Impacts of Changes in Land Use, Climate, and Emissions on Global Air Quality by 2050 following the Shared Socioeconomic Pathways

Surface ozone (O3) and fine particulate matter (PM2.5) are major air pollutants, whose concentrations can be substantially influenced by changes in land use, climate, and most importantly anthropogenic emissions. However, the complex and interactive influence all these factors have on air quality are highly uncertain. Thus, we performed a series of model experiments for the present day (2010) and future (2050) using the Community Earth System Model (CESM v2.1.3) following two Shared Socioeconomic Pathways (SSPs): SSP1 (best-case scenario) and SSP3 (business-as-usual scenario). We found that between 2010 and 2050, land use and land cover change (LULCC) alone increases boreal summer (JJA) maximum daily average 8 h (MDA8) O3 by 2-5 ppb over high-NOx urban areas (e.g., eastern China, eastern US, western Europe) with larger changes in SSP1 (compared to SSP3), the scenario with more afforestation. Greenhouse gas-induced temperature changes enhance isoprene emission, which further increases O3 by 10-20 ppb particularly in SSP3 over urban environment, whereas, in the remote environment (such as tropical rainforests over the Amazon and central Africa), O3 is reduced by 5-10 ppb in both scenarios. Future changes in anthropogenic emissions reduce O3 by 10-15 ppb over the US and Europe, while in central Africa it would elevate O3 by ~10 ppb in both scenarios. The combined effect of LULCC, climate, and emission would worsen the O3 pollution in many parts of Asia and Africa, but improve it in many parts of North America and Europe. Climate change was in particular found to be as important as anthropogenic emissions in shaping future O3 pollution, oftentimes counteracting the benefits of emission controls. LULCC only has significant effects in some regions with heavy afforestation or deforestation. Meanwhile, annual surface PM2.5 changes by the mid-21st century (2050) are mainly influenced by changes in anthropogenic emissions. The US and Europe will experience reduced PM2.5 in all future scenarios (reduction of ~5 µg m-3; ~50%) in SSP1. PM2.5 in highly polluted regions such as India and China is reduced by around 20 µg m-3 (~35%) in SSP1, whereas it increases by a similar magnitude in SSP3. The impacts of LULCC and climate are relatively weaker compared to anthropogenic emissions on future PM2.5.