Browsing by Subject "GAS-PHASE REACTIONS"

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  • Lambe, Andrew; Massoli, Paola; Zhang, Xuan; Canagaratna, Manjula; Nowak, John; Daube, Conner; Yan, Chao; Nie, Wei; Onasch, Timothy; Jayne, John; Kolb, Charles; Davidovits, Paul; Worsnop, Douglas; Brune, William (2017)
    Oxidation flow reactors that use low-pressure mercury lamps to produce hydroxyl (OH) radicals are an emerging technique for studying the oxidative aging of organic aerosols. Here, ozone (O-3) is photolyzed at 254 nm to produce O(D-1) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O-3 hinders the ability of oxidation flow reactors to simulate NOx-dependent secondary organic aerosol (SOA) formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NOx (NO + NO2) to nitric acid (HNO3), making it impossible to sustain NOx at levels that are sufficient to compete with hydroperoxy (HO2) radicals as a sink for organic peroxy (RO2) radicals. We developed a new method that is well suited to the characterization of NOx-dependent SOA formation pathways in oxidation flow reactors. NO and NO2 are produced via the reaction O(D-1) + N2O -> 2NO, followed by the reaction NO + O-3 -> NO2 + O-2. Laboratory measurements coupled with photochemical model simulations suggest that O(D-1) + N2O reactions can be used to systematically vary the relative branching ratio of RO2 + NO reactions relative to RO2 + HO2 and/or RO2 + RO2 reactions over a range of conditions relevant to atmospheric SOA formation. We demonstrate proof of concept using high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate (NO3-) reagent ion to detect gas-phase oxidation products of isoprene and alpha-pinene previously observed in NOx-influenced environments and in laboratory chamber experiments.
  • Lambe, Andrew T.; Krechmer, Jordan E.; Peng, Zhe; Casar, Jason R.; Carrasquillo, Anthony J.; Raff, Jonathan D.; Jimenez, Jose L.; Worsnop, Douglas R. (2019)
    Oxidation flow reactors (OFRs) are an emerging technique for studying the formation and oxidative aging of organic aerosols and other applications. In these flow reactors, hydroxyl radicals (OH), hydroperoxyl radicals (HO2), and nitric oxide (NO) are typically produced in the following ways: photolysis of ozone (O-3) at), = 254 nm, photolysis of H2O at), = 185 nm, and via reactions of O(D-1) with H2O and nitrous oxide (N2O); O(D-1) is formed via photolysis of O-3 at = 254 nm and/or N2O at = 185 nm. Here, we adapt a complementary method that uses alkyl nitrite photolysis as a source of OH via its production of HO2 and NO followed by the reaction NO + HO2 -> NO2 + OH. We present experimental and model characterization of the OH exposure and NO, levels generated via photolysis of C3 alkyl nitrites (isopropyl nitrite, perdeuterated isopropyl nitrite, 1,3-propyl dinitrite) in the Potential Aerosol Mass (PAM) OFR as a function of photolysis wavelength (7, = 254 to 369 nm) and organic nitrite concentration (0.5 to 20 ppm). We also apply this technique in conjunction with chemical ionization mass spectrometer measurements of multifunctional oxidation products generated following the exposure of a-Pinene to HO, and NO, obtained using both isopropyl nitrite and O-3 + H2O + N2O as the radical precursors.
  • Taipale, Ditte; Kerminen, Veli-Matti; Ehn, Mikael; Kulmala, Markku; Niinemets, Ülo (2021)
    Most trees emit volatile organic compounds (VOCs) continuously throughout their life, but the rate of emission and spectrum of emitted VOCs become substantially altered when the trees experience stress. Despite this, models to predict the emissions of VOCs do not account for perturbations caused by biotic plant stress. Considering that such stresses have generally been forecast to increase in both frequency and severity in the future climate, the neglect of stress-induced plant emissions in models might be one of the key obstacles for realistic climate change predictions, since changes in VOC concentrations are known to greatly influence atmospheric aerosol processes. Thus, we constructed a model to study the impact of biotic plant stresses on new particle formation and growth throughout a full growing season. We simulated the influence on aerosol processes caused by herbivory by the European gypsy moth (Lymantria dispar) and autumnal moth (Epirrita autumnata) feeding on pedunculate oak (Quercus robur) and mountain birch (Betula pubescens var. pumila), respectively, and also fungal infections of pedunculate oak and balsam poplar (Populus balsamifera var. suaveolens) by oak powdery mildew (Erysiphe alphitoides) and poplar rust (Melampsora larici-populina), respectively. Our modelling results indicate that all the investigated plant stresses are capable of substantially perturbing both the number and size of aerosol particles in atmospherically relevant conditions, with increases in the amount of newly formed particles by up to about an order of magnitude and additional daily growth of up to almost 50 nm. We also showed that it can be more important to account for biotic plant stresses in models for local and regional predictions of new particle formation and growth during the time of infestation or infection than significant variations in, e.g. leaf area index and temperature and light conditions, which are currently the main parameters controlling predictions of VOC emissions. Our study thus demonstrates that biotic plant stress can be highly atmospherically relevant. To validate our findings, field measurements are urgently needed to quantify the role of stress emissions in atmospheric aerosol processes and for making integration of biotic plant stress emission responses into numerical models for prediction of atmospheric chemistry and physics, including climate change projection models, possible.
  • Garmash, Olga; Rissanen, Matti P.; Pullinen, Iida; Schmitt, Sebastian; Kausiala, Oskari; Tillmann, Ralf; Zhao, Defeng; Percival, Carl J.; Bannan, Thomas; Priestley, Michael; Hallquist, Asa M.; Kleist, Einhard; Kiendler-Scharr, A.; Hallquist, Mattias; Berndt, Torsten; McFiggans, Gordon; Wildt, Jürgen; Mentel, Thomas F.; Ehn, Mikael (2020)
    Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Julich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1% to 14.0%, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.
  • Liu, Yang; Schallhart, Simon; Taipale, Ditte; Tykkä, Toni; Räsänen, Matti; Merbold, Lutz; Hellén, Heidi; Pellikka, Petri (2021)
    The East African lowland and highland areas consist of water-limited and humid ecosystems. The magnitude and seasonality of biogenic volatile organic compounds (BVOCs) emissions and concentrations from these functionally contrasting ecosystems are limited due to a scarcity of direct observations. We measured mixing ratios of BVOCs from two contrasting ecosystems, humid highlands with agroforestry and dry lowlands with bushland, grassland, and agriculture mosaics, during both the rainy and dry seasons of 2019 in southern Kenya. We present the diurnal and seasonal characteristics of BVOC mixing ratios and their reactivity and estimated emission factors (EFs) for certain BVOCs from the African lowland ecosystem based on field measurements. The most abundant BVOCs were isoprene and monoterpenoids (MTs), with isoprene contributing > 70 % of the total BVOC mixing ratio during daytime, while MTs accounted for > 50 % of the total BVOC mixing ratio during nighttime at both sites. The contributions of BVOCs to the local atmospheric chemistry were estimated by calculating the reactivity towards the hydroxyl radical (OH), ozone (O-3), and the nitrate radical (NO3). Isoprene and MTs contributed the most to the reactivity of OH and NO3, while sesquiterpenes dominated the contribution of organic compounds to the reactivity of O-3. The mixing ratio of isoprene measured in this study was lower than that measured in the relevant ecosystems in western and southern Africa, while that of monoterpenoids was similar. Isoprene mixing ratios peaked daily between 16:00 and 20:00 (all times are given as East Africa Time, UTC+3), with a maximum mixing ratio of 809 pptv (parts per trillion by volume) and 156 pptv in the highlands and 115 and 25 pptv in the lowlands during the rainy and dry seasons, respectively. MT mixing ratios reached their daily maximum between midnight and early morning (usually 04:00 to 08:00), with mixing ratios of 254 and 56 pptv in the highlands and 89 and 7 pptv in the lowlands in the rainy and dry seasons, respectively. The dominant species within the MT group were limonene, alpha-pinene, and beta-pinene. EFs for isoprene, MTs, and 2-Methyl-3-buten-2-ol (MBO) were estimated using an inverse modeling approach. The estimated EFs for isoprene and beta-pinene agreed very well with what is currently assumed in the world's most extensively used biogenic emissions model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN), for warm C-4 grass, but the estimated EFs for MBO, alpha-pinene, and especially limonene were significantly higher than that assumed in MEGAN for the relevant plant functional type. Additionally, our results indicate that the EF for limonene might be seasonally dependent in savanna ecosystems.
  • Yang, Yuan; Wang, Yonghong; Yao, Dan; Zhao, Shuman; Yang, Shuanghong; Ji, Dongsheng; Sun, Jie; Wang, Yinghong; Liu, Zirui; Hu, Bo; Zhang, Renjian; Wang, Yuesi (2020)
    To what extent anthropogenic emissions could influence volatile organic compound (VOCs) concentrations and related atmospheric reactivity is still poorly understood. China's 70th National Day holidays, during which anthropogenic emissions were significantly reduced to ensure good air quality on Anniversary Day, provides a unique opportunity to investigate these processes. Atmospheric oxidation capacity (AOC), OH reactivity, secondary transformation, O-3 formation and VOCs-PM2.5 sensitivity are evaluated based on parameterization methods and simultaneous measurements of VOCs, O-3, NOx, CO, SO2, PM2.5, JO(1)D, JNO(2), JNO(3) carried out at a suburban site between Beijing and Tianjin before, during, and after the National Day holiday 2019. During the National Day holidays, the AOC, OH reactivity, O-3 formation potential (OFP) and secondary organic aerosol formation potential (SOAP) were 1.6 x 10(7) molecules cm(-3) s(-1), 41.8 s(-1), 299.2 mg cm(-3) and 1471.8 mg cm(-3), respectively, which were 42%, 29%, 47% and 42% lower than pre-National Day values and -12%, 42%, 36% and 42% lower than post-National Day values, respectively. Reactions involving OH radicals dominated the AOC during the day, but OH radicals and O-3 reactions at night. Alkanes (the degree of unsaturation = 0, (D, Equation (1)) accounted for the largest contributions to the total VOCs concentration, oxygenated VOCs (OVOCs; D
  • Kontkanen, Jenni; Paasonen, Pauli; Aalto, Juho; Bäck, Jaana; Rantala, Pekka; Petäjä, Tuukka; Kulmala, Markku (2016)
    The oxidation products of monoterpenes likely have a crucial role in the formation and growth of aerosol particles in boreal forests. However, the continuous measurements of monoterpene concentrations are usually not available on decadal timescales, and the direct measurements of the concentrations of monoterpene oxidation product have so far been scarce. In this study we developed proxies for the concentrations of monoterpenes and their oxidation products at a boreal forest site in Hyytiala, southern Finland. For deriving the proxies we used the monoterpene concentration measured with a proton transfer reaction mass spectrometer (PTR-MS) during 2006-2013. Our proxies for the monoterpene concentration take into account the temperature-controlled emissions from the forest ecosystem, the dilution caused by the mixing within the boundary layer and different oxidation processes. All the versions of our proxies captured the seasonal variation of the monoterpene concentration, the typical proxy-to-measurements ratios being between 0.8 and 1.3 in summer and between 0.6 and 2.6 in winter. In addition, the proxies were able to describe the diurnal variation of the monoterpene concentration rather well, especially in summer months. By utilizing one of the proxies, we calculated the concentration of oxidation products of monoterpenes by considering their production in the oxidation and their loss due to condensation on aerosol particles. The concentration of oxidation products was found to have a clear seasonal cycle, with a maximum in summer and a minimum in winter. The concentration of oxidation products was lowest in the morning or around noon and highest in the evening. In the future, our proxies for the monoterpene concentration and their oxidation products can be used, for example, in the analysis of new particle formation and growth in boreal environments.
  • Hakola, Hannele; Tarvainen, Virpi; Praplan, Arnaud P.; Jaars, Kerneels; Hemmilä, Marja; Kulmala, Markku; Bäck, Jaana; Hellen, Heidi (2017)
    We present spring and summer volatile organic compound (VOC) emission rate measurements from Norway spruce (Picea abies L. Karst) growing in a boreal forest in southern Finland. The measurements were conducted using in situ gas chromatograph with 1 to 2 h time resolution to reveal quantitative and qualitative short-term and seasonal variability of the emissions. The measurements cover altogether 14 weeks in years 2011, 2014 and 2015. Monoterpene (MT) and sesquiterpene (SQT) emission rates were measured all the time, but isoprene only in 2014 and 2015 and acetone and C-4-C-10 aldehydes only in 2015. The emission rates of all the compounds were low in spring, but MT, acetone, and C-4-C-10 aldehyde emission rates increased as summer proceeded, reaching maximum emission rates in July. Late summer mean values (late July and August) were 29, 17, and 33 ng g(dw)(-1) h(-1) for MTs, acetone, and aldehydes respectively. SQT emission rates increased during the summer and highest emissions were measured in late summer (late summer mean value 84 ng g(dw)(-1) h(-1)) concomitant with highest linalool emissions most likely due to stress effects. The between-tree variability of emission pattern was studied by measuring seven different trees during the same afternoon using adsorbent tubes. Especially the contributions of limonene, terpinolene, and camphene were found to vary between trees, whereas proportions of alpha-pinene (25 +/- 5 %) and beta-pinene (7 +/- 3 %) were more stable. Our results show that it is important to measure emissions at canopy level due to irregular emission pattern, but reliable SQT emission data can be measured only from enclosures. SQT emissions contributed more than 90% of the ozone reactivity most of the time, and about 70% of the OH reactivity during late summer. The contribution of aldehydes to OH reactivity was comparable to that of MT during late summer, 10-30% most of the time.