New particle formation from agricultural recycling of organic waste products

Abstract

Secondary organic aerosols (SOA) are one of the main sources of uncertainty in the current understanding of the Earth’s climate. Agriculture contributes to primary aerosol emissions, but there is no estimate of SOA formation from gaseous precursors. Organic waste products such as sewage sludge are applied to cropland as fertilizers. In this work, we show that sewage sludge is an unaccounted source of nucleation precursors, such as skatole (C₉H₉N). Skatole emission and nucleation rates up to 1.1 × 10⁶ cm⁻³ s⁻¹ owing to ozone reactivity were measured in the laboratory. Our results show that SO₂ plays a key role in the oxidation of skatole and leads to intensive new particle formation. The products of ozone reactions with skatole and the possible ozonolysis reaction mechanism are discussed. This nucleation mechanism might aid our understanding of the organic waste agricultural recycling contribution to the aerosol balance in the atmosphere. Based on the measured particle emission flux, the surface area of sewage sludge spread in France and the time before sewage sludge incorporation into the soil, a rough estimate of the annual quantity of particles generated by this agricultural activity is in the range of one ton, which represents ~0.03% of the total PM_1.0 emissions from the agricultural and forestry sectors in France. As spreading occurs over only a few days (mid-summer), these emissions may locally be of great concern for local and regional air quality during this period of the year.