Global Soil Nitrous Acid (HONO) Emission Has Aggravated PM_2.5 Pollution and Health Risks over the Last Century

Soil nitrous acid (HONO) emissions represent an important source of atmospheric reactive nitrogen, significantly influencing air quality and climate change. However, the long-term global patterns of soil HONO emissions and their contributions to fine particulate matter (PM_2.5) and its key components remain poorly understood. In this study, we developed a new module within a process-based model (LPJ-GUESS) to estimate global soil HONO emissions across all major land-use types over the past century and also quantified their impacts on PM_2.5 and associated health outcomes. Our model demonstrated robust performance in estimating global soil HONO emissions (R² = 0.91). The results revealed that global warming and increased fertilizer use have driven a substantial rise in soil HONO emissions, escalating from 2.2 Tg of N in 1901 to 4.7 Tg of N in 2015. Then, the GEOS-Chem model simulations suggested that the contribution of soil HONO emission to the global average concentrations of PM_2.5, sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), black carbon (BC), and organic aerosol (OA) reached 0.015 (0.3% (range: 0-6%) accounting for all-source concentration), 0.003 (0.6% (0-7%)), 0.008 (3.4% (0-52%)), 0.002 (1% (0-26%)), ∼0, and 0.002 (0.6% (0-4%)) μg/m³ in 2015, respectively. Notably, exposure to soil HONO-driven PM_2.5 was associated with an estimated 113,137 global mortalities. As anthropogenic emissions continue to decline in the future, targeted measures to mitigate soil HONO emissions will become increasingly critical to reducing their adverse effects on air pollution and public health.