Trees remove PM2.5 when particulate matter is deposited on leaf surfaces ( Nowak et al 2013 ). This deposited PM2.5 can be resuspended to the atmosphere or removed during rain events and dissolved or transferred to the soil . This combination of events can lead to positive or negative pollution removal and value depending on various atmospheric factors . Generally , PM2.5 removal is positive with positive benefits . However , there are some cases when net removal is negative or resuspended particles lead to increased pollution concentrations and negative values . During some months ( e . g ., with no rain ), trees resuspend more particles than they remove . Resuspension can also lead to increased overall PM2.5 concentrations if the boundary layer conditions are lower during net resuspension periods than during net removal periods . Since the pollution removal value is based on the change in pollution concentration , it is possible to have situations when trees remove PM2.5 but increase concentrations and thus have negative values during periods of positive overall removal . These events are not common , but can happen .
For reports in the United States , default air pollution removal value is calculated based on local incidence of adverse health effects and national median externality costs . The number of adverse health effects and associated economic value is calculated for ozone , sulfur dioxide , nitrogen dioxide , and particulate matter less than 2.5 microns using data from the U . S . Environmental Protection Agency ' s Environmental Benefits Mapping and Analysis Program ( BenMAP ) ( Nowak et al 2014 ). The model uses a damage-function approach that is based on the local change in pollution concentration and population . National median externality costs were used to calculate the value of carbon monoxide removal ( Murray et al 1994 ).
For international reports , user-defined local pollution values are used . For international reports that do not have local values , estimates are based on either European median externality values ( van Essen et al 2011 ) or BenMAP regression equations ( Nowak et al 2014 ) that incorporate user-defined population estimates . Values are then converted to local currency with user-defined exchange rates .
For this analysis , pollution removal value is calculated based on the prices of $ 1,327 per ton ( carbon monoxide ), $ 24,885 per ton ( ozone ), $ 2,170 per ton ( nitrogen dioxide ), $ 956 per ton ( sulfur dioxide ), $ 1,213,188 per ton ( particulate matter less than 2.5 microns ).
Carbon Storage and Sequestration :
Carbon storage is the amount of carbon bound up in the above-ground and below-ground parts of woody vegetation . To calculate current carbon storage , biomass for each tree was calculated using equations from the literature and measured tree data . Open-grown , maintained trees tend to have less biomass than predicted by forest-derived biomass equations ( Nowak 1994 ). To adjust for this difference , biomass results for open-grown urban trees were multiplied by 0.8 . No adjustment was made for trees found in natural stand conditions . Tree dry-weight biomass was converted to stored carbon by multiplying by 0.5 .
Carbon sequestration is the removal of carbon dioxide from the air by plants . To estimate the gross amount of carbon sequestered annually , average diameter growth from the appropriate genera and diameter class and tree condition was added to the existing tree diameter ( year x ) to estimate tree diameter and carbon storage in year x + 1 .
Carbon storage and carbon sequestration values are based on estimated or customized local carbon values . For international reports that do not have local values , estimates are based on the carbon value for the United States ( U . S . Environmental Protection Agency 2015 , Interagency Working Group on Social Cost of Carbon 2015 ) and
I-Tree Report of Ecosystem Analysis at Mount Auburn Cemetery Insert to the Climate Action and Sustainability Plan
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