Constraints on the relative roles of N2O5 heterogeneous and NO3 gas-phase chemistry in secondary aerosol production

Nitrogen oxides (NOx) play a controlling role in the photochemical production of ozone (O3) at Earth�??s surface. Laboratory and field measurements have demonstrated that NOx can also significantly impact the production rate of organic and inorganic aerosol mass during the daytime. More recently, the importance of nocturnal chemistry involving N2O5 and nitrate radicals (NO3) has emerged. Nocturnal reactive nitrogen chemistry impacts ozone concentrations and aerosol particle mass in at least three ways: i) efficient, terminal removal of NOx and volatile organic compounds (VOC) at night reduces O3 precursors, ii) the nocturnal oxidation of VOC by NO3 radicals is an efficient pathway for organic aerosol production, and iii) the hydrolysis of N2O5 on aerosol surfaces leads to the production of inorganic nitrate aerosol. Central to these nocturnal processes is the equilibrium between N2O5 and NO3, where the extent of NO3 chemistry can be regulated by the efficiency of N2O5 loss rates, and vice versa.

The primary scientific objective of this proposal is the direct, simultaneous measurement of NO3 gas-phase reactivity and N2O5 heterogeneous reactivity in ambient air for both urban and remote environments. Ambient measurements of NO3 and N2O5 reactivity provide unique constraints on the nighttime oxidation rates of VOCs and the extent by which NO3 reactions impede NOx loss and particulate nitrate formation via N2O5 heterogeneous reactions. To achieve this objective, we propose the following specific tasks:

1) Analysis of existing measurements of NO3 and N2O5 reactivity in ambient air at a remote site made during the Southern Oxidant and Aerosol Study (SOAS) at Look Rock, TN.

2) Conduct new measurements of NO3 and N2O5 reactivity in ambient air at an urban site with variable influence from biogenic volatile organic emissions (Madison, WI).

Measurements of NO3 gas-phase reactivity and N2O5 heterogeneous reactivity in ambient air will be determined using existing, well-characterized flow reactors equipped with a high purity and controllable N2O5/NO3 source and chemical ionization mass spectrometer for reactant and product detection. Ancillary measurements of nitric oxide, speciated volatile organic compounds, and aerosol surface area and chemical composition will be used to apportion observed loss rates and test model parameterizations.