Abstract
Abstract. Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of this chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find that the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, accounting for 26%, 2%, 7%, 20%, and 45%, of the production rate on average during the campaign, respectively. At high temperatures, when upwind isoprene emissions are highest, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate, with methylvinyl ketone being the most important of the isoprene-derived sources. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.
Highlights
Acyl peroxy nitrates (APNs, known as PANs) are an important class of reactive nitrogen species having the general structure: RC(O)OONO2
Measurements of acetaldehyde, propanal, biacetyl (2,3butanedione), methacrolein, methylvinyl ketone, isoprene and other organics were obtained on the same tower at 6 m using a 2-channel gas chromatograph/mass spectrometer (GC/MS) (Goldan et al, 2004; Bouvier-Brown et al, 2008)
Deviations of the steady-state predictions from observations at low temperatures can be explained by significant differences in the loss rates for MPANT relative to PPNT and PANT as a result of its fast loss to OH, which are especially apparent at low temperatures where loss to thermal decomposition for APNs is slow
Summary
Acyl peroxy nitrates (APNs, known as PANs) are an important class of reactive nitrogen species having the general structure: RC(O)OONO2. Detailed chemical mechanisms indicate that in addition to acetaldehyde, the isoprene oxidation products methyl glyoxal (MGLY), methylvinyl ketone (MVK), and methacrolein (MACR) are potential sources of the peroxy acetyl (PA) radical, the precursor to PAN Consistent with this expectation, Roberts et al (2001, 2002) found that for Nashville, TN, a region heavily impacted by isoprene emissions, production of PA radicals was significantly higher than could be accounted for by acetaldehyde alone. The primary objective of this study is to evaluate a comprehensive chemical mechanism for production of the PA radical by comparison to observations We approach this objective by making use of a steady-state model to determine whether mixing ratios of PAN, MPAN, and PPN, are in balance with their known sources and sinks. Et al (2007) study, which used the APN steady-state model to predict OH concentrations, the BEARPEX measurements include OH and HO2, providing an opportunity to thoroughly evaluate the APN steady-state model and to test the mechanisms of PA radical production
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