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Historical trends of biogenic SOA tracers in an ice core from Kamchatka Peninsula Pingqing Fu, Kimitaka Kawamura, Osamu Seki, Yusuke Izawa, Takayuki Shiraiwa, and Kirsti Ashworth Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00275 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016
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Historical trends of biogenic SOA tracers in an ice
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core from Kamchatka Peninsula
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Pingqing Fu1,2,*, Kimitaka Kawamura1, Osamu Seki1, Yusuke Izawa1,
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Takayuki Shiraiwa1 & Kirsti Ashworth3
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Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
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LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
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Biosphere-Atmosphere Interactions Group, Climate and Space Sciences and Engineering (CLaSP), University of Michigan, Ann Arbor, Michigan 48109-2143, USA
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Short Title: Ice core records of biogenic secondary organic aerosols
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*Corresponding author e-mail:
[email protected] 16
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TOC Art:
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Abstract:
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Biogenic secondary organic aerosol (SOA) is ubiquitous in the Earth’s atmosphere,
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influencing climate and air quality. However, the historical trend of biogenic SOA is not well
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known. Here we report for the first time the major isoprene- and monoterpene-derived SOA
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tracers preserved in an ice core from the Kamchatka Peninsula. Significant variations are
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recorded during the past 300 years with lower concentrations in the early-to-mid 19th century
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and higher concentrations in the preindustrial period and the present day. We discovered that
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isoprene-SOA tracers were more abundant in the preindustrial period than the present day,
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while monoterpene-SOA tracers stay almost unchanged. The causes of the observed
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variability are complex, depending on atmospheric circulation, changes in emissions, and
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other factors such as tropospheric oxidative capacity. Our data presents an unprecedented
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opportunity to shed light on the formation, evolution and fate of atmospheric aerosols and to
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constrain the uncertainties associated with modeling their atmospheric concentrations.
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INTRODUCTION
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Palaeoclimate archives containing annual layers (e.g., ice cores, tree rings, speleothems, and
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coral reefs) have played a central role in reconstructing decadal-scale climatic oscillation of
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the past (1). This insight has proved an invaluable tool to constrain climate model projections of
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future climate change by validating model hindcasts. Similarly, analysis of particles preserved in
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ice cores provides an unprecedented opportunity to elucidate the distribution, concentration,
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size distribution and even chemical composition of atmospheric aerosols in the past. Such data
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would allow us to deduce the influence of aerosol radiative forcing on past climate change.
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Previously, aerosol particles preserved in high altitudinal or high latitudinal ice cores have
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been examined for inorganic species (e.g., sulfate), black carbon, and organic species such as
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polycyclic aromatic hydrocarbons, carboxylic acids, biomass burning tracers, and humic-like
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substances (2-7). To date, little was known about the historical trends of secondary organic
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aerosols at a molecular level (3, 8). Here, we present the findings of the analysis of ice cores for
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evidence of organic compounds formed from biogenic trace gases.
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Terrestrial vegetation emits large quantities (~1 Pg C y–1) of biogenic volatile organic
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compounds (BVOCs), including reactive species such as isoprene and monoterpenes, to the
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atmosphere (9). The role of their atmospheric reactions in governing the production and loss of
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tropospheric ozone is well-studied and relatively well understood, but BVOC oxidation has also
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been shown to lead to aerosol formation (10-12). Organic particles formed by the
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photooxidation of BVOCs are considered “secondary” organic aerosols (SOA), and are
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believed to be more abundant than directly emitted “primary” organic aerosols (POA) in the
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Earth’s atmosphere (11, 13-16). It is believed that SOA could be a significant source of new
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nanoscale particles, especially in pristine remote regions (17, 18), that can grow into the
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accumulation mode and act as CCN, influencing local climate and radiative forcing. However,
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the uncertainties are substantial. Estimates of biogenic SOA production range from 9–910 Tg
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C y–1 with a best estimate of 60–240 Tg C y–1 (11, 19-21). The radiative forcing effect of
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has been estimated as –0.03 W m–2 (–0.27 to +0.20 W m–2) (22, 23), but this is highly
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dependent on assumptions of the total atmospheric burden of SOA. Given the ubiquity and influence of organic particles in the atmosphere, there is an urgent
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need to better understand and constrain the processes leading to the formation of SOA, and to
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elucidate the role of aerosols in governing global and regional climate. Studies of production,
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transformation and removal processes have been extensively conducted for ambient aerosols
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and simulated in laboratory conditions (24-28). Relationships between the phases of organic
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aerosols and their reactivity (29, 30) have been investigated. Model simulations have been
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performed to identify trends in SOA concentrations and distributions, and to quantify modern
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and past SOA budgets (31, 32). However, the uncertainties of such estimates are substantial,
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and better constraints are required.
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Here, we report 300 years of ice core records of biogenic SOA based on organic marker
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compounds produced by the oxidation of isoprene and monoterpenes from the Ushkovsky ice
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cap in Northeast Asia (Fig. 1). Such data represent a potential source of direct evidence of
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biogenic SOA concentrations and chemical properties that could be used to evaluate model
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hindcasts and constrain model projections of future budgets and radiative forcing of
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atmospheric aerosols.
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MATERIALS AND METHODS Study Area. The ice core (211.7 m long) was drilled from the ice cap of the Gorshkov
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crater at Ushkovsky volcano (56°04'N, 160°28'E; 3903 m a.s.l.) in the central part of the
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Kamchatka Peninsula, Russia (Fig. 1). Detailed ice core chronology (33) and analytical
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methodology are provided in the Supporting Information (SI). Here, seventy-five sections
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were cut off using a band saw. Ice core sections (50 cm long, 1/4 cut) were taken at every one
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meter for the upper 25 m and at every 4–5 meter for the layers deeper than 25 m.
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Approximately 1.0 cm thickness of the outer core surface was mechanically removed using a
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pre-cleaned ceramic knife in a cold clean room to avoid potential contamination.
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Bulk Analysis. Each sample section was melted in a pre-cleaned Pyrex beaker (2 l). The
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samples were poisoned with HgCl2 to prevent potential microbial degradation of organic
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compounds, and stored at 4°C in pre-cleaned brown glass bottles prior to analysis. In this
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study, we use 59 samples collected from 1.1 to 152.6 m in depth (1997–1693); the data of
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deeper sections (Table S1) were not used because of the presence of many sand layers.
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The melt water samples were transferred to a pear-shape flask and concentrated to
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almost complete dryness using a rotary evaporator under a vacuum. The total organic matter
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in the dried samples was extracted with a 2:1 v/v solution of CH2Cl2/CH3OH using an
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ultrasonic bath. The extracts were concentrated and passed through a glass column packed
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with quartz wool and further eluted with CH2Cl2 and CH3OH to extract the organics
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potentially adsorbed on the particles. The eluents were then combined with the extracts,
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transferred to 1.5 ml glass vials and dried under a pure nitrogen gas stream. Polar organic
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markers in the extracts were derivatized with 99% N, O-bis-(trimethylsilyl)trifluoroacetamide
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(BSTFA) and 1% trimethylsilyl chloride for 2 hours at 70°C in a sealed glass vial (1.5 ml).
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The derivatives were then diluted by the addition of n-hexane containing C13 n-alkane as an
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internal standard prior to the determination by gas chromatography-mass spectrometry
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(GC-MS).
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GC-MS Measurement. GC-MS analyses were performed on a Hewlett-Packard model
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6890 GC coupled to a Hewlett-Packard model 5973 MSD with a programmed GC oven
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temperature. Target compounds were identified by comparing the mass spectra with those of
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authentic standards or data in the literature (24, 34). Recoveries for the standards or
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surrogates were better than 80%. The analytical errors in triplicate analyses were within 15%.
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A laboratory blank was measured using Milli-Q water and showed no contamination for any
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target species.
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RESULTS
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The total concentrations of biogenic SOA tracers (Fig. 2A,B and Table S2) detected in the
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Ushkovsky ice cores range widely (50.2–18,400 pg/g-ice; mean 2,890 pg/g-ice), covering the
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period between 1693 and present day (1997). The enlarged figure covering the period of
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1950–1997 is provided in the supporting information (Fig. S2). The data are strongly
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positively skewed (median 3750 pg/g-ice; 10th and 90th percentiles 179 pg/g-ice and 6,230
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pg/g-ice respectively) due to anomalously high concentrations in a handful of years during
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the preindustrial (1693–1790) and the 20th century (1908–1997) periods, notably 1768 and
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1949. Concentrations of total organic carbon (Fig. 2C) as well as those of individual tracers
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were lowest in the 19th century and in particular during the peak of the Little Ice Age in
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Europe (early-mid 1800s), when temperatures were low throughout the biogenic source
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regions suppressing emissions (35, 36). On the whole, there is a strong correlation between
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ice-core SOA tracer concentrations and the Northern Hemisphere high-latitude temperature
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anomaly and a weak correlation with solar irradiance (Fig. 2A-E). While concentrations of
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the individual tracers fit this general pattern, there are also notable differences, which will be
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discussed later.
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Isoprene-SOA Tracers. Oxidation products of isoprene, the most prevalent
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non-methane hydrocarbon emitted to the atmosphere (37), have been shown to be significant
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contributors to global organic aerosol mass (24, 38). Concentrations of 2-methyltetrols
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(2-MT), the sum of 2-methylthreitol and 2-methylerythritol, in the Ushkovsky ice core ranged
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from 3.8 to 9,710 pg/g-ice (median 587.5 pg/g-ice; Table S2). 2-MT, together with C5-alkene
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triols and 3-methyltetrahydrofuran-3, 4-diols (3-MeTHF-3, 4-diols) neither of which were
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detected in the Ushkovsky ice core, are higher generation products formed from the
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photooxidation of epoxydiols of isoprene (IEPOX = β-IEPOX + α-IEPOX) under low-NOx
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(NOx = NO + NO2) or NOx-free conditions (39). In this study, 2-MT concentrations in the
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preindustrial period were about double those in the 20th century and more than thirty times
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higher than those during the end of Little Ice Age (early–mid 1800s). As expected, a strong
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correlation (R2 = 0.95, p
