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.
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Secondary organic aerosol formation from the β-pinene+NO<sub>3</sub> system: effect of humidity and peroxy radical fate
Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO2) fate on aerosol formation, "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C10H17NO4, C10H15NO5, C10H17NO5, and C10H15NO6, respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO+ (at m/z 30) and NO2+ (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol mass spectrum, with the NO+ : NO2+ ratio ranging from 4.8 to 10.2 in all experiments conducted. The SOA yields in the "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are comparable. For a wide range of organic mass loadings (5.1–216.1 μg m−3), the aerosol mass yield is calculated to be 27.0–104.1 %. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45–74 % of the organic aerosol. The extent of organic nitrate hydrolysis is significantly lower than that observed in previous studies on photooxidation of volatile organic compounds in the presence of NOx. It is estimated that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO3 reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Results from this laboratory chamber study provide the fundamental data to evaluate the contributions of monoterpene + NO3 reaction to ambient organic aerosol measured in the southeastern United States, including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.
more » « less- Award ID(s):
- 1255290
- NSF-PAR ID:
- 10107227
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 15
- Issue:
- 13
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 7497 to 7522
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. It has been widely observed around the world that the frequency and intensityof new particle formation (NPF) events are reduced during periods of highrelative humidity (RH). The current study focuses on how RH affects theformation of highly oxidized molecules (HOMs), which are key components ofNPF and initial growth caused by oxidized organics. The ozonolysis ofα-pinene, limonene, and Δ3-carene, with and without OHscavengers, were carried out under low NOx conditions undera range of RH (from ∼3 % to ∼92 %) in atemperature-controlled flow tube to generate secondary organic aerosol (SOA).A Scanning Mobility Particle Sizer (SMPS) was used to measure the sizedistribution of generated particles, and a novel transverse ionizationchemical ionization inlet with a high-resolution time-of-fight massspectrometer detected HOMs. A major finding from this work is that neitherthe detected HOMs nor their abundance changed significantly with RH, whichindicates that the detected HOMs must be formed from water-independentpathways. In fact, the distinguished OH- and O3-derived peroxyradicals (RO2), HOM monomers, and HOM dimers could mostly beexplained by the autoxidation of RO2 followed by bimolecularreactions with other RO2 or hydroperoxy radicals (HO2),rather than from a water-influenced pathway like through the formation of astabilized Criegee intermediate (sCI). However, as RH increased from ∼3 % to ∼92 %, the total SOA number concentrations decreased bya factor of 2–3 while SOA mass concentrations remained relatively constant. These observations show that, whilehigh RH appears to inhibit NPF as evident by the decreasing numberconcentration, this reduction is not caused by a decrease inRO2-derived HOM formation. Possible explanations for these phenomenawere discussed.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-15-7497-2015
