Abstract
Abstract. Experiments performed in laboratory chambers have contributed significantly to the understanding of the fundamental kinetics and mechanisms of the chemical reactions occurring in the atmosphere. Two chemical regimes, classified as “high-NO” vs. “zero-NO” conditions, have been extensively studied in previous chamber experiments. Results derived from these two chemical scenarios are widely parameterized in chemical transport models to represent key atmospheric processes in urban and pristine environments. As the anthropogenic NOx emissions in the United States have decreased remarkably in the past few decades, the classic “high-NO” and “zero-NO” conditions are no longer applicable to many regions that are constantly impacted by both polluted and background air masses. We present here the development and characterization of the NCAR Atmospheric Simulation Chamber, which is operated in steady-state continuous flow mode for the study of atmospheric chemistry under “intermediate NO” conditions. This particular chemical regime is characterized by constant sub-ppb levels of NO and can be created in the chamber by precise control of the inflow NO concentration and the ratio of chamber mixing to residence timescales. Over the range of conditions achievable in the chamber, the lifetime of peroxy radicals (RO2), a key intermediate from the atmospheric degradation of volatile organic compounds (VOCs), can be extended to several minutes, and a diverse array of reaction pathways, including unimolecular pathways and bimolecular reactions with NO and HO2, can thus be explored. Characterization experiments under photolytic and dark conditions were performed and, in conjunction with model predictions, provide a basis for interpretation of prevailing atmospheric processes in environments with intertwined biogenic and anthropogenic activities. We demonstrate the proof of concept of the steady-state continuous flow chamber operation through measurements of major first-generation products, methacrolein (MACR) and methyl vinyl ketone (MVK), from OH- and NO3-initiated oxidation of isoprene.
Highlights
A particular research focus has been understanding the influence of nitrogen oxides (NOx = NO+NO2) on the atmospheric oxidation cascades of biogenic volatile organic compounds (BVOCs), which generate ozone (O3) and secondary organic aerosols (SOA)
Nitrogen oxides alter the distribution of BVOC oxidation products by primarily modulating the fate of peroxy radicals (RO2), a key intermediate produced from the atmospheric degradation of volatile organic compounds (VOCs) by major oxidants including OH, O3, and NO3
Reaction of peroxyacyl radicals (RC(O)O2) with NO2 produces peroxyacyl nitrates that constitute a large reservoir of reactive nitrogen and a potentially important SOA precursor (Singh and Hanst, 1981; Nguyen et al, 2015)
Summary
With the discovery of the role of biogenic volatile organic compounds (BVOCs) in urban photochemical smog (Chameides et al, 1988), the interactions of biogenic emissions with manmade pollution and their subsequent impact on the atmosphere’s oxidative capacity and aerosol burden have received extensive studies in the ensuing decades (De Gouw et al, 2005; Ng et al, 2007; Goldstein et al, 2009; Surratt et al, 2010; Rollins et al, 2012; Shilling et al, 2013; Xu et al, 2015). Recent theoretical and laboratory studies have found that the hydroxy peroxy radical conformers produced from isoprene photooxidation decompose readily to allylic radicals on timescales faster than bimolecular processes under atmospherically relevant NO and HO2 levels (tens to hundreds of parts per trillion by volume) This highly dynamic system leads to formation of distinctly different products that depend on the concentrations of bimolecular reaction partners from those observed in chamber experiments under “highNO” and “zero-NO” conditions (Teng et al, 2017). We focus on establishing an “intermediate NO” regime characterized by a constant steady-state NO level ranging from tens of ppt to a few ppb in the chamber This particular chemical regime is well suited for the study of atmospheric behavior of RO2 radicals, as they can survive up to minutes and embrace various reaction possibilities as opposed to reaction with NO, NO3, HO2, and RO2 as their dominant fate observed in most batch-mode chamber. We employ the “intermediate NO” regimes to reexamine the daytime and nocturnal chemistry of isoprene through the measurements of two first-generation products, methacrolein (MACR) and methyl vinyl ketone (MVK)
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