Effects of Future Land Use and Land Cover Change on Surface Ozone Air Quality Over the Mid-21st Century

Ozone (O3), a secondarily formed air pollutant, is ubiquitous throughout the globe. Surface O3 air quality is of great concern because of its adverse effects on human and vegetation health, and its climatic effect as a greenhouse gas. In addition to climate change and anthropogenic emissions, land use and land cover change (LULCC) are critical factors influencing O3. LULCC affects the emissions and chemistry of O3 precursors (e.g., NOx, volatile organic compounds or VOCs) as well as meteorological conditions (e.g., temperature, wind), which ultimately determine surface O3 concentrations. Here we investigated the impact of future (year-2050) LULCC on global O3 air quality. The Community Earth System Model (CESM v2.1.3) in a fully coupled land-atmosphere mode with active biogeochemistry was driven by future LULCC projections based on the Shared Socioeconomic Pathways (SSPs) (in particular, SSP1-2.6, SSP2-4.5 and SSP3-7.0) from the present day (2010). Results revealed that from now to the mid-21st century, annual O3 concentration can change by up to ~3 ppb and northern summertime O3 can change by up to ~7 ppb in some areas due to LULCC alone while keeping climate and anthropogenic emissions constant, with distinct seasonal variations; as climate change is taken into account as well, O3 concentration can change by up to ~10 ppb. For the LULCC-only scenarios, the main driver of O3 is the counteracting effects of isoprene emission vs. O3 dry deposition linked to LULCC, as well as the associated meteorological changes. Most high-NOx areas in the US, Europe and East Asia are expected to experience an increase in O3 with increased isoprene emissions in the scenarios where forests are expected to expand, particularly in summer, and vice versa. We also found that summertime O3 can increase substantially by up to ~6 ppb in places where LULCC is minimal, and this is attributable to boundary-layer meteorological and circulation changes brought about by LULCC in the adjacent regions. Our results indicate that LULCC could be an important but overlooked driver of O3 air quality in the future that substantially modulates the generally more dominant effect of climate change per se.