Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.3.4.2 Air Quality Regulation ====

<div id="h3-22-siblings" class="h3-siblings"></div>

NBS in cities can help regulate air quality by absorbing air pollutants ( ''medium evidence'' , ''medium agreement'' ). For example, planting trees or vegetated barriers along streets or in urban forests can reduce particulate matter, the ambient air pollutant with the largest global health burden ( [[#Janhäll--2015|Janhäll, 2015]] ; Tiwary, Reff and Colls, 2008; Matos et al., 2019; McDonald et al., 2016). However, findings show that trees can also positively affect ground-level ozone (Calfapietra et al., 2013; Kroeger et al., 2014), airborne pollen concentrations ( [[#Willis--2017|Willis and Petrokofsky, 2017]] ) and indirectly affect air quality through reduced emissions from energy production offset by shade provision (Keeler et al., 2019). Certain tree species however can also be detrimental to urban ozone formation by emitting significant amounts of reactive biogenic volatile organic compounds (VOCs). Decreasing urban emissions of VOCs is an increasingly important ozone mitigation strategy in urban areas (Fitzky et al., 2019).

Trees can also have negative effects by increasing pedestrian exposure to pollution if they are introduced in heavily travelled street canyons where air pollutants can be trapped (Vos et al., 2013; [[#Gromke--2015|Gromke and Blocken, 2015]] ). To maximise the adaptation benefits of NBS for improving air quality, planners and managers can target tree selection for species with low VOC emissions, low allergen emissions and high pollutant deposition potential (Keeler et al., 2019), and combine with low pollution transportation policies. Studies suggest sensitive planting of roadside tree canopies can have positive effects on air pollutants (Beckett, Freer Smith and Taylor, 2000; Yang, Chang and Yan, 2015). For example, [[#Xue--2021|Xue et al. (2021)]] found that the PM2.5 reduction between 2013 and 2017 in China was associated with a saving of approximately USD 111 billion yr -1 nationally. Tree planting near schools, nursing homes and hospitals can ensure that benefits provided by trees are delivered to the local populations that stand to benefit the most from improved air quality, but species need to be adapted to regional climate to provide benefits over time ( [[#Donovan--2017|Donovan, 2017]] ; Nowak et al., 2018).

<div id="6.3.4.3" class="h3-container"></div>

<span id="stormwater-regulation-and-sanitation"></span>