by Dan McCabe
One aspect of urban ecology that is often overlooked in development is the biological benefit of vegetation in cities. In order to quantify the environmental impact of urban plants, Baró et al. (2014) analyzed the effect of green spaces on air quality and carbon sequestration in the city of Barcelona, Spain. The authors randomly selected nearly 600 small plots of land within the city limits and collected field data on the plant life and pollutant levels in each. This information, along with meteorological data, was then processed using i-Tree Eco software, which quantified the biological and economic effects of vegetation on both air quality and climate change. In this software model, green space is treated as providing two kinds of ecosystem benefits—defined as air purification and global climate regulation—as well as one harmful consequence, the emission of biogenic volatile organic compounds (BVOCs). For this case study, the model focused only on the levels of particulate matter (PM10) and NO2 and no other pollutants that harm air quality, because Barcelona has recently had exceedingly high concentrations of these two pollutants.
The i-Tree Eco model found that the impact of green spaces in Barcelona was substantial both for air pollutant removal and for carbon sequestration, but small relative to total emissions. Air pollutant removal totaled hundreds of tons, but this total amounted to only 0.52% of Barcelona’s total NO2 emissions and 22.31% of PM10 emissions—but less than 3% of urban PM10 levels when including pollution that did not originate in the city. Likewise, climate change mitigation due to carbon sequestration in plants was substantial, estimated at nearly 20,000 tons of CO2 per year, but this figure represents only 0.47% of Barcelona’s annual citywide greenhouse gas emissions.
The authors’ results display both the impacts and limitations that urban vegetation has on air quality and carbon emissions mitigation. Although urban forests appear to have a meager impact on airborne pollutants and greenhouse gas emissions in Barcelona, they can still be a valuable tool for urban planning. Green spaces in cities may not be able to offset air pollution and climate change on their own, but they can provide other services such as recreation opportunities and local climate regulation while simultaneously capturing harmful pollutants and carbon emissions, which promotes human health and can save millions of dollars. Mindful management and expansion of urban green space therefore appears to be a viable strategy for fighting air pollution and climate change, but a truly successful urban sustainability plan must integrate green space preservation and development into a multifaceted plan to enhance sustainability.
Baró, F., Chaparro, L., Gómez-Baggethun, E., Langemeyer, J., et al. 2014. Contribution of ecosystem services to air quality and climate change mitigation policies: The case of urban forests in Barcelona, Spain. AMBIO 43, 466-479.