The aging of aerosol nitrate and implications for the global nitrogen cycle

The atmospheric chemistry of nitrogen oxides (NOx), biogenic volatile organic compounds (BVOCs), and secondary organic aerosol (SOA) are closely linked: NOx has a governing role on the mechanism of BVOC oxidation and SOA formation, and SOA in turn can sequester NOx in the particle phase in the form of nitrates. Particulate nitrate (which includes both organic and inorganic nitrate) can make up an important fraction of reactive nitrogen in the atmosphere, but the fate of nitrogen in the particle phase is poorly understood. Nitrate particles have traditionally been considered to be a sink of reactive oxidized nitrogen (NOy) via wet removal, though recent studies suggest that particulate nitrate can photolyze, emitting NOx and HONO to the gas phase. This so-called �??renoxification�?� can have important implications for atmospheric chemistry away from anthropogenic sources, but little is known about the detailed chemistry of the process, or its dependence on key parameters such as relative humidity, wavelength, etc. Similarly, the fate of particulate organic nitrates is poorly understood �?? the further �??aging�?� (photolysis or heterogeneous oxidation) of organic nitrates within SOA may be an important component of the overall BVOC oxidation mechanism and the atmospheric cycling of reactive nitrogen, but there exist few constraints on either the rates or products of such aging reactions.

Here we propose a joint laboratory-modeling study to explore the importance of aging of aerosol nitrate to the atmospheric lifecycles of both BVOC-derived SOA and NOy. Laboratory studies will focus on the kinetics and products of aging reactions, which will be examined by generating nitrate-containing particles in a large environmental chamber, sampling them into a flow reactor to simulate several days of photolytic/oxidative aging, and measuring reactants and products (in the gas and particle phases) with a suite of state-of-the-art analytical instruments. The focus of these experiments will be on particulate organic nitrates generated from BVOC oxidation (by OH in the presence of NO, or by NO3), though the photolysis of inorganic nitrates, a simpler chemical system, will also be explored. Such laboratory measurements will provide new constraints on the chemistry of particulate nitrate, which will then be incorporated into a global chemical-transport model (GEOS-Chem). Results from this model will be tested against field measurements of NOy species, most importantly particulate nitrates (both organic and inorganic), and will provide insight into the impact of this nitrate aging chemistry (e.g., renoxification) on global particulate nitrate loadings and, more generally, the chemistry of the troposphere.