On the competing effects of contemporary land management vs. land\ncover changes on global air quality

Abstract

Abstract. Our work explores the impact of two important dimensions of land system changes, land use and land cover change (LULCC) and direct agricultural reactive nitrogen (Nr) emissions from soils, on ozone (O3) and fine particulate matter (PM2.5) air quality over contemporary (1992 to 2014) time scales. We account for LULCC and agricultural Nr emissions changes with consistent remote sensing products and new global emission inventories, respectively, estimating their impacts on global surface O3 and PM2.5 concentrations and Nr deposition using the GEOS-Chem global chemical transport model. Over this time period, our model results show that agricultural Nr emission changes cause reduction of annual mean PM2.5 level over Europe and northern Asia (up to −2.1\u2009μg\u2009m−3), while increasing PM2.5 level India, China and eastern US (up to +3.5\u2009μg\u2009m−3). Land cover changes induce small reductions in PM2.5 (up to −0.7\u2009μg\u2009m−3) over Amazonia, China and India due to reduced biogenic volatile organic compounds (BVOC) emissions and enhanced deposition of aerosol precursor gases (e.g. NO2, SO2). Agricultural Nr emission changes only lead to minor changes (up to ±0.6\u2009ppbv) in annual mean surface O3 level, mainly over China, India and Myanmar. Meanwhile, our model result suggests a stronger impact of LULCC on surface O3 over the time period Across South America, the combination of changes in dry deposition and isoprene emissions results in −0.8 to +1.2\u2009ppbv surface ozone changes. The enhancement of dry deposition reduces surface ozone level (up to −1\u2009ppbv) over southern China, eastern US and central Africa. The enhancement of soil NOx emission due to crop expansion also contribute to surface ozone changes (up to +0.6\u2009ppbv) over sub-Saharan Africa. In certain regions, the combined effects of LULCC and agricultural Nr emission changes on O3 and PM2.5 air quality can be comparable (>\u200920\u2009%) to that of anthropogenic emission changes over the same time period. Finally, we calculate that the increase in global agricultural Nr emissions leads to a net increase in global land area (+3.67\u2009×\u2009106\u2009km2) that potentially faces exceedance in critical Nr load (>\u20095\u2009kgN\u2009ha−1\u2009yr−1). Our result demonstrates the possible impacts of contemporary LULCC and agricultural Nr emission changes on PM2.5 and O3 air quality, which also implies the potential importance of land system changes on air quality over multi-decadal timescales.