Abstract. Oxidation of aromatic volatile organic compounds (VOCs) leads to the formation of tropospheric ozone and secondary organic aerosol, for which gaseous oxygenated products are important intermediates. We show,
herein, the experimental results of highly oxygenated organic molecules (HOMs) produced by the oxidation of benzene and toluene in a wide range of OH exposure and NOx conditions. The results suggest that multigeneration OH oxidation plays an important role in the product distribution, which likely proceeds more preferably via H subtraction than OH addition for early generation products from light aromatics. More oxygenated products present in our study than in previous flow tube studies, highlighting the impact of experimental conditions on product distributions. The formation of dimeric products, however, was suppressed and might be unfavorable under conditions of high OH exposure and low NOx in toluene oxidation. Under high-NOx conditions, nitrogen-containing multifunctional products are formed, while the formation of other HOMs is suppressed. Products containing two nitrogen atoms become more important as the NOx level increases, and the concentrations of these compounds depend significantly on NO2.
The highly oxygenated nitrogen-containing products might be peroxyacyl nitrates, implying a prolonged effective lifetime of RO2 that
facilitates regional pollution. Our results call for further investigation
on the roles of high-NO2 conditions in the oxidation of aromatic VOCs.
Nitrated phenols (NPs) are important atmospheric pollutants that affect air quality, radiation, and health. The recent development of the time-of-flight chemical ionization mass spectrometer (ToF-CIMS) allows quantitative online measurements of NPs for a better understanding of their sources and environmental impacts. Herein, we deployed nitrate ions as reagent ions in the ToF-CIMS and quantified six classes of gaseous NPs in Beijing. The concentrations of NPs are in the range of 1 to 520 ng m −3 . Nitrophenol (NPh) has the greatest mean concentration. Dinitrophenol (DNP) shows the greatest haze-to-clean concentration ratio, which may be associated with aqueous production. The high concentrations and distinct diurnal profiles of NPs indicate a strong secondary formation to overweigh losses, driven by high emissions of precursors, strong oxidative capacity, and high NO x levels. The budget analysis on the basis of our measurements and box-model calculations suggest a minor role of the photolysis of NPs (<1 ppb h −1 ) in producing OH radicals. NPs therefore cannot explain the underestimated OH production in urban environments. Discrepancies between these results and the laboratory measurements of the NP photolysis rates indicate the need for further studies aimed at understanding the production and losses of NPs in polluted urban environments.
Naphthalene
(Nap) and methylnaphthalene (MN) are the most abundant
polycyclic aromatic hydrocarbons (PAHs) in atmosphere and have been
proposed to be important precursors of anthropogenic secondary organic
aerosol (SOA) derived from laboratory chamber experiments. In this
study, atmospheric Nap/MN and their gas-phase photooxidation products
were quantified by a Proton Transfer Reaction-Quadrupole interface
Time-of-Flight Mass Spectrometer (PTR-QiTOF) during the 2016 winter
in Beijing. Phthalic anhydride, a late generation product from Nap
under high-NO
x
conditions, appeared to
be more prominent than 2-formylcinnamaldehyde (early generation product),
possibly due to more sufficient oxidation during the haze. 1,2-Phthalic
acid (1,2-PhA), the hydrated form of phthalic anhydride, was capable
of partitioning into aerosol phase and served as a tracer to explore
the contribution of Nap to ambient SOA. The measured fraction in particle
phase (F
p) of 1,2-PhA averaged at 73 ±
13% with OA mass loadings of 52.5–87.8 μg/m3, lower than the value predicted by the absorptive partitioning model
(100%). Using tracer product-based and precursor consumption-based
methods, 2-ring PAHs (Nap and MN) were estimated to produce 14.9%
(an upper limit) of the SOA formed in the afternoon during the wintertime
haze, suggesting a comparable contribution of Nap and MN with
monocyclic-aromatics on urban SOA formation.
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