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This chapter introduces the aim of the NBS Impact Evaluation Handbook as a reference for evaluating the impacts of nature-based solutions (NBS). It provides a general framework on the value of NBS to the community, investors, and policy makers, and illustrates how the NBS impact evaluation framework can be used. Chapter 1 describes the global context in which NBS operate. Two infographics help visualise the definition of NBS and provide an in-depth explanation of the concept’s origin and evolution. Another infographic describes the full life cycle of NBS including monitoring, evaluation, and cost-benefit analysis. The chapter concludes by describing the content of each section of the handbook.

Nature‐Based Solutions (NBS) are solutions to societal challenges that are inspired and supported by nature. The European Commission requested the EKLIPSE project to help building up an evidence and knowledge base on the benefits and challenges of applying NBS. In response to the request, the EKLIPSE Expert Working Group on Nature‐based Solutions to Promote Climate Resilience in Urban Areas (EWG) devised the following objectives: 1) To develop an impact evaluation framework with a list of criteria for assessing the performance of NBS in dealing with challenges related to climate resilience in urban areas; 2) To prepare an application guide for measuring how NBS projects fare against the identified indicators in delivering multiple environmental, economic and societal benefits; 3) To make recommendations to improve the assessment of the effectiveness of NBS projects, including the identification of knowledge gaps according to the criteria presented in the impact evaluation framework. This document reports on these three objectives. It is intended to be used as a reference document by members of current and future European projects with an interest in NBS in urban areas, and by practitioners seeking to compare the effectiveness of NBS design, implementation and evaluation. The EWG recognises that the type and intensity of NBS impacts may vary according to the characteristics of the NBS and the context in which they are applied. The intent of this report is not to define NBS, but rather provide examples of indicators and methods for assessing impacts of NBS that may be applied in a range of different ways across urban areas in Europe. As such, the report also identifies the scale at which the indicators are relevant, to guide an assessment of impacts.

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A crucial issue in urban environments is the interaction between urban trees and atmospheric pollution, particularly ozone (O3). Ozone represents one of the most harmful pollutants in urban and peri-urban environments, especially in warm climates. Besides the large interest in reducing anthropogenic and biogenic precursors of O3 emissions, there is growing scientific activity aimed at understanding O3 removal by vegetation, particularly trees. The intent of this paper is to provide the state of the art and suggestions to improve future studies of O3 fluxes and to discuss implications of O3 flux studies to maximize environmental services through the planning and management of urban forests. To evaluate and quantify the potential of O3 removal in urban and peri-urban forests, we describe experimental approaches to measure O3 fluxes, distinguishing laboratory experiments, field measurements, and model estimates, including recent case studies. We discuss the strengths and weaknesses of the different approaches and conclude that the combination of the three levels of investigation is essential for estimating O3 removal by urban trees. We also comment on the implications of these findings for planning and management of urban forests, suggesting some key issues that should be considered to maximize O3 removal by urban and peri-urban forests.