Response of Surface Ozone to Future Agricultural Ammonia Emission and Subsequent Nitrogen Deposition through Vegetation Feedbacks

With the rising food need for the future world population, more intense agricultural activities are expected to cause substantial perturbations to the global nitrogen cycle, aggravating surface air pollution and imposing stress on terrestrial ecosystems. Much less studied, however, is how the ecosystem changes induced by agricultural nitrogen deposition may modify biosphere-atmosphere exchange and further exert secondary feedback effects on global air quality. In this work, therefore, we examine the responses of surface ozone air quality to vegetation changes caused by 2000-to-2050 changes in agricultural ammonia emission and nitrogen deposition by asynchronously coupling between the Community Atmospheric Model with Chemistry (CAM-Chem) and the Community Land Model (CLM) within the Community Earth System Model (CESM) framework. We find that global gross primary production (GPP) is enhanced by 2.1 Pg C yr^-1 following a 19.6% (19.8 Tg N yr^-1) increase in global nitrogen deposition by the end of year 2050 in response to rising agricultural ammonia emission as projected by the Food and Agriculture Organization. Leaf area index (LAI) is simulated to be higher by up to 0.3-0.4 m² m^-2 over most tropical grasslands and croplands, and 0.1-0.2 m² m^-2 across boreal and temperate forests at midlatitudes. Around 0.1-0.4 m increases in canopy height are found in boreal and temperate deciduous forests, and 0.1 m increases in from tropical grasslands and croplands. We also find that such vegetation changes could lead to a reduction of ozone by 0.5 ppbv when prescribed meteorology is used (i.e., when meteorology is not simultaneously changed by the vegetation changes), and most of the reduction is caused by LAI changes (80-90%). In contrast, when meteorology is dynamically simulated in response to vegetation changes, surface ozone could typically be modified by about 2-3 ppbv, accounting for 60-90% of the overall ozone changes. Therefore, we find that meteorological variations induced by vegetation changes can be as important as or even more important than the vegetation changes per se in terms of modulating future surface ozone air quality.