Biogenic VOCs in the Portland Metro Area as Potential Air Quality HazardsSarah Cutts and Ryan Guidry

Introduction:

Isoprenoids are a primary contributor to the formation of tropospheric

ozone, ground level ozone. The Environmental Protection Agency (EPA) states

that higher daily ozone concentrations are associated with more frequent

Asthma attacks, as well as the inflammation of airways, the decrements of lung

functions and increased cardiac effects. These symptoms are most common in

people who already suffer from chronic respiratory conditions.

Trees can emit Biogenic Volatile Organic Compounds (bVOCs), such as

Isoprene and Monoterpenes, which play a large role in air quality. Isoprene is a

particular cause for concern due to it being a naturally occurring carcinogen.

Plant foliar VOC’s can have both positive and negative effects on air quality. In

the presence of high concentrations of NOx, such as what would be observed

in an urban environment, isoprene will be a contributor to tropospheric Ozone

formation (Sharkey et al., 2008). Yearly production of isoprene emissions by

vegetation is around 600 metric tons (Guenther, et al., 2006). This is about

equivalent to methane emissions and accounts for almost 1/3 of all

hydrocarbons released into the atmosphere, both anthropogenic and biogenic.

Emission rates are more pronounced in the warm summer months as

the temperature increases. This is a concern due to the increase in summer

temperatures due to global warming. Trees emitting isoprene have been shown

to have a greater heat tolerance (Sharkey, et al., 2007). Isoprene emissions are

significantly less in the winter due to the deciduous trees being bare and

therefore incapable of photosynthesis.

For this project we wanted to determine which areas in Portland have

the greatest potential air quality threat resulting from Isoprenoids, a biogenic

VOC emission.

Methods

To isolate areas with a large presence of isoprenoid emitting trees we

first used a street trees data set from civic apps, and assigned isoprenoid

emission values by tree species based on the emission values listed in USFS

2002 Brooklyn’s Urban Forest technical report (Nowak, et al., 2002). The report

gave estimated bVOCs emission rates for common US trees and shrubs. We

then isolated our study area by running the kernel density tool on the Portland

street tree point layer using the inverse distance weighted method, a cell size of

200 ft and a search radius of 300 ft. Using the identify tool on the kernel

density raster we determined a density value of 100 which include all of our

tree data points. Using the raster calculator, we isolated our study area with the

function Con(Kernel Raster > 100, 1), which generated a raster that assigned a

value of 1 to all cells with a density value greater than 100 and no data to all

other cell values.

We performed a spatial join between the street tree point layer and the

2010 census tract boundary layer in order to determine the tree count within

each tract. Using the spatial analyst tool “Extract By Mask” we limited the

joined census tract and tree layer to the areas where tree location data was

available. We defined our threshold for highly emissive trees as those whose

isoprene emissions were greater than 11 µg per gram of dry leaf weight per

hour or whose monoterpene emissions were greater than 2 µg per gram of dry

leaf weight per hour and isolated them using select by attribute and created a

new layer. Based on our research we separated the trees based on their trunk

size, defining trees with a DBH less than 4in as “young”(low emitting) trees,

and trees with a DBH greater than 4in as “mature” (high emitting) using the

select by attributes. With the spatial analyst tool “Nearest Neighbor” we

determined that there was clustering present in both the small young highly

emissive trees and large mature highly emissive trees, We then ran the kernel

density tool on the small highly emissive trees layer using the inverse distance

weighted method with an output cell size of 10ft and a search radius of 1231ft

to determine the locations with a future air quality problem resulting from high

isoprenoid emissions. We then ran the kernel density tool again using the same

parameters on the mature highly emissive trees to determine the locations with

current potential air quality concerns. Next we overlaid these two layers to show

the current and future locations with potential air quality concerns due to

isoprenoid concentrations (fig. 1).

Using the original study area which included the tree species and

emission rates by census tract we generated two choropleth maps. Fig. 2.

Symbolizes the census tracts by the number of highly emissive trees present in

each tract regardless of size. Fig. 3. Symbolizes the census tracts by the total

number of trees present in each tract regardless of their emissivity.

Results:

We found that as of 2014, 16% of Portland's inventoried street trees

were highly emissive tree species. The 8 most highly emissive tree species present

in Portland, which emit isoprene at a rate of 70µg per gram of dry leaf weight

per hour, are Poplar, Sweetgum, Oak, Black Locust, Sycamore, Willow, Black

Gum, and Eucalyptus. We found that these trees were fairly prevalent

throughout the city but there were clear area of higher concentration.

We found that the Neighborhoods with the highest number of

Isoprenoid emitting trees were, Laurelhurst, Concordia, Sellwood, East

Moreland, Roseway, Kenton, Portsmouth, and Downtown. This was not

surprising, for Sellwood, East Moreland and Laurelhurst as those neighborhoods

also have the highest overall tree populations.

Our analysis allowed us to identify the neighborhoods whose air quality is

potentially threatened by the clustering of highly emissive trees. The

neighborhoods who currently have the greatest potential air quality concerns are

Irvington, Chinatown, the Lloyd District, South Portland, Laurelhurst and Kerns.

The neighborhoods who currently display clustering of small highly emissive

trees and will potentially have future air quality concerns are Boise, Concordia,

Roseway, Cully, Powellhurst-Gilbert, Arbor Lodge, and Kenton. The

neighborhood which have both current and future potential air quality concerns

are Eliot and Downtown.

Surprisingly Sellwood, East Moreland, and Portsmouth, while having a

large number of Isoprenoid emitting trees, did not show the highest degree of

clustering, indicating that the trees in question were more evenly dispersed

throughout the neighborhoods.

Conclusions:

While the presence of any tree is better for air quality than no tree,

because of the environmental services they provide, some trees are more

desirable than others. The human health risk associated with isoprenoid emitting

trees in an urban environment should be factored into tree planting regulations.

Our researched indicated that the prevalence of isoprenoid emitting trees

throughout the city did not appear to be correlated with any identifiable variable.

This lead us to the conclusion that there is a lack of education surrounding the

potential air quality implications associated with bVOCs. This conclusion is

supported by the fact that many Isoprenoid emitting trees are currently listed on

the Urban Forestry Departments approved planting list and by the fact that

Friends of Trees may even plant and maintain an emissive tree, free of charge,

upon request.

Our recommendation would be to limit new planting to non-emissive or

low isoprenoid emitting trees whenever possible. Currently the determining

factors listed in the Urban Forestry Departments planting criteria are plot size

and presence of power lines. While we believe there is additional unlisted criteria,

our hope is that the emission of bVOCs would be incorporated into the cities air

quality considerations thereby reducing the risk of preventable respiratory illness

for current and future generations.

References:

Nowak, D.J., Crane, D. E., Stevens, J.C., Ibarra, M. (2002). Brooklyn’s Urban Forest.(pp 107, Tech. No. NE-

290). Newtown Square, PA: U.S. Department of Agriculture. Retrieved February 02, 2016

Air Quality Modeling. (n.d.). Retrieved February 02, 2016, from

http://www3.epa.gov/airquality/modeling.html

Pirjola, L. (1998). Effects of the increased UV radiation and biogenic VOC emissions on ultrafine aerosol

formation. Journal of Aerosol Science, 29. Retrieved February 2, 2016, from

http://www.sciencedirect.com/science/article/pii/S002185029890711X

Sharkey, T. D., Wiberley, A. E., & Donohue, A. R. (2007). Isoprene Emission from Plants: Why and How.

Annals of Botany, 101(1), 5-18. Retrieved February 2, 2016.

Vallero, D. A. (2008). Fundamentals of air pollution. Amsterdam: Elsevier

Health Effects of Ozone in the General Population. (n.d.). Retrieved March 10, 2016, from

http://www3.epa.gov/apti/ozonehealth/population.html

Materić, D., Bruhn, D., Turner, C., Morgan, G., Mason, N., & Gauci, V. (2015). Methods in Plant Foliar

Volatile Organic Compounds Research. Applications in Plant Sciences, 3(12), 1500044.

Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., & Geron, C. (2006). Estimates of global

terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from

Nature). Atmospheric Chemistry and Physics Discussions Atmos. Chem. Phys. Discuss., 6(1), 107-173.

Datsets:

2010 Census Tracts: polygon shapefile from Metro RLIS

City Limits: polygon shapefile from Metro RLIS

Major Rivers: polygon shapefile from Metro RLIS

Neighborhood Organizations: polygon shapefile from Metro RLIS

Street Trees: polygon shapefile from CivicApps

Fig. 2

Fig. 1

Fig. 3