Abstract
Abstract. The production of atmospheric organic nitrates (RONO2) has a large impact on air quality and climate due to their contribution to secondary organic aerosol and influence on tropospheric ozone concentrations. Since organic nitrates control the fate of gas phase NOx (NO + NO2), a byproduct of anthropogenic combustion processes, their atmospheric production and reactivity is of great interest. While the atmospheric reactivity of many relevant organic nitrates is still uncertain, one significant reactive pathway, condensed phase hydrolysis, has recently been identified as a potential sink for organic nitrate species. The partitioning of gas phase organic nitrates to aerosol particles and subsequent hydrolysis likely removes the oxidized nitrogen from further atmospheric processing, due to large organic nitrate uptake to aerosols and proposed hydrolysis lifetimes, which may impact long-range transport of NOx, a tropospheric ozone precursor. Despite the atmospheric importance, the hydrolysis rates and reaction mechanisms for atmospherically derived organic nitrates are almost completely unknown, including those derived from α-pinene, a biogenic volatile organic compound (BVOC) that is one of the most significant precursors to biogenic secondary organic aerosol (BSOA). To better understand the chemistry that governs the fate of particle phase organic nitrates, the hydrolysis mechanism and rate constants were elucidated for several organic nitrates, including an α-pinene-derived organic nitrate (APN). A positive trend in hydrolysis rate constants was observed with increasing solution acidity for all organic nitrates studied, with the tertiary APN lifetime ranging from 8.3 min at acidic pH (0.25) to 8.8 h at neutral pH (6.9). Since ambient fine aerosol pH values are observed to be acidic, the reported lifetimes, which are much shorter than that of atmospheric fine aerosol, provide important insight into the fate of particle phase organic nitrates. Along with rate constant data, product identification confirms that a unimolecular specific acid-catalyzed mechanism is responsible for organic nitrate hydrolysis under acidic conditions. The free energies and enthalpies of the isobutyl nitrate hydrolysis intermediates and products were calculated using a hybrid density functional (ωB97X-V) to support the proposed mechanisms. These findings provide valuable information regarding the organic nitrate hydrolysis mechanism and its contribution to the fate of atmospheric NOx, aerosol phase processing, and BSOA composition.
Highlights
The atmospheric oxidation of biogenic volatile organic compounds (BVOCs), which have annual emission rates (∼ 1100 Tg yr−1 total) roughly 1 order of magnitude larger than anthropogenic non-methane VOCs (Guenther et al, 1995), has a significant impact on air quality and climate
The production of secondary organic aerosol (SOA) from BVOC oxidation products influences the radiative balance of the planet by directly interacting with both solar and terrestrial radiation, as well as indirectly through their role as cloud condensation nuclei (e.g., Ramanathan et al, 2001)
For the tertiary an α-pinene-derived organic nitrate (APN), hydrolysis rate constants ranged from 3.2 × 10−5 s−1 at neutral pH (6.9) to 2.0 × 10−3 s−1 at low pH (0.25)
Summary
The atmospheric oxidation of biogenic volatile organic compounds (BVOCs), which have annual emission rates (∼ 1100 Tg yr−1 total) roughly 1 order of magnitude larger than anthropogenic non-methane VOCs (Guenther et al, 1995), has a significant impact on air quality and climate. The production of secondary organic aerosol (SOA) from BVOC oxidation products influences the radiative balance of the planet by directly interacting with both solar and terrestrial radiation, as well as indirectly through their role as cloud condensation nuclei (e.g., Ramanathan et al, 2001). The production of SOA from BVOCs has a cooling effect on global climate, estimated to have a combined radiative forcing as large as −1.5 W m−2 (Scott et al, 2014). The inhalation of SOA has a significant impact on the hu-. Despite the importance of SOA, the chemical mechanisms that explain the composition of aerosol particles and their chemical processes are still highly uncertain
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