Volatile organic compounds from logwood combustion: Emissions and

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Volatile organic compounds from logwood combustion: Emissions and transformation under dark and photochemical aging conditions in a smog chamber Anni Hartikainen, Pasi Yli-Pirilä, Petri Tiitta, Ari Leskinen, Miika Kortelainen, Jürgen Orasche, Jürgen Schnelle-Kreis, Kari Lehtinen, Ralf Zimmermann, Jorma Jokiniemi, and Olli Sippula Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Environmental Science & Technology

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Volatile organic compounds from logwood

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combustion: Emissions and transformation under

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dark and photochemical aging conditions in a smog

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chamber

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Anni Hartikainen*†, Pasi Yli-Pirilä † ‡, Petri Tiitta †, Ari Leskinen ‡ §, Miika Kortelainen †, Jürgen

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Orasche∥⊥, Jürgen Schnelle-Kreis∥, Kari E. J. Lehtinen ‡ §, Ralf Zimmermann∥⊥, Jorma

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Jokiniemi†, and Olli Sippula †#

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† Department of Environmental and Biological Sciences, University of Eastern Finland, P.O.

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Box 1627, 70211 Kuopio, Finland

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‡ Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio,

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Finland

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§ Finnish Meteorological Institute, P.O. Box 1627, 70211 Kuopio, Finland

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∥ Cooperation Group Comprehensive Molecular Analytics, Helmholtz Zentrum München,

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German Research Center for Environmental Health, 85764 Oberschleißheim, Germany

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⊥ Analytical Chemistry, Institute of Chemistry, University of Rostock, 18059 Rostock, Germany

ACS Paragon Plus Environment

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# Department of Chemistry, University of Eastern Finland, P.O. Box 111, 80101 Joensuu,

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Finland

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* Corresponding author. Email: [email protected]

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ABSTRACT

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Residential wood combustion (RWC) emits high amounts of volatile organic compounds

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(VOCs) into ambient air, leading to formation of secondary organic aerosol (SOA) and various

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health and climate effects. In this study, the emission factors of VOCs from a logwood-fired

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modern masonry heater were measured using a Proton-Transfer-Reactor Time-of-Flight Mass

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Spectrometer (PTR-ToF-MS). Next, the VOCs were aged in a 29 m3 Teflon chamber equipped

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with UV black lights, where dark and photochemical atmospheric conditions were simulated.

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The main constituents of the VOC emissions were carbonyls and aromatic compounds, which

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accounted for 50%-52% and 30%-46% of the detected VOC emission, respectively. Emissions

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were highly susceptible to different combustion conditions, which caused a 2.4-fold variation in

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emission factors. The overall VOC concentrations declined considerably during both dark and

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photochemical aging, with simultaneous increase in particulate organic aerosol mass. Especially

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furanoic and phenolic compounds decreased, and they are suggested to be the major precursors

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of RWC-originated SOA in all aging conditions. On the other hand, dark aging produced

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relatively high amounts of nitrogen-containing organic compounds in both gas and particulate

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phase, while photochemical aging increased especially the concentrations of certain gaseous

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carbonyls, particularly acid anhydrides.

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INTRODUCTION


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Residential wood combustion (RWC) remains an important source of energy worldwide,

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creating substantial gaseous and particulate emissions with adverse health and climate effects.1, 2

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Overall, the emissions from logwood batch combustion contain high levels of particulate primary

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organic aerosol (POA) and numerous volatile organic compounds (VOCs).3-5 The composition of

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emissions depends on various factors, such as operating conditions, combustion technology, and

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fuel.6-9 Furthermore, the different phases of batch combustion, including the ignition, stable, and

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burnout phases, have distinct emission profiles.10, 11 The emissions are also transformed in the

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atmosphere, where the predominant chemistry determines the aging processes and its products.

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In atmosphere VOCs go through photolysis and react with atmospheric oxidants, namely, ozone

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(O3), hydroxyl radicals (OH·), and nitrate radicals (NO3·). These reactions create secondary

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organic aerosol (SOA), which makes up a major portion of the total particulate organic aerosol

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(OA) mass from residential biomass combustion.12-14 In addition, oxidation transforms the POA

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in biomass combustion plumes during aging.14-16 OH· is created from photolysis products, and it

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governs the daytime oxidation of VOCs. O3 stems from reactions of nitrogen oxides (NOx) with

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VOCs, and it is available throughout the day reacting mainly with unsaturated VOCs. NO3·, on

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the other hand, is produced from O3 oxidizing NO2. It has an oxidative role mainly in dark

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conditions, as it is very susceptible to photolysis. NO3· is, however, estimated to control the

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nighttime oxidation of VOCs and the production of SOA from anthropogenic sources with high

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NOx emissions.17 The proportion of VOC mass converted into particulate mass, i.e., the SOA

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yield (∆SOA/∆VOC), depends largely on the availability of oxidants, VOC species, and the

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amount of NOx and the initial particulate aerosol.18-20 Thus, the variability of the primary

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emissions from wood combustion complicates the estimations of SOA formation. Furthermore,

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the SOA yields of most of the emitted VOCs remain unassessed.3


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In this study, we identify the VOC emissions from combustion of a batch of spruce logs in a

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modern masonry heater. Two distinct ignition methods were used in order to consider different

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combustion conditions. Particularly, the used ignition procedure affects combustion rate of batch

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burning, which can result in different quantity and composition of emissions. Aging experiments

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were carried out in a smog chamber with ambient dark or photochemical aging conditions. In

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addition, the effect of diurnal, sequential aging was assessed by exposing dark aged plume to

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photochemical aging conditions. A Proton-Transfer-Reactor Time-of-Flight Mass Spectrometer

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(PTR-MS) and a variety of other aerosol and gas measurement instruments (Figure S1) were

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used to monitor the physico-chemical evolution of the emissions and formation of secondary

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species. In the study conducted by Tiitta et al.14 simultaneously with this study, we discovered a

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doubling of the OA mass in both dark and photochemical conditions. In this study, we provide

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emission factors for VOCs produced in logwood combustion, and an evaluation of their potential

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to act as precursors for SOA.

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MATERIALS AND METHODS

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Combustion and sampling

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The experiments were conducted in the ILMARI research facility at University of Eastern

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Finland, using a 29 m3 smog chamber specifically designed for combustion aerosol aging

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studies.21 A modern masonry heater, equipped with an air staging system, was used as the

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combustion source.22 The experimental setup is explained in-depth in Tiitta et al.,14 and schemes

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of the setup and the masonry heater are available in Figure S1. The chamber in use is a deflatable

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Teflon chamber, with a movable top ensuring constant overpressure in the chamber to prevent

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the aging sample from being diluted or contaminated. The design of the chamber is described in


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detail in Leskinen et al.21 Prior to each experiment, the chamber was flushed with purified air for

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16 hours. In the photochemical aging experiments, ambient-like UV-irradiance of 29.7 Wm-2

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was created with 47 blacklight lamps (Sylvania F 40 W/350 BL lamps) with irradiance centered

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at 350 nm. The temperature in the chamber was 18±2 °C and relative humidity (RH) 60±5%,

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which are representative of atmospheric conditions (Willet et al 2014).23 The experimental

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conditions in the five experiments are described in Table 1.

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In each experiment, ten 235 g logs of spruce (Picea abies) were laid crosswise and ignited

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from the top using 150 g of wood sticks as kindling. The experiments were carried out with two

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different kindling sizes, in order to produce varying combustion conditions. For slow ignition

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(SI) the sticks were twice the size of fast ignition (FI) kindling. The sampling of the combustion

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lasted 35 minutes, from cold-start ignition to the residual char burning of one batch.

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A multicomponent Fourier Transform Infrared Spectrometer (FTIR, Gasmet Technologies

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Inc.) was incorporated to the stack to characterize the contents of the fresh exhaust. The FTIR

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measurements included online monitoring of O2, CO2, CO, NO, NO2, CH4, and 28 organic

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compounds typical for combustion (Table S3). The exhaust was sampled throughout the

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combustion period with a PM10 pre-cyclone and a heated (150 °C) probe. Sample was diluted

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with clean air of ambient temperature with a porous tube diluter followed by an ejector diluter

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(DI-1000, Dekati Ltd). The prediluted sample was continuously led to the chamber pre-filled

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with clean air. Final dilution ratio (DR) in the chamber was 229-266. The degree of dilution was

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defined from CO2 concentrations in the raw gas (FTIR), in the sampling line (ABB CO2

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analyzer), and in the chamber (Vaisala GMP 343, Finland). The concentrations of NOx and O3 in

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the chamber were also continuously traced (Figure S7). Once chamber was filled, the uniform

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mixing of the sample was guaranteed with a 40-minute stabilization period.


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Aging procedure

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After stabilization, ozone was injected into the chamber to oxidize all NO in the chamber into

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NO2. In the three experiments with dark (D) aging (FI-D-PcA, FI-D-HONO and SI-D-PcA) the

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initial O3 concentration was adjusted to 40 ppb. The samples were then aged in the dark for four

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hours before turning the blacklights on for three hours for photochemical aging (PcA). In

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experiments FI-PcA and SI-PcA the photochemical aging was initiated directly after NO

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oxidation, and continued for four hours. The levels of NOx and O3 in the chamber are shown in

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Figure S7. In experiment FI-D-HONO, nitrous acid (HONO) was added into the chamber prior

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to photochemical aging to enhance OH production and simulate conditions with high amounts of

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oxidants. Simultaneously, propene was added to the chamber. Consequently, the VOC:NOx ratio

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remained similar to the ratio before HONO addition.

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The OH exposure in the chamber was traced by injecting 1 µl (approximately 9 ppb) of

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butanol-d9 into the chamber and monitoring its decay during experiments.24 The extent of

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photochemical aging is portrayed as atmospheric hours (atm. h), which corresponds to hours

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spent in ambient atmosphere with average OH concentration of 1E6 cm-3.

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Measurements in the chamber

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The VOCs were monitored with a high-resolution Proton-Transfer-Reactor Time-of-Flight

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Mass Spectrometer (PTR-TOF 8000, Ionicon Analytik, Innsbruck, Austria) with H3O+ as reagent

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ion. The data were analyzed with the Ionicon Analytik GmbH software (PTR MS Viewer,

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version 3.2.0). Calibration and use of PTR-MS is described in Section S1. The ratio of electric

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field to gas density in drift tube (E/N) was 132 Td. When available, reaction rates from Cappellin

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et al.25 for E/N 120 Td were applied in peak data analysis; for the other 77% of compounds,


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which covered approximately 30% of total emission factor, the reaction rate was assumed to be

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2E9 cm3 s-1 (Table S4). Fragmentation of VOCs in PTR-MS is discussed in Section S2. Due to

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the complexity and mostly minor effect of fragmentation, we only applied fragmentation

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correction (correction factor 2.1) for monoterpenes. The PTR-MS measurement of FI-D-PcA

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was excluded due to contamination of the instrument.

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Background concentrations in the chamber (Table S6) were determined for each experiment by

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monitoring the chamber before sample input. The detection limits of PTR-MS for each peak

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were set at three times the standard deviation during the background measurement. The

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measured compounds were background-corrected, identified based on ion chemical formulas and

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previous combustion studies, and grouped based on their chemical composition (Tables S4 and

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S6).4, 5, 9, 26 Emission factors (EFs) were calculated following the procedure explained in Section

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S6. To portray the different speeds of reactions in the chamber during aging treatments, we

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calculated the times (t-50%, minutes in chamber and atm. h for photochemical aging) when half

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of the change in concentration had taken place for each compound and group.

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The particulate-phase OA concentration and composition in the chamber was measured with a

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Soot Particle Aerosol Mass Spectrometer (SP-HR-ToF-AMS, Aerodyne Research Inc).27 The

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processing and results of the AMS measurement are described in Tiitta et al.14 The number and

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surface area concentrations of particles in the size range 13.9-749.9 nm were measured with a

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Scanning Mobility Particle Sizer (SMPS 3080, CPC 3022, TSI), while a Tapered Element

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Oscilliating Microbalance (TEOM, Model 1405, Thermo Scientific) was used for measuring the

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total particulate mass in the chamber. Estimations of SOA yields during aging are based on

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studies utilizing, e.g., the two-product model by Odum et al.28 See Section S2 for further

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information on SOA yield estimations.


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In FI-D-PcA, FI-D-HONO, SI-D-PcA, and FI-PcA, offline particulate samples were collected

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on quartz fiber filters (T-293, Munktell, Sweden). A flow rate of 0.2 lpm was fed to filters from

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the chamber for 35 minutes during the stabilization period, 90 minutes at the end of dark aging,

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and 90 minutes at the end of photochemical aging. The nitrophenol contents of the samples were

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analyzed with in situ derivatization thermal desorption gas chromatography and time-of-flight

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mass spectrometry (IDTD-GC-TOFMS).29

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Deposition of aerosols onto walls of the chamber is a disadvantage for every environmental

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chamber.30, 31 Wall loss correction for the particle concentration was estimated based on the

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decay of refractory black carbon.14 Gas-to-wall losses are, however, more challenging to

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estimate although they may have a noticeable effect on formation of SOA. To assess gas-to-wall

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losses, we calculated the condensation sinks caused by the particles (CS) in the chamber,32 which

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were high throughout the experiments (0.14-0.26 s-1; Figure S8). These sinks are two orders of

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magnitude higher than gas-to-wall losses determined by Ye et al.33 for a 10 m3 Teflon chamber.

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Furthermore, the ILMARI chamber is considerably larger and has a low surface-to-volume-ratio

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(58 m2:29 m3). As a conclusion, we estimate the losses onto walls to be minor compared to the

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condensation onto particles.

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3. RESULTS AND DISCUSSION

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Primary emission factors

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In total, 105 ions were observed from the chamber with PTR-MS (Figure 1). The EFs of all the

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observed ions are available in Table S4. Carbonyls and aromatic compounds had the highest

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emission factors, and have also previously been widely reported as the major VOC groups in

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RWC emissions.34-36 Aromatics were divided into four groups: aromatic hydrocarbons, furans,


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phenols, and other oxygenated aromatics. Carbonyls, on the other hand, were divided into two

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groups (Carbonyls-A and -B) based on their behavior during aging.

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VOC emissions from RWC were clearly affected by the change in ignition procedure, with SI

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emitting, in total, 2.4 times higher concentration of the VOCs detected with PTR-MS than FI.

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This difference was clearly observed in the time-dependent measurement of VOCs (Figure S2) in

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which SI was characterized by a longer ignition period with enhanced VOC emissions. Between

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the 11 VOC groups, the ratio of the EFs from SI to the EFs from FI (SI/FI-ratio) ranged between

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1.5 for nitrogen containing compounds to 2.7 for furanoic compounds and 3.4 for aliphatic

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hydrocarbons. Also the 28 VOCs measured with FTIR from the fresh exhaust during combustion

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(Table S3, Figure S6) clearly increased due to slower ignition. The total FTIR-based SI/FI-ratio

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was 1.7, with especially oxidized compounds increasing as they peaked at the end of ignition

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phase (SI/FI ratio 2.4, Figure S5). The total duration of the combustion cycle was longer in SI

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experiments, compared to the FI experiments where no flames were visible at the end of the 35

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minute sampling period.

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Overall, the VOCs measured with PTR-MS had considerably smaller EFs than previously

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measured from logwood combustion in less advanced residential combustion appliances,5, 34, 36

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but were in line with previous studies of the same appliance.11, 22 The average modified

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combustion efficiencies (MCE) during batch combustion were between 0.96-0.98 (Table 1,

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Figure S3), indicating good operation of the stove and, consequently, relatively efficient

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combustion conditions for a logwood operated appliance. However, note that the MCE did not

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correlate with the VOC emissions, which is in agreement with earlier studies.5, 37


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Carbonyls were the most abundant group in fresh emissions, with both the highest

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concentration and the highest number of identified compounds. Acetaldehyde (m/z 45.03) and

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acetic acid and glycolaldehyde (both at m/z 61.03), accounted for over a fifth of the total

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identified VOC EF. They have been the main carbonyls also in previous studies, where carbonyls

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such as ketones and acids routinely dominate VOC emissions.5, 9, 11 Based on FTIR-

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measurements, the formaldehyde EF was 31-35 mg kg-1 for FI experiments, and 59-72 mg kg-1

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for SI experiments. While the formaldehyde peak at m/z 31.02 was strongly visible in the PTR-

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MS spectrum, the humidity-dependence in PTR-MS impedes its quantification.38 Omitting

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formaldehyde, our PTR-MS based carbonyl EF (55-58 for FI, 120-140 mg kg-1 for SI), is

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consistent with previous determinations,22 and accounts for 44%-51% of the identified VOC

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emission. Reda et al.22 identified 12 gaseous carbonyls with gas chromatography−selective ion

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monitoring−mass spectrometry (GC-SIM-MS) from the same appliance. The total EF of those

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carbonyls from spruce combustion was 171±19 mg kg-1, of which formaldehyde accounted for

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108±9.5 mg kg-1.

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Benzene was clearly the most prominent aromatic compound with an EF of 10.6-22.3 mg kg-1.

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The EFs of the naphthalene and toluene were 23%-30% and 19%-23% of the benzene EFs, with

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higher shares in slow ignition cases. Interestingly, alkylbenzenes had higher SI/FI-ratios than

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benzene. We also observed substantial emission of furans, which is a relatively little studied

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group despite its abundance in RWC emissions.3, 9, 11 With the same appliance and fuel, Czech et

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al.40 found furanoic compounds releasing during both ignition and stable combustion phases, as

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they form during pyrolysis of the cellulose in the burned wood. Concentrations of furanoic

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compounds approached that of aromatic hydrocarbons, especially in SI experiments. Furanoic

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emission was dominated by furfural, which was the second most emitted of all aromatic


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compounds, with an EF of 3.2-10.3 mg kg-1. It was emitted 1.75-2.29 times than the furan

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emission (EF 1.4-5.1 mg kg-1). While the EFs of furanoic compounds are considerably (2-100

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times) lower than in e.g. Bruns et al.5 and Hatch et al.3, the ratios between the concentrations of

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these different furanoic compounds remain remarkably similar.

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Phenols were relatively susceptible to changing combustion conditions (SI/FI-ratio 2.3). The

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main phenolic compounds were phenol and benzenediols, followed by cresols and guaiacol, with

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EFs of 3.7-9.3, 2.1-5.6, 1.3-3.8, and 0.8-2.9 mg kg-1, respectively. Phenolic compounds are

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generally formed from thermal decomposition of lignin and have also previously been assessed

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as a significant constituent of biomass burning emissions.5, 9, 26 Nitrogen-containing VOCs

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(NVOCs) in the fresh emission were scarce, and relatively unaffected by combustion conditions.

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Overall, the SI/FI-ratios of NVOC groups were lower (1.5-1.6) than for other compound groups.

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The most prevalent NVOC was CH3NO2 (e.g. nitromethane or methyl nitrite, EF 7.7-11.9 mg kg-

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1

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nitrophenols (EF 0.5-0.6 mg kg-1) had an SI/FI-ratio of 0.8.

), which has not been observed in prior studies. It had an SI/FI-ratio of 1.3, while, for example,

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Monoterpenes and cyclopentadiene were the most abundant of the aliphatic hydrocarbons

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detected with PTR-MS, with EFs 0.6-3.4 and 0.6-1.6 mg kg-1, respectively. The measured

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monoterpenes may include at least 32 possible isomers previously identified from combustion of

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coniferous wood.26 The emissions of the numerous isomers depend highly on conditions, such as

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the fuel and appliance in use.9, 26 For example. Bruns et al.5 found no monoterpenes with PTR-

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MS from stable phase of beech log combustion. The ignition conditions had a major effect on

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monoterpene emissions, which had one of the highest SI/FI-ratios (4.2) of all the identified,

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individual VOCs. This could be due to evaporation of monoterpenes during ignition of the

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logwood batch, which was longer in SI experiments. Similar condition dependence has


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previously been seen by, e.g., Stockwell et al.,9 who found open cooking to emit 20 times more

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monoterpenes than a more developed appliance.

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Transformation of VOCs during dark aging

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The concentrations of most compound groups decreased upon exposure to ozone and NO3·

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(Figures 2 and 3, Table S6). During the dark aging experiments, the most extensive decay was

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observed for phenolic and furanoic compounds. The t-50% for phenolic compounds was 32-50

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minutes, and after four hours of dark aging only 13%-20% of the phenolic mass remained.

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Concentrations of furanoic compounds, on the other hand, decreased to 59%-63% of the initial

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concentration, with t-50% of 43-48 minutes. As the reaction rates of phenolic and furanoic

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compounds with ozone are very slow, NO3· is estimated to be their main oxidant in dark

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conditions (Table S8).39, 40

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The reactions of hydrocarbons in dark conditions were relatively quick, with t-50%s of 25 and

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22 minutes for aromatic and aliphatic hydrocarbons, respectively. Unsaturated hydrocarbons

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diminished completely during dark aging. For example, monoterpenes had t-50% of 17-27

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minutes and were below the detection limit after an hour of dark aging, most likely due to NO3·

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driven reactions (Table S8). Aromatic hydrocarbons were clearly more prone to react if they had

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alkyl substituents: toluene and naphthalene decreased by a third during dark aging, where

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naphthalene had also the highest absolute decrease (0.7-1.2 µg m-3). This is in line with their

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faster reaction rates towards NO3· and O3 than benzene (Table S8), which did also decrease

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noticeably (0.8-1.1 µg m-3) although the changes relative to initial concentrations were low.

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We witnessed substantial formation of nitrogen-containing VOCs (with m/z>100) during dark aging. Nitrophenols in the gas phase (C6H5NO3-H+, m/z 140.03) increased by 1-2 µg m-3 in the


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chamber, leading to a 290%-650% increase relative to the mass concentrations in the fresh

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sample. The production of nitrophenols was confirmed from the filter samples, which show their

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mass in particulate phase increasing by 530% of the initial concentration in SI-D-PcA, and by

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640% in FI-D-HONO (Table S5). Furthermore, formation of particulate organonitrate

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compounds was indicated by the AMS results.16 In addition to the nitrogenated compounds

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observed in the fresh emissions, new NVOCs with ion formulas such as C5H9NO-H+ (m/z

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100.07) and C5H7NO2-H+ (m/z 114.05) formed during dark aging. Generally, NVOCs, and their

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particulate counterparts, may originate from reactions of NO3· with, e.g., phenolic compounds or

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methylfurans,41-43 or reactions of NO2 with compounds originating from aromatic VOCs.44 While

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biomass combustion has been associated with the organic nitrogen compounds in nighttime

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ambient air in, for instance, Germany,45 United Kingdom,46 and Australia,47 this study is among

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the first to track the formation of gaseous nitrogen-containing organic compounds from wood

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combustion emissions.

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Transformation of VOCs during photochemical aging

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The exposure to OH· in the chamber during photochemical aging was equivalent to exposures

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of 14 and 18 atm. h for SI-PcA and FI-PcA, respectively. This, together with the enhanced

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ozonolysis, rapidly transformed the VOCs in the chamber (Figures 2 and 3, Table S7). In both

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FI-D-PcA and SI-D-PcA the OH· exposure corresponded to 14 atm. h after three hours of

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photochemical aging. The VOCs remaining after dark aging in SI-D-PcA behaved similarly to

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the experiments with only photochemical aging (Figure 3) and had similar t-50%s (Table S7). In

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FI-D-HONO, we tested the effect of HONO and propene on photochemical aging. The amount

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of OH· was expected to increase due to the addition of HONO and propene, as they are often

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used to accelerate atmospheric oxidation in chamber studies. Their addition expectedly led to


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high concentrations of NOx and VOCs, namely, carbonyls such as acetaldehyde and

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hydroxypropanone. However, the OH· exposure in FI-D-HONO corresponded to only 7 atm. h,

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indicating a decline in OH· availability. This may be due to propene reducing the availability of

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oxidants and consequently slowing the degradation of VOCs, as was suggested by Song et al.48

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Aromatic compounds went through a major decay during photochemical aging (Table S7), as

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they are highly reactive towards OH· (Table S8). Aromatic hydrocarbons, furanoic compounds,

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phenolic compounds, and other oxygen containing aromatic compounds decreased by 32%, 70%,

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80%, and 34% from their initial concentrations, respectively. These changes had t-50%s of 15,

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18, 14 and 27 minutes, respectively, which correspond to 3.7, 4.5, 3.8 and 6.1 atm. h. Similarly,

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aliphatic hydrocarbons decreased considerably, with t-50% of 10-16 minutes (2.4-4.6 atm. h.)

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and only 22% of concentration remaining at the fourth hour of photochemical aging. Degradation

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of aromatic compounds was clearly suppressed in the photochemical aging during the FI-D-

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HONO-experiment, where the conversion of especially phenols and aromatic hydrocarbons was

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restrained. The degradation of furanoic compounds during photochemical aging also slowed

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down due to HONO+propene addition, although they were the least affected by this change in

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chemistry.

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The photooxidation of aromatic hydrocarbons may also lead to formation of additional

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phenols, especially in high-NOx conditions.49 Additionally, the formation of NVOCs in

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photochemical conditions, when not preceded by dark aging, may originate from reactions of

303

OH· with aromatic hydrocarbons and phenols in the presence of NOx.49, 50 In SI-D-PcA, phenolic

304

compounds had already been consumed by dark aging, and the increase in NVOCs was

305

negligible during photochemical aging, although particulate nitrophenols increased considerably.


14

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306


Carbonyls had distinct transformation patterns under photochemical conditions. The

307

‘Carbonyls-A’ group decreased by 25%-55%, with some of the most prevalent carbonyls, such

308

as acrolein (m/z 57.03) and methyl vinyl ketone and methacrolein (both at m/z 71.05), decaying

309

by half. While the total concentration of carbonyls decreased, photochemical aging increased

310

concentrations of the ‘Carbonyls-B’ group, which consists of, e.g., acid anhydrides. This

311

enhancement was highest for acetic acid, which increased by 2.7 µg m-3 (+19%) and 5.4 µg m-3

312

(+13%) and maleic anhydride, increasing by 1.7 µg m-3 (+82%) and 5.6 µg m-3 (+64%) in FI-

313

PcA and SI-PcA, respectively. Most of the increase of Carbonyls-B took place during the first

314

24-41 minutes of photochemical aging (5.9-6.9 atm. h). Overall, the formation of these carbonyls

315

was most rapid at the beginning of photochemical aging, simultaneous with the decline of

316

aromatic compounds, which are potential precursors for carbonyl production.51, 52 However, the

317

increase continued throughout the experiments, and for example degradation of SOA may also

318

have participated in the formation of volatile organic acids.53

319

Fate of organic emissions under different conditions

320

Overall SOA yields. During dark aging, the particulate OA in the chamber increased by 14.9,

321

15.4 and 26.1 µg m-3 in FI-D-PcA, FI-D-HONO and SI-D-PcA, respectively, leading to OA

322

concentrations 1.6-1.7 times the initial concentrations (Figure S9). Simultaneously, the total

323

concentration of the VOCs measured with PTR-MS decreased by 20.7 µg m-3 (FI-D-HONO) and

324

56.0 µg m-3 (SI-D-PcA) indicating a total SOA yield of 47%-74% during dark aging if only the

325

identified compounds are considered. However, these values do not include, for instance, larger

326

compounds out of the mass range of PTR-MS.


15


Page 16 of 38

327

The OA mass increase during photochemical aging was 14.5 and 39.1 µg m-3 in FI-PcA and

328

SI-PcA, respectively, while the observed VOCs decreased by 19.3 and 47.3 µg m-3. This led to

329

final OA concentrations of 1.7 and 2.1 times the initial concentrations and total SOA yields of

330

75% and 83% for the observed VOCs, whereas the two-product parameters proposed by Barsanti

331

et al.54 result in estimated yield of 70%. During the photochemical aging of SI-D-PcA, SOA

332

increased by 22.6 µg m-3, while the identified VOCs decreased by 31.1 µg m-3 (SOA yield 73%).

333

During photochemical aging in FI-D-HONO, the mass of OA increased to 1.6 times the initial

334

OA (+13.8 µg m-3), similarly to FI-D-PcA . This SOA was, however, clearly more oxidized than

335

in other experiments with a final oxidation state of 0.6, while other experiments resulted in final

336

oxidation states between 0.1-0.3.14 The observed high oxidation state can also partly be due to

337

POA oxidizing more completely in FI-D-HONO than in other experiments. The addition of

338

propene seems to limit SOA formation, due to propene consuming oxidants without formation of

339

SOA.48, 55 At the same time, the conversion of some of the precursors, such as aromatic

340

compounds, was repressed. This points to fewer, but more oxidized reaction products

341

partitioning to particulate phase.

342

Effect of experimental conditions. The amount of SOA produced in this study is similar to

343

those from RWC of beech and various other fuels.12, 13, 60 However, SOA emissions from RWC

344

vary depending on both combustion and experimental conditions. For example, aging of an

345

exhaust from a complete batch is likely to differ from aging of separate phases of combustion.60

346

The ignition phase typically generates a major part of the VOC emissions , which leads to major

347

difference between this study and, for example, Bruns et al.,61 where only the stable phase of

348

beech-log combustion was considered. In order to fully grasp the SOA potential of RWC, a

349

broader set of conditions, together with a larger selection of appliances, fuels and operating


16

Page 17 of 38


350

practices, should be assessed. Furthermore, the setup of the experiments influences SOA

351

production: for example, the dilution of the exhaust affects the partitioning of compounds and

352

the formation SOA.62 DR of experiments (229-266) remain lower than in most ambient plumes,

353

where the dilution after a few hours can range from 10E5.62 Dilution enhances

354

evaporation of OA, and low dilution may lead to overestimation of POA.63 In contrast, high

355

dilution can enhance SOA formation due to reactions of the evaporated precursors.62

356

The formation of SOA was enhanced in SI experiments, due to the higher amount of initial

357

precursors and seed particles (M0 in Table 1). The transformation rates of VOCs, however, were

358

generally similar in both SI and FI (Figure 3). Furthermore, the different SI/FI-ratios led to

359

differences in the relative proportions of compounds groups; for example, furanoic compounds

360

were more and phenolic compounds less prevailing in SI experiments. The prevalent chemistry

361

was further impacted by differences in the VOC:NOx-ratios, which were higher in SI

362

experiments.

363

In this study, all NO was oxidized into NO2 prior to the aging procedures, which influences

364

the reaction pathways of VOCs.20 According to other studies, the relatively high NOx

365

concentration and low NO:NO2–ratio enhance SOA formation from aldehydes and furanoic

366

compounds,56, 57 while high-NOx may suppress SOA formation from other precursors, such as

367

aromatic hydrocarbons or α-pinene.58-59 Furthermore, the relatively high condensation sinks of

368

the existing particulate matter may lead to higher SOA yields in this study, compared to studies

369

with less seed particles or a smaller chamber, where more VOCs condense on the walls instead

370

of the particles.


17


371

Page 18 of 38

Phenolic compounds. Phenol was one of the major single precursors for SOA in both dark and

372

photochemical aging. In addition to its high decay rate, the decrease of phenolic compounds

373

during dark aging correlated with the formation of the portion of SOA initiated by NO3· (R2

374

0.98-0.99, Figure S10). Furthermore, the strong increase in nitrophenols in both particulate and

375

gas phases indicates that the phenolic compounds are important precursors for nitrogen

376

containing SOA. In photochemical low-NOx conditions, the SOA yield of phenol has been

377

assessed to be approximately 40%.64, 65 Conservatively estimating, phenol would account for

378

5%-9% of the measured photochemical SOA, although the higher NOx may enhance the yield. In

379

addition, we propose cresols and benzenediols as important precursors for SOA due to their high

380

decay rates and relatively high SOA yields: around 40% for cresols,66 and 39% for the

381

benzenediol catechol,65 and they are estimated to produce 1.5%-1.6% and 2.5%-3.2% of the total

382

SOA, respectively. In total, if all phenolic compounds would have a SOA yield of 40%, their

383

transformation would explain 12%-14% of all photochemical SOA. This is noticeably less than

384

in Bruns et al.,61 who found phenol alone to account for, on average, 26% of SOA formation

385

from the stable phase of RWC batch with slightly higher NOx concentrations.

386

Furanoic compounds. During photochemical aging, the absolute decrease of furanoic

387

compounds was slightly higher than that of phenolic compounds. In dark conditions their decay

388

correlated with the increase of NO3·-initiated SOA (R2 0.94-0.95, Figure S10), and, supported by

389

Hatch et al.,3 we propose furanoic compounds as a potential major group of SOA precursors in

390

both aging modes. However, the SOA yields of, for instance, furfural and methylfurans should

391

be confirmed in order to ascertain the impact of their conversion in the atmosphere.

392 393

Aromatic hydrocarbons are well-recognized SOA precursors under photochemical conditions.58, 67 Benzene and naphthalene had the highest absolute decay rates, and can be


18

Page 19 of 38


394

considered the most important SOA precursors of aromatic hydrocarbons, as the SOA yields of

395

aromatics have been reported to decrease with increasing amounts of alkyl groups.68 We estimate

396

the SOA yield of naphthalene during photooxidation to be 37% in all experiments, and the yield

397

of benzene to range from 55% in FI-PcA to 61% in SI-PcA, based on the two-product parameters

398

presented by Barsanti et al.54 and Ng et al.58 See Section S3 for information on the procedure on

399

yield estimation. The contributions of benzene and naphthalene to SOA formation under

400

photochemical conditions were similar: benzene accounted for 1.9%-2.6%, naphthalene for

401

1.9%-3.3% of the SOA, with higher relative contributions in the fast ignition experiment.

402

However, since the SOA yield of aromatic hydrocarbons depends on humidity, the relatively

403

high RH likely enhanced their SOA yield in these experiments.69-71

404

Monoterpenes were the major aliphatic SOA precursors observed. According to literature, the

405

SOA precursor potentials of monoterpenes are strongly dependent on NOx level under both dark

406

and photochemical conditions, and their yields are very structure-dependent, especially from

407

reactions with NO3·.72-75 Thus, estimating the SOA formation from monoterpenes is

408

complicated; see Section S3 for further information. Combining the isomer distribution presented

409

by Hatch et al.26 for black spruce with the SOA yield evaluations for monoterpenes+NO3·

410

collected by Ng et al.76, we estimate monoterpenes to account for 0.7% to 1.4% of SOA in FI-D-

411

HONO and SI-D-PcA, respectively. In photochemical conditions, we estimate that monoterpenes

412

provided 1%-3% of SOA, based on yields reported by Lee et al.77

413

Assessing the SOA formation in the atmosphere based on VOC decay is limited by unknown

414

precursors and the diversity of yields in different conditions. There is a particular lack of

415

information on SOA yields under dark conditions. However, based on these results, the major

416

SOA precursors from residential wood combustion are surprisingly similar in both night- and


19


Page 20 of 38

417

daytime: namely, furans and phenols. On the other hand, we observed that the products of VOC

418

transformation can differ under different atmospheric conditions. In particular, we found

419

relatively high production of nitrogen-containing VOCs especially during dark aging, while acid

420

anhydrides were more prominent in photochemical aging. These changes may also alter the

421

health effects of the emissions. Sequential photooxidation of the dark aged exhaust showed that

422

changes in ambient chemistry may renew VOC decay and enhance SOA production of biomass

423

exhaust hours after being emitted. The considerable increase of organic aerosol and several

424

gaseous carbonyls upon atmospheric aging may significantly change the potential of residential

425

wood combustion to cause adverse health and climate effects. Therefore, it is highly important to

426

consider also the transformation of emissions in ambient air in order to evaluate the effects of

427

emissions on air quality and public health.

428 429

TOC art.

430


20

Page 21 of 38


431 432

Figure 1. Average emission factors of VOCs from combustion of a spruce logwood batch for

433

each slow (SI) and fast ignition (SI) experiment. EFs are based on initial VOC concentrations,

434

measured from the chamber with PTR-MS, and the procedure fully explained in Section S6. The

435

number of compounds in each group is given in parentheses.


21


Page 22 of 38

436 437

Figure 2. Changes in the mass concentrations of VOCs measured with PTR-MS during dark

438

(FI-D-HONO, SI-D-PcA) and photochemical aging (FI-PcA, SI-PcA), relative to the initial

439

emission. *not including methanol

440


22

Page 23 of 38


441

Figure 3. Development of the mass concentrations of VOC groups traced with PTR-MS in the

442

chamber, relative to the initial concentration. *Time from I: ignition, D: start of dark aging, PcA:

443

turning on the UV-lights in the chamber. aCHO not including methanol.

444

Table 1. Experimental conditions and initial concentrations of particulate mass (M0, measured

445

with TEOM), primary organic aerosol (POA, AMS), particle surface area (A, SMPS), and NOx

446

in the chamber. The extent of OH· exposure (# cm-3 s) is expressed in parenthesis as the

447

atmospheric equivalent age, which corresponds to hours spent in atmosphere.

ECF Experiment

Ignition

Aging treatment

DR

VOC:NOx

M0

A

POA

NOx

OH exp.

MCE 3

-1

-1

-3

2

-3

-3

[m kg ]

[ppbv ppbv ]

[µg m ]

[mm cm ]

[µg m ]

[ppb]

[# cm-3 s (atm.h)]

259

0.96

5.8

3.1

150±19

4.6±0.09

28±0.8

105±0.3

5.0E10 (14)

Dark and photochemical

245

0.98

6.2

4.9

192±25

6.2±0.08

37±1.2

86±0.2

5.2E10 (14)

Fast

Dark and photochemical addition of HONO+propene

245

0.96

6.0

4.0

151±23

5.1±0.06

26±0.8

79±0.2

2.6E10 (7)

FI-PcA

Fast

Photochemical

266

0.97

6.1

2.7

129±22

3.6±0.05

21±1.9

118±0.3

5.1E10 (14)

SI-PcA

Slow

Photochemical

229

0.97

5.8

4.8

172±21

5.8±0.06

37±1.7

94±0.2

6.6E10 (18)

FI-D-PcA

Fast

Dark and photochemical

SI-D-PcA

Slow

FI-D-HONO

448 449 450

a

The final dilution ratio of the exhaust gas in the chamber. b Emission conversion factor, in m3 kg-1 fuel. c The ratio of VOCs to NOx in the fresh exhaust, measured with FTIR.

451 452

ASSOCIATED CONTENT

453

Supportive information contains

454

Scheme of the experimental setup and further information about the progression and efficiency

455

of a combustion cycle; further information on the use and calibration of PTR-MS, measured

456

spectra, and fragmentation in the instrument; discussion about the estimation of the SOA yields;

457

table of the compounds measured with FTIR from the fresh emission, and figures of the EFs and

458

timeseries of the VOC emissions measured with FTIR; figures of concentrations of NOx and O3


23


Page 24 of 38

459

and CS in the chamber; figure of the OA concentrations and oxidation states during the

460

experiments; figure showing the correlation of NO3 initiated SOA to the decay of phenolic and

461

furanoic compounds; procedure for emission factor calculation; tables with precise listing of the

462

identified compounds, their emission factors and concentrations in different phases, and of the

463

nitrophenols in particulate phase, and a table of reaction rates of oxidants with selected VOCs.

464 465

ACKNOWLEDGMENTS

466

Funding by the Academy of Finland (ASTRO-project, Grant No. 304459 and NABCEA-

467

project, Grant No. 296645), Doctoral School of University of Eastern Finland, and the Helmholtz

468

Foundation for the HICE virtual institute (www.hice-vi.eu) is gratefully acknowledged.

469

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