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Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A modelAbstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such asremote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, itsevolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicitGenerator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidationof several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading tomore »
Organic peroxy radical chemistry in oxidation flow reactors and environmental chambers and their atmospheric relevance
Abstract. Oxidation flow reactors (OFRs) are a promising complement toenvironmental chambers for investigating atmospheric oxidation processes andsecondary aerosol formation. However, questions have been raised about howrepresentative the chemistry within OFRs is of that in the troposphere. Weinvestigate the fates of organic peroxy radicals (RO2), which playa central role in atmospheric organic chemistry, in OFRs and environmentalchambers by chemical kinetic modeling and compare to a variety of ambientconditions to help define a range of atmospherically relevant OFR operatingconditions. For most types of RO2, their bimolecular fates in OFRsare mainly RO2+HO2 and RO2+NO, similar to chambers andatmospheric studies. For substituted primary RO2 and acylRO2, RO2+RO2 can make a significant contribution tothe fate of RO2 in OFRs, chambers and the atmosphere, butRO2+RO2 in OFRs is in general somewhat less important than inthe atmosphere. At high NO, RO2+NO dominates RO2 fate inOFRs, as in the atmosphere. At a high UV lamp setting in OFRs,RO2+OH can be a major RO2 fate and RO2isomerization can be negligible for common multifunctional RO2,both of which deviate from common atmospheric conditions. In the OFR254operation mode (for which OH is generated only from the photolysismore »
Exploration of oxidative chemistry and secondary organic aerosol formation in the Amazon during the wet season: explicit modeling of the Manaus urban plume with GECKO-AAbstract. The GoAmazon 2014/5 field campaign took place in Manaus, Brazil, and allowed the investigation of the interaction between background-level biogenic air masses and anthropogenic plumes.We present in this work a box model built to simulate the impact of urban chemistry on biogenic secondary organic aerosol (SOA) formation and composition.An organic chemistry mechanism is generated with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) to simulate the explicit oxidation of biogenic and anthropogenic compounds.A parameterization is also included to account for the reactive uptake of isoprene oxidation products on aqueous particles.The biogenic emissions estimated from existing emission inventories had to be reduced to match measurements.The model is able to reproduce ozone and NOx for clean and polluted situations.The explicit model is able to reproduce background case SOA mass concentrations but does not capture the enhancement observed in the urban plume.The oxidation of biogenic compounds is the major contributor to SOA mass.A volatility basis set (VBS) parameterization applied to the same cases obtains better results than GECKO-A for predicting SOA mass in the box model.The explicit mechanism may be missing SOA-formation processes related to the oxidation of monoterpenes that could be implicitly accounted for in themore »
An omnipresent diversity and variability in the chemical composition of atmospheric functionalized organic aerosol
The atmospheric evolution of organic compounds encompasses many thousands of compounds with varying volatility, polarity, and water solubility. The molecular-level chemical composition of this mixture plays a major, yet uncertain, role in its transformations and impacts. Here we perform a non-targeted molecular-level intercomparison of functionalized organic aerosol from three diverse field sites and a chamber. Despite similar bulk composition, we report large molecular-level variability between multi-hour organic aerosol samples at each site, with 66 ± 13% of functionalized compounds differing between consecutive samples. Single precursor environmental laboratory chamber experiments and fully chemically-explicit modeling confirm this variability is due to changes in emitted precursors, chemical age, and/or oxidation conditions. These molecular-level results demonstrate greater compositional variability than is typically observed in less-speciated measurements, such as bulk elemental composition, which tend to show less daily variability. These observations should inform future field and laboratory studies, including assessments of the effects of variability on aerosol properties and ultimately the development of strategic organic aerosol parameterizations for air quality and climate models.