Impacts of future land use and land cover change on mid-21st-century surface ozone air quality: distinguishing between the biogeophysical and biogeochemical effects

Lang Wang,Peng Wang,Amos P K Tai,Kevin K W Cheung,Chi-Yung Tam,Mehliyar Sadiq

doi:10.5194/acp-20-11349-2020

Abstract

Abstract. Surface ozone (O3) is an important air pollutant and greenhouse gas. Land use and land cover is one of the critical factors influencing ozone, in addition to anthropogenic emissions and climate. Land use and land cover change (LULCC) can on the one hand affect ozone “biogeochemically”, i.e., via dry deposition and biogenic emissions of volatile organic compounds (VOCs). LULCC can on the other hand alter regional- to large-scale climate through modifying albedo and evapotranspiration, which can lead to changes in surface temperature, hydrometeorology, and atmospheric circulation that can ultimately impact ozone “biogeophysically”. Such biogeophysical effects of LULCC on ozone are largely understudied. This study investigates the individual and combined biogeophysical and biogeochemical effects of LULCC on ozone and explicitly examines the critical pathway for how LULCC impacts ozone pollution. A global coupled atmosphere–chemistry–land model is driven by projected LULCC from the present day (2000) to the future (2050) under RCP4.5 and RCP8.5 scenarios, focusing on the boreal summer. Results reveal that when considering biogeochemical effects only, surface ozone is predicted to have slight changes by up to 2 ppbv maximum in some areas due to LULCC. It is primarily driven by changes in isoprene emission and dry deposition counteracting each other in shaping ozone. In contrast, when considering the combined effect of LULCC, ozone is more substantially altered by up to 5 ppbv over several regions in North America and Europe under RCP4.5, reflecting the importance of biogeophysical effects on ozone changes. In boreal and temperate mixed forests with intensive reforestation, enhanced net radiation and sensible heat induce a cascade of hydrometeorological feedbacks that generate warmer and drier conditions favorable for higher ozone levels. In contrast, reforestation in subtropical broadleaf forests has minimal impacts on boundary-layer meteorology and ozone air quality. Furthermore, significant ozone changes are also found in regions with only modest LULCC, which can only be explained by “remote” biogeophysical effects. A likely mechanism is that reforestation induces a circulation response, leading to reduced moisture transport and ultimately warmer and drier conditions in the surrounding regions with limited LULCC. We conclude that the biogeophysical effects of LULCC are important pathways through which LULCC influences ozone air quality both locally and in remote regions even without significant LULCC. Overlooking the effects of hydrometeorological changes on ozone air quality may cause underestimation of the impacts of LULCC on ozone pollution.

Highlights

Surface ozone (O3), as a harmful air pollutant, has negative consequences for human health (Occupational and Environmental Health Team, 2006; Jerrett et al, 2009; Malley et al, 2017), decreases plant gross primary productivity (e.g., Yue and Unger 2014), and leads to substantial reductions in global crop yields (Avnery et al, 2011; Tai et al, 2014; Tian et al, 2016; Tai and Val Martin, 2017; Mills et al, 2018)
Transition from the present day to 2050 in RCP4.5 highlights the global growth of forest from 71.8×106 to 74.0×106 km2, at the expense of croplands; grasslands slightly increase in area from 33.7×106 to 33.8×106 km2
Land use and land cover change (LULCC) is expected to continue to co-occur with future socioeconomic development and anthropogenic emission reduction strategies

Summary

Introduction

Surface ozone (O3), as a harmful air pollutant, has negative consequences for human health (Occupational and Environmental Health Team, 2006; Jerrett et al, 2009; Malley et al, 2017), decreases plant gross primary productivity (e.g., Yue and Unger 2014), and leads to substantial reductions in global crop yields (Avnery et al, 2011; Tai et al, 2014; Tian et al, 2016; Tai and Val Martin, 2017; Mills et al, 2018) It is an important greenhouse gas, contributing to climate change (Myhre et al, 2013). Recent studies found that decreases in anthropogenic emissions alone might not necessarily decrease ozone in some polluted regions if factors such as climatic and land cover changes act to enhance ozone and offset emission control efforts (Zhou et al, 2013; Zhang et al, 2014; Xue et al, 2014)

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Results

Conclusion