% n=0.1% n=1.5% n=0.9% n=0.4% n=0.1% n=1%

Educational Services Health Care & Social Assistance Finance & Insurance Real Estate Rental & Leasing

n=44.2%

Agriculture, Forestry, Fishing, Hunting

n=0.3%

75% 0% 25% 50% 75% 25% 50% Percentage of locally sustainable facilities for defined air pollutant(s) PM10 NO2, SO2 & PM 10 NO2 SO2 Total (Current + Potential) Current

Figure 3: Percentage of sites with local sustainability based on current and total vegetation (after land restoration) to the east and west of the Mississippi river. Numbers to the left and right of each panel represent the percentage of facilities in each sector, to the west and east of Mississippi respectively. Refer to Table S1 for sector numbers. the restoration period. That is, the emissions from the point source are assumed to remain unchanged.

Results

9


n=2.1% n=4.9% n=0.3% n=0.7%


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Table 1: Sector-wise emissions of NO2 , PM10 and SO2 from facilities located East of the Mississippi river. Numbers in bracket represent the fractional contribution of each sector to the total emissions from facilities analyzed in this study for each. Sector Name and Number Ag., Forestry, Fishing & Hunting (11) Mining, Quarrying, & Oil and Gas Extraction (21) Utilities (22) Construction (23) Manufacturing (31-33) Wholesale Trade (42) Retail Trade (44-45) Transportation & Warehousing (48-49) Information (51) Finance & Insurance (52) Real Estate & Rental and Leasing (53) Professional, Scientific & Technical Services (54) Mgmt. of Companies & Enterprises (55) Admin., Support, Waste Management & Remediation Services (56) Educational Services (61) Health Care & Social Assistance (62) Arts, Entertainment & Recreation (71) Acco. & Food Services (72) Other Services (81) Public Administration (92) Total

NO2 (kg)

PM10 (kg)

SO2 (kg)

105,252 (0.12%)

149,391 (0.77%)

4,635 (0.08%)

7,541,476 (8.48%)

1,303,251 (6.72%)

718,299 (11.78%)

9,839,978 (11.06%) 13,252 (0.01%) 21,129,766 (23.75%) 319,933 (0.36%) 98,433 (0.11%)

898,871 (4.63%) 121,475 (0.63%) 13,597,453 (70.09%) 308,858 (1.59%) 13,424 (0.07%)

894,677 (14.68%) 336 (0.01%) 3,184,848 (52.24%) 88,628 (1.45%) 17,865 (0.29%)

41,686,229 (46.86%)

1,875,472 (9.67%)

210,030 (3.45%)

184,427 (0.21%) 104,444 (0.12%)

15,219 (0.08%) 7,499 (0.04%)

10,197 (0.17%) 2,477 (0.04%)

391,079 (0.44%)

18,621 (0.10%)

76,888 (1.26%)

521,359 (0.59%)

165,850 (0.85%)

25,703 (0.42%)

9,181 (0.01%)

1,145 (0.01%)

544 (0.01%)

2,554,858 (2.87%)

329,105 (1.70%)

353,010 (5.79%)

937,653 (1.05%)

153,320 (0.79%)

154,430 (2.53%)

2,220,222 (2.50%)

178,507 (0.92%)

125,645 (2.06%)

73,151 (0.08%)

2,135 (0.01%)

635 (0.01%)

93,890 (0.11%)

6,032 (0.03%)

44,845 (0.74%)

118,242 (0.13%)

89,844 (0.46%)

4,206 (0.07%)

1,021,184 (1.15%)

163,380 (0.84%)

178,299 (2.92%)

88,964,009

19,398,864

6,096,197

All the lower 48 states were divided into 9 regions based on the climatological map developed by the National Oceanic Atmospheric Association (NOAA) as shown in Figure S6 in the Appendix and further aggregated into two regions. Facilities in East North Central (ENC), North East (NE), Central (C) and South East (SE) are located to the East of Mississippi while the rest of the facilities are located mostly to the West of Mississippi. Figure 3 represents the fraction of facilities with local sustainability index (Vi,f ≥ 0) to the east and west of the Mississippi river. Green lines represent sustainability index for NO2 , red lines for PM10 , black lines for SO2 and blue lines for all the three pollutants combined. From the right panel in Figure 3, we can see that vegetation including current and restored vegetation has a large potential to sequester SO2 and PM10 emissions followed by NO2 . As 10


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mentioned previously, sequestration or removal of pollutants from the atmosphere indicate the removal of emissions equivalent to the emissions from different industrial facilities. For NO2 , only the transportation and warehousing sector, retail trade, wholesale trade, admin., support, waste management and remediation services, and manufacturing sectors have a small fraction of facilities that can reach local sustainability (Vi,f > 0) based on current and restored vegetation cover. About 30% of the facilities classified under the transportation and warehousing sector in the East have vegetation cover to sequester all the NO2 emissions, while an additional 4% of facilities benefit from restoration. This sector primarily includes industries that transport passengers and cargo; warehouses for storing goods; and other support activities related to transportation. Sources of NO2 emissions from this sector include NO2 from freight trucks docked in warehouse and distribution facilities, emission from air transport of passengers and cargo, and refrigerated warehouses. These sites are mainly located in the Central region including states like North Carolina, Michigan, Ohio and Illinois, in suburban areas with larger land availability. This sector also has the largest contribution to NO2 emissions demand in the east (47%), followed by the manufacturing sector that contributes to 24% of emissions as described in Table 1. Sectors with no dark green points have no sites that have local sustainability for NO2 . These results indicate that the transportation and warehousing sector benefits the most from NO2 sequestration by vegetation. For PM10 (red line, Figure 3), the top 5 sectors that benefit from current and restored vegetation east of the Mississippi include mining, quarrying, and oil and gas extraction followed by the arts, entertainment and recreation facilities, transportation and warehousing sector, admin., support, waste mgmt. and remediation services sector and utilities. The mining and quarrying industry contributes to 7% of PM10 emissions in the East and 50% of these sites are locally sustainable, based on the current vegetation cover surrounding these sites. An additional 10% of the sites benefit from restoration. These sites are mostly located in Michigan, with some sites in Kentucky and Ohio. Several factors affect the restoration of land around mined sites including the soil contamination and stability, hydrology and climatological conditions. Studies on tree species that are commonly planted on coal mines in eastern USA recommend using species that are native to the local area. 22 Facilities in the arts and recreation industry are located in urban regions in states like Washington DC, Maine, Massachusetts and Pennsylvania and these sites have no land available for restoration. Only 4 out of the 8 sites in this sector have enough land area within a 500 m radius to reach local sustainability. For transportation and warehousing sector, 38% of the facilities reach ’local sustainability’ based on current vegetation while an additional 6% of facilities benefit from restoration. These sites are mainly located in states like Illinois, Ohio, Michigan and New York. However, the transportation and warehousing sector contributes to only 10% of total PM10 emissions in the east. The manufacturing sector contributes to 70% of PM10 emissions demand in the east, but only 15% of the sites are locally sustainable based on current and restored vegetation. These numbers indicate that land availability around manufacturing sites is not enough to reach mitigate PM10 emissions from individual sites. Facilities in the mining and quarrying sector (sector 21), transportation and warehousing sector (sector 48-49), arts and recreation sector (sector 71), admin. and support sector (sector 56) and manufacturing sector (sector 31-33) have the highest benefit for SO2 sequestration by current and potential vegetation (black line, Figure 3) . Almost 76% of facilities in mining 11



and quarrying, and 49% of facilities in transportation sector already have enough vegetation cover to mitigate all the SO2 emissions. Some of the states in which these industrial sites are located include Pennsylvania, Michigan, Illinois, and Ohio. However, these two sectors contribute to only 15% of the total SO2 emissions demand in the east. Among the 8,717 facilities analyzed in the east, facilities in the manufacturing sector contribute to a total of 52% of SO2 emissions. In addition, 27% of the facilities are locally sustainable with an additional 5% benefiting from restoration, indicating that among the larger emitters, manufacturing sectors benefit the most from land restoration. Source of SO2 emissions from the manufacturing sector include industries that combust fuels containing SO2 for internal use, metal smelting industries and other industrial processes. The blue line in Figure 3 represents the percentage of facilities with local sustainability for all the three pollutants. About 31% of sites in the transportation and warehousing sector and 11% of sites in the construction sector reach local sustainability for all three pollutants after land restoration. We observed that emissions demand of NO2 is highest, compared to PM10 and SO2 for all the facilities to the East of Mississippi. NO2 emissions represent 45% of total emissions from all facilities analyzed in this work, while PM10 and SO2 emissions from facilities in the East represent 55% and 54%, respectively. Accommodation and food services sector (sector 72), agriculture, forestry, fishing and hunting sector (sector 11), finance and insurance sector (sector 52) that are much smaller sources of emissions benefit the least from sequestration of pollutants by vegetation. This is mainly because most of these sectors are located in urban regions with very little land area available around the boundary of the facility. These sectors also collectively contribute to less than 1% of US emissions.

12


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Table 2: Sector-wise emissions demand of NO2 , PM10 and SO2 from facilities located west of the Mississippi river. Numbers in bracket represent the fractional contribution of each sector to the total emissions from facilities analyzed in this study for each. Sector Name and Number Ag., Forestry, Fishing and Hunting (11) Mining, Quarrying, & Oil and Gas Extraction (21) Utilities (22) Construction (23) Manufacturing (31-33) Wholesale Trade (42) Retail Trade (44-45) Transportation & Warehousing (48-49) Information (51) Finance & Insurance (52) Real Estate & Rental and Leasing(53) Professional, Scientific & Technical Services (54) Mgmt. of Companies & Enterprises (55) Admin., Support, Waste Management & Remediation Services (56) Educational Services (61) Health Care & Social Assistance (62) Arts, Entertainment & Recreation (71) Acco. & Food Services (72) Other Services (81) Public Administration (92) Total

NO2 (kg)

PM10 (kg)

SO2 (kg)

478,592 (0.43%)

897,814 (5.69%)

113,003 (2.18%)

48,539,077 (43.77%)

2,861,387 (18.14%)

2,435,204 (46.98%)

5,540,205 (5.00%) 137,656 (0.12%) 8,235,627 (7.43%) 302,439 (0.27%) 263,912 (0.24%)

574,030 (3.64%) 32,252 (0.20%) 7,853,273 (49.79%) 272,473 (1.73%) 21,834 (0.14%)

431,601 (8.33%) 12,103 (0.23%) 1,393,236 (26.88%) 14,066 (0.27%) 4,834 (0.09%)

43,476,671 (39.20%)

1,783,273 (11.31%)

405,091 (7.82%)

419,924 (0.38%) 71,781 (0.06%)

21,542 (0.14%) 7,282 (0.05%)

21,108 (0.41%) 3,237 (0.06%)

53,099 (0.05%)

19,918 (0.13%)

806 (0.02%)

212,428 (0.19%)

18,562 (0.12%)

9,502 (0.18%)

5,153 (0.0046%)

140 (0.0033%)

173 (0.00%)

294,717 (0.27%)

463,847 (2.94%)

56,427 (1.09%)

570,588 (0.51%)

66,346 (0.42%)

26,603 (0.51%)

1,239,683 (1.12%)

100,399 (0.64%)

173,320 (3.34%)

102,989 (0.09%)

7,049 (0.04%)

5,795 (0.11%)

176,247 (0.16%)

52,401 (0.33%)

3,905 (0.08%)

125,206 (0.11%)

60,355 (0.38%)

30,156 (0.58%)

652,867 (0.59%)

658,444 (4.17%)

42,948 (0.83%)

110,898,861

15,772,621

5,183,118

The left panel in Figure 3 represents the fraction of facilities with local sustainability for sites located West of the Mississippi and Table 2 contains sector-wise emissions demand for each pollutant. These facilities benefit the most from PM10 sequestration, followed by SO2 and NO2 . For NO2 , a small fraction of facilities in the transportation and warehousing sector; professional, scientific and technical services; admin. support services sector; and accommodation and food services sector can reach local sustainability based on current and potential vegetation cover. Almost 13% of facilities in the transportation and warehousing sector have land cover that can sequester all the NO2 emissions, and this increases to 35% of sites after land restoration. These facilities are located in states like Texas, California and Montana. 13



The transportation and warehousing sector contributes to about 39% of NO2 in the west, while the largest contribution to NO2 emissions are from mining, quarrying and oil and gas extraction sector (44%). However, only 3% of the sites in the mining sector can become locally sustainable on land restoration, indicating that this sector does not benefit much from land restoration, despite the availability of barren and under-developed land around these sites. The main reason for this is because the emission of NO2 from this sector is much larger than the capacity of restored ecosystems to sequester emissions. The total contribution to NO2 emissions in the west from facilities in other sectors that reach local sustainability (accommodation and food services, admin. and support service sectors) are less than 2%. Facilities in the mining and quarrying sector; transportation and warehousing sector; utilities sector and arts and recreation sector benefit the most from PM10 and SO2 sequestration by current and potential vegetation. For PM10 , highest contribution to emissions from facilities in the west of Mississippi are from the manufacturing sector (50%) followed by mining and quarrying (18%). However, more than 55% of facilities in mining and quarrying and only about 14% of all manufacturing facilities can reach local sustainability for PM10 after land restoration, indicating that facilities in the mining and quarrying sector have the highest benefit for PM10 . Transportation and warehousing sector contributes to only 11% of total PM10 emissions in the west, indicating that this sector has the second highest benefit. Additionally, facilities in utilities and construction also have a large potential to reach local sustainability after land restoration. However, PM10 emissions from the utilities and construction sectors contribute to only 6% of emissions in the west. For SO2 , highest contribution to emissions from facilities in the west of Mississippi is from mining and quarrying sector (47%) and manufacturing (27%). Almost 48% of all facilities in the mining sector and only 14% of facilities in the manufacturing sector can reach local sustainability on land restoration, indicating that the mining and quarrying sectors have the largest benefit. Comparing the left and right panels in Figure 3, we see that facilities located to the West of Mississippi benefit the most from land restoration while facilities located in the East already have enough land cover that provides air quality regulation ecosystem service. This is represented in Figure S7 in the Supporting Information. The gray and light orange bars represent the current land area and land area available for restoration around industrial sites, respectively. These numbers are based on the land cover analysis of the 18,500 facilities that were analyzed and within the 500 m radius. Current canopy cover around sites is significantly higher in Central, East North Central, and North Eastern states while facilities in South, West and South West have a large fraction of land available for restoration. Overall, by maximizing the use of open space around industrial sites for air quality regulation, the fraction of industrial sites that result in local sustainability increases substantially for all the pollutants (14%, 35%, 41% for NO2 , PM10 , and SO2 respectively). Figures S8 and S9 in the appendix represent the fraction of facilities (out of all facilities in the same NAICS category) to the East and West of Mississippi, respectively, classified based on the sustainability index. Points in orange, yellow, light green, dark green and light blue represent sites where vegetation can sequester less than 25% of emissions, between 25-50% of emissions, between 50-75%, between 75-100% and more than 100% of emissions, respectively. Points in light blue are the sites that can reach local sustainability, as discussed previously. The reversal of circles and triangle for the orange lines in these figures indicate 14


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that with restoration, the percentage of facilities with the least sustainability index (-1 < Vi,f

Role of Vegetation in Mitigating Air Emissions Across Industrial Sites

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