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DOE OSTI2012

DISCOVERY OF A VERY LOW MASS TRIPLE WITH LATE-M AND T DWARF COMPONENTS: LP 704-48/SDSS J0006-0852AB

We report the identification of the M9 dwarf SDSS J000649.16-085246.3 as a spectral binary and radial velocity (RV) variable with components straddling the hydrogen-burning mass limit. Low-resolution near-infrared spectroscopy reveals spectral features indicative of a T dwarf companion, and spectral template fitting yields component types of M8.5 {+-} 0.5 and T5 {+-} 1. High-resolution near-infrared spectroscopy with Keck/NIRSPEC reveals pronounced RV variations with a semi-amplitude of 8.2 {+-} 0.4 km s{sup -1}. From these we determine an orbital period of 147.6 {+-} 1.5 days and eccentricity of 0.10 {+-} 0.07, making SDSS J0006-0852AB the third tightest very low mass binary known. This system is also found to have a common proper motion companion, the inactive M7 dwarf LP 704-48, at a projected separation of 820 {+-} 120 AU. The lack of H{alpha} emission in both M dwarf components indicates that this system is relatively old, as confirmed by evolutionary model analysis of the tight binary. LP 704-48/SDSS J0006-0852AB is the lowest-mass confirmed triple identified to date, and one of only seven candidate and confirmed triples with total masses below 0.3 M{sub Sun} currently known. We show that current star and brown dwarf formation models cannot produce triple systems like LP 704-48/SDSS J0006-0852AB, and we rule out Kozai-Lidov perturbations and tidal circularization as a viable mechanism to shrink the inner orbit. The similarities between this system and the recently uncovered low-mass eclipsing triples NLTT 41135AB/41136 and LHS 6343ABC suggest that substellar tertiaries may be common in wide M dwarf pairs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Determining Empirical Stellar Masses and Radii from Transits and Gaia Parallaxes as Illustrated by Spitzer Observations of KELT-11b

Using the Spitzer Space Telescope, we observed a transit at 3.6 μm of KELT-11b. We also observed three partial planetary transits from the ground. We simultaneously fit these observations, ground-based photometry from Pepper et al., radial velocity data from Pepper et al., and a spectral energy distribution (SED) model using catalog magnitudes and the Hipparcos parallax to the system. The only significant difference between our results and those of Pepper et al. is that we find the orbital period to be shorter by 37 s, 4.73610 ± 0.00003 versus 4.73653 ± 0.00006 days, and we measure a transit center time of BJD{sub TDB} 2457483.4310 ± 0.0007, which is 42 minutes earlier than predicted. Using our new photometry, we precisely measure the density of the star KELT-11 to 4%. By combining the parallax and catalog magnitudes of the system, we are able to measure the radius of KELT-11b essentially empirically. Coupled with the stellar density, this gives a parallactic mass and radius of 1.8 M{sub ⊙} and 2.9 R{sub ⊙}, which are each approximately 1σ higher than the adopted model-estimated mass and radius. If we conduct the same fit using the expected parallax uncertainty from the final Gaia data release, this difference increases to 4σ. The differences between the model and parallactic masses and radii for KELT-11 demonstrate the role that precise Gaia parallaxes, coupled with simultaneous photometric, radial velocity, and SED fitting, can play in determining stellar and planetary parameters. With high-precision photometry of transiting planets and high-precision Gaia parallaxes, the parallactic mass and radius uncertainties of stars become 1% and 3%, respectively. TESS is expected to discover 60–80 systems where these measurements will be possible. These parallactic mass and radius measurements have uncertainties small enough that they may provide observational input into the stellar models themselves.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Improving Collision Induced Dissociation (CID), High Energy Collision Dissociation (HCD), and Electron Transfer Dissociation (ETD) Fourier Transform MS/MS Degradome-Peptidome Identifications Using High Accuracy Mass Information

Identification of peptides through analysis of peptide ion fragmentation mass spectra is key to proteomic/peptidomic studies. Advanced mass spectrometry has enabled measurement of peptide fragments with {approx}ppm mass error levels; however, mass accuracy has not been widely utilized in broad aspects of peptide identification. In this work we describe how peptide high-precision fragments measured with Fourier transform mass spectrometry can provide a powerful discriminative capability to distinguish true peptide 'hits' from possible incorrect peptides. For identification of peptides having multiple termini and carrying multiple charges, we show that the simple utilization of high-precision mass fragments information results in improved peptide analysis coverage and consistency, with coverage of >95% peptides identified totally from different spectral scoring and peptide statistic probability methods developed for moderate accuracy mass spectral peptide identification.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Hubble space telescope near-ir transmission spectroscopy of the super-Earth HD 97658B

Recent results from the Kepler mission indicate that super-Earths (planets with masses between 1-10 times that of the Earth) are the most common kind of planet around nearby Sun-like stars. These planets have no direct solar system analogue, and are currently one of the least well-understood classes of extrasolar planets. Many super-Earths have average densities that are consistent with a broad range of bulk compositions, including both water-dominated worlds and rocky planets covered by a thick hydrogen and helium atmosphere. Measurements of the transmission spectra of these planets offer the opportunity to resolve this degeneracy by directly constraining the scale heights and corresponding mean molecular weights of their atmospheres. We present Hubble Space Telescope near-infrared spectroscopy of two transits of the newly discovered transiting super-Earth HD 97658b. We use the Wide Field Camera 3's (WFC3) scanning mode to measure the wavelength-dependent transit depth in 30 individual bandpasses. Our averaged differential transmission spectrum has a median 1σ uncertainty of 23 ppm in individual bins, making this the most precise observation of an exoplanetary transmission spectrum obtained with WFC3 to date. Our data are inconsistent with a cloud-free solar metallicity atmosphere at the 10σ level. They are consistent at the 0.4σ level with a flat line model, as well as effectively flat models corresponding to a metal-rich atmosphere or a solar metallicity atmosphere with a cloud or haze layer located at pressures of 10 mbar or higher.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

BROWN DWARFS IN YOUNG MOVING GROUPS FROM PAN-STARRS1. I. AB DORADUS

Substellar members of young (≲150 Myr) moving groups are valuable benchmarks to empirically define brown dwarf evolution with age and to study the low-mass end of the initial mass function. We have combined Pan-STARRS1 (PS1) proper motions with optical–IR photometry from PS1, Two Micron All Sky Survey (2MASS), and WISE to search for substellar members of the AB Dor Moving Group within ≈50 pc and with spectral types of late M to early L, corresponding to masses down to ≈30 M {sub Jup} at the age of the group (≈125 Myr). Including both photometry and proper motions allows us to better select candidates by excluding field dwarfs whose colors are similar to young AB Dor Moving Group members. Our near-IR spectroscopy has identified six ultracool dwarfs (M6–L4; ≈30–100 M {sub Jup}) with intermediate surface gravities (int-g) as candidate members of the AB Dor Moving Group. We find another two candidate members with spectra showing hints of youth but consistent with field gravities. We also find four field brown dwarfs unassociated with the AB Dor Moving Group, three of which have int-g gravity classification. While signatures of youth are present in the spectra of our ≈125 Myr objects, neither their J – K nor W 1 – W 2 colors are significantly redder than field dwarfs with the same spectral types, unlike younger ultracool dwarfs. We also determined PS1 parallaxes for eight of our candidates and one previously identified AB Dor Moving Group candidate. Although radial velocities (and parallaxes, for some) are still needed to fully assess membership, these new objects provide valuable insight into the spectral characteristics and evolution of young brown dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

L-band spectroscopy with magellan-AO/Clio2: first results on young low-mass companions

L-band spectroscopy is a powerful probe of cool low-gravity atmospheres: the P, Q, and R branch fundamental transitions of methane near 3.3 μm provide a sensitive probe of carbon chemistry; cloud thickness modifies the spectral slope across the band; and H{sub 3}{sup +} opacity can be used to detect aurorae. Many directly imaged gas-giant companions to nearby young stars exhibit L-band fluxes distinct from the field population of brown dwarfs at the same effective temperature. Here we describe commissioning the L-band spectroscopic mode of Clio2, the 1–5 μm instrument behind the Magellan adaptive-optics system. We use this system to measure L-band spectra of directly imaged companions. Our spectra are generally consistent with the parameters derived from previous near-infrared spectra for these late M to early L type objects. Therefore, deviations from the field sequence are constrained to occur below 1500 K. This range includes the L–T transition for field objects and suggests that observed discrepancies are due to differences in cloud structure and CO/CH{sub 4} chemistry.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AN ALMA DISK MASS FOR THE CANDIDATE PROTOPLANETARY COMPANION TO FW TAU

We present ALMA observations of the FW Tau system, a close binary pair of M5 stars with a wide-orbit (300 AU projected separation) substellar companion. The companion is extremely faint and red in the optical and near-infrared, but boasts a weak far-infrared excess and optical/near-infrared emission lines indicative of a primordial accretion disk of gas and dust. The component-resolved 1.3 mm continuum emission is found to be associated only with the companion, with a flux (1.78 ± 0.03 mJy) that indicates a dust mass of 1-2 M {sub ⊕}. While this mass reservoir is insufficient to form a giant planet, it is more than sufficient to produce an analog of the Kepler-42 exoplanetary system or the Galilean satellites. The mass and geometry of the disk-bearing FW Tau companion remains unclear. Near-infrared spectroscopy shows deep water bands that indicate a spectral type later than M5, but substantial veiling prevents a more accurate determination of the effective temperature (and hence mass). Both a disk-bearing ''planetary-mass'' companion seen in direct light or a brown dwarf tertiary viewed in light scattered by an edge-on disk or envelope remain possibilities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE COOLEST ISOLATED BROWN DWARF CANDIDATE MEMBER OF TWA

We present two new late-type brown dwarf candidate members of the TW Hydrae association (TWA): 2MASS J12074836-3900043 and 2MASS J12474428-3816464, which were found as part of the BANYAN all-sky survey (BASS) for brown dwarf members of nearby young associations. We obtained near-infrared (NIR) spectroscopy for both objects (NIR spectral types are respectively L1 and M9), as well as optical spectroscopy for J1207-3900 (optical spectral type is L0γ), and show that both display clear signs of low gravity, and thus youth. We use the BANYAN II Bayesian inference tool to show that both objects are candidate members to TWA with a very low probability of being field contaminants, although the kinematics of J1247-3816 seem slightly at odds with that of other TWA members. J1207-3900 is currently the latest-type and the only isolated L-type candidate member of TWA. Measuring the distance and radial velocity of both objects is still required to claim them as bona fide members. Such late-type objects are predicted to have masses down to 11-15 M {sub Jup} at the age of TWA, which makes them compelling targets to study atmospheric properties in a regime similar to that of currently known imaged extrasolar planets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A VLA SURVEY FOR FAINT COMPACT RADIO SOURCES IN THE ORION NEBULA CLUSTER

We present Karl G. Jansky Very Large Array 1.3, 3.6, and 6 cm continuum maps of compact radio sources in the Orion Nebular Cluster (ONC). We mosaicked 34 arcmin{sup 2} at 1.3 cm, 70 arcmin{sup 2} at 3.6 cm and 109 arcmin{sup 2} at 6 cm, containing 778 near-infrared detected young stellar objects and 190 Hubble Space Telescope -identified proplyds (with significant overlap between those characterizations). We detected radio emission from 175 compact radio sources in the ONC, including 26 sources that were detected for the first time at these wavelengths. For each detected source, we fitted a simple free–free and dust emission model to characterize the radio emission. We extrapolate the free–free emission spectrum model for each source to ALMA bands to illustrate how these measurements could be used to correctly measure protoplanetary disk dust masses from submillimeter flux measurements. Finally, we compare the fluxes measured in this survey with previously measured fluxes for our targets, as well as four separate epochs of 1.3 cm data, to search for and quantify the variability of our sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

PLANETS AROUND LOW-MASS STARS (PALMS). I. A SUBSTELLAR COMPANION TO THE YOUNG M DWARF 1RXS J235133.3+312720

We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.''4 ({approx}120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0{sup +2}{sub -1}. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of {approx}10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 {+-} 10 pc) indicate it is likely a member of the {approx}50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the {approx}200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 {+-} 6 M{sub Jup} for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Transformation of Mo and W thiosalts into unsupported sulfide catalysts: A temperature dependent in-situ spectroscopic investigation

Highlights: • TPD–MS results suggest that the majority of S remains in the solid sample after decomposition. • Decomposition of the (CH{sub 3}){sub 4}N is not directly correlated with that of the MS{sub 4}{sup 2−} species. • Surface of materials does not track directly with the bulk at various treatment temperatures. • ATT-derived materials retained a significant amount of +6 state consistent with the precursor. - Abstract: The thermal decomposition of ammonium thiomolybdate (ATM), ammonium thiotungstate (ATT), tetramethylammonium thiomolybdate (TMATM) and tetramethylammonium thiotungstate (TMATT) are investigated as a function of temperature by using thermogravimetric analysis (TGA), temperature-programmed decomposition with mass spectroscopy (TPS–MS), in-situ Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray photoelectron spectroscopy (XPS). The results allow for correlations to be made between the changes in the bulk and surface structures of the materials, and the evolution of gas-phase decomposition products. The major difference between the ammonium and tetramethylammonium precursors is the complexity of the thermal decomposition profile, which is found to follow two steps over a wide temperature range for the former, but one step over a relatively narrow range for the latter materials. Raman and FTIR spectra reveal the decomposition of the ammonium and tetramethylammonium groups, along with the decomposition of the initial sulfide structures to metal disulfides. For both sets of precursors, XPS results show that the surface of the resulting materials at various temperatures of treatment does not track directly with the state of the bulk material. While the ATM, TMATM, and TMATT-derived material surfaces are reduced to the 4{sup +} state at the highest temperature, indicating disulfides, the ATT-derived materials still retained a significant amount of W{sup 6+} state consistent with the starting precursor.

36 MATERIALS SCIENCE↗
DOE OSTI2015

SEARCHING FOR PLANETS IN HOLEY DEBRIS DISKS WITH THE APODIZING PHASE PLATE

We present our first results from a high-contrast imaging search for planetary mass companions around stars with gapped debris disks, as inferred from the stars' bright infrared excesses. For the six considered stars, we model the disks' unresolved infrared spectral energy distributions in order to derive the temperature and location of the disk components. With VLT/NaCo Apodizing Phase Plate coronagraphic L'-band imaging, we search for planetary mass companions that may be sculpting the disks. We detect neither disks nor companions in this sample, confirmed by comparing plausible point sources with archival data. In order to calculate our mass sensitivity limit, we revisit the stellar age estimates. One target, HD 17848, at 540 ± 100 Myr old is significantly older than previously estimated. We then discuss our high-contrast imaging results with respect to the disk properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE NEAR-INFRARED IDENTIFICATION OF THE INTEGRAL SOURCE IGR J16328–4726

The aim of this work is to identify the infrared (IR) counterpart of the Galactic high-mass X-ray binary IGR J16328–4726 discovered by the INTEGRAL satellite, and to derive the extinction and distance to the system. We present new deep sub-arcsecond JHK{sub s} imaging and low-resolution near-IR spectroscopy in the 1.5 and 2.4 μm range of IGR J16328–4726. We report the presence of two near-IR stellar sources separated by about 1.″8 at the location of the unresolved 2MASS source J16323791–4723409, previously considered to be the near-IR counterpart of the X-ray source. From the analysis of their near-IR colors and spectra as well as accurate positions, we uniquely identify the true IR counterpart of IGR J16328–4726. Our 1.5–2.4 μm spectrum of this star is consistent with the published classification O8Iafpe. Assuming this, and in combination with new JHK{sub s} photometry, a reddening A{sub V} = 23.6 ± 0.7 and a distance of 7.2 ± 0.3 kpc from the Sun are derived.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Stability of a tachyon braneworld

Within the braneworld paradigm the tachyonic scalar field has been used to generate models that attempt to solve some of the open problems that physics faces nowadays, both in cosmology and high energy physics as well. When these field configurations are produced by the interplay of higher dimensional warped gravity with some matter content, braneworld models must prove to be stable under the whole set of small fluctuations of the gravitational and matter fields background, among other consistency tests. Here we present a complete proof of the stability under scalar perturbations of tachyonic thick braneworlds with an embedded maximally symmetric 4D space-time, revealing its physical consistency. This family of models contains a recently reported tachyonic de Sitter thick braneworld which possesses a series of appealing properties. These features encompass complete regularity, asymptotic flatness (instead of being asymptotically dS or AdS) even when it contains a negative bulk cosmological constant, a relevant 3-brane with dS metric which naturally arises from the full set of field equations of the 5D background (it is not imposed), qualitatively describing the inflationary epochs of our Universe, and a graviton spectrum with a single zero mode bound state that accounts for the 4D graviton localised on the brane and is separated from the continuum of Kaluza-Klein massive graviton excitations by a mass gap. The presence of this mass gap in the graviton spectrum makes the extra-dimensional corrections to Newton’s law decay exponentially. Gauge vector fields with a single massless bound state in its mass spectrum are also localised on this braneworld model a fact that allows us to recover the Coulomb’s law of our 4D world. All these properties of the above referred tachyonic braneworld together with the positive stability analysis provided in this work, constitute a firm step towards the construction of realistic cosmological models within the braneworld paradigm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Outflow from Outer-arm Starburst in a Grazing Collision between Galaxies

Gemini NIFS K-band spectra and Atacama Large Millimeter/submillimeter Array {sup 12}CO J=1→0, HCO{sup +}, and 100 GHz continuum observations are used to study a bright starburst clump on an outer arm of the interacting galaxy NGC 2207. This clump emits 23% of the total 24 μm flux of the galaxy pair and has an optically opaque dust cone extending out of its 170 pc core. The measured CO accounts for the dark cone extinction if almost all the gas and dust there are in front of the star clusters. An associated approaching CO outflow has v {sub z} ~ 16 km s{sup -1}, an estimated molecular mass 8 × 10{sup 6} M {sub ⊙}, and rises to heights ~0.9 kpc. A receding CO outflow on the far side with v {sub z} ~ 28 km s{sup -1} is less extensive. The observed star formation in the core over 10 Myr can supply the dark cone kinetic energy of roughly 2 × 10{sup 52} erg via supernovae and stellar winds. Other signs of intense activity are a variable radio continuum, suggesting an embedded supernova or other outburst; X-ray emission possibly from an X-ray binary or intermediate-mass black hole, depending on the extinction; and Brγ and He i lines with 82 km s{sup -1} line widths and fluxes consistent with excitation by embedded O-type stars. According to previous models, the retrograde encounter suffered by NGC 2207 caused the loss of angular momentum. This compressed its outer disk. We suggest that the resulting inward crashing stream hit a massive H i clump on the preexisting spiral arm and triggered the observed starburst.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Hα star formation rates of z > 1 galaxy clusters in the IRAC shallow cluster survey

We present Hubble Space Telescope near-IR spectroscopy for 18 galaxy clusters at 1.0 <z < 1.5 in the IRAC Shallow Cluster Survey. We use Wide Field Camera 3 grism data to spectroscopically identify Hα emitters in both the cores of galaxy clusters as well as in field galaxies. We find a large cluster-to-cluster scatter in the star formation rates within a projected radius of 500 kpc, and many of our clusters (∼60%) have significant levels of star formation within a projected radius of 200 kpc. A stacking analysis reveals that dust reddening in these star-forming galaxies is positively correlated with stellar mass and may be higher in the field than the cluster at a fixed stellar mass. This may indicate a lower amount of gas in star-forming cluster galaxies than in the field population. Also, Hα equivalent widths of star-forming galaxies in the cluster environment are still suppressed below the level of the field. This suppression is most significant for lower mass galaxies (log M {sub *} < 10.0 M {sub ☉}). We therefore conclude that environmental effects are still important at 1.0 <z < 1.5 for star-forming galaxies in galaxy clusters with log M {sub *} ≲ 10.0 M {sub ☉}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7

We recently used near-infrared spectroscopy to improve the characterization of 76 low-mass stars around which K2 had detected 79 candidate transiting planets. 29 of these worlds were new discoveries that had not previously been published. We calculate the false positive probabilities that the transit-like signals are actually caused by non-planetary astrophysical phenomena and reject five new transit-like events and three previously reported events as false positives. We also statistically validate 17 planets (7 of which were previously unpublished), confirm the earlier validation of 22 planets, and announce 17 newly discovered planet candidates. Revising the properties of the associated planet candidates based on the updated host star characteristics and refitting the transit photometry, we find that our sample contains 21 planets or planet candidates with radii smaller than 1.25 R {sub ⊕}, 18 super-Earths (1.25–2 R {sub ⊕}), 21 small Neptunes (2–4 R {sub ⊕}), three large Neptunes (4–6 R {sub ⊕}), and eight giant planets (>6 R {sub ⊕}). Most of these planets are highly irradiated, but EPIC 206209135.04 (K2-72e, 1.29{sub −0.13}{sup +0.14} R{sub ⊕}), EPIC 211988320.01 (R{sub p}=2.86{sub −0.15}{sup +0.16} R{sub ⊕}), and EPIC 212690867.01 (2.20{sub −0.18}{sup +0.19} R{sub ⊕}) orbit within optimistic habitable zone boundaries set by the “recent Venus” inner limit and the “early Mars” outer limit. In total, our planet sample includes eight moderately irradiated 1.5–3 R {sub ⊕} planet candidates (F {sub p} ≲ 20 F {sub ⊕}) orbiting brighter stars (Ks < 11) that are well-suited for atmospheric investigations with the Hubble, Spitzer, and/or James Webb Space Telescopes. Five validated planets orbit relatively bright stars (Kp < 12.5) and are expected to yield radial velocity semi-amplitudes of at least 2 m s{sup −1}. Accordingly, they are possible targets for radial velocity mass measurement with current facilities or the upcoming generation of red optical and near-infrared high-precision RV spectrographs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: seasonal and diel variations and impact of anthropogenic emissions

Abstract. During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign, size-resolved cloud condensation nuclei (CCN) spectra were characterized at a research site (T3) 60 km downwind of the city of Manaus, Brazil, in central Amazonia for 1 year (12 March 2014 to 3 March 2015). Particle hygroscopicity (κCCN) and mixing state were derived from the size-resolved CCN spectra, and the hygroscopicity of the organic component of the aerosol (κorg) was then calculated from κCCN and concurrent chemical composition measurements. The annual average κCCN increased from 0.13 at 75 nm to 0.17 at 171 nm, and the increase was largely due to an increase in sulfate volume fraction. During both wet and dry seasons, κCCN, κorg, and particle composition under background conditions exhibited essentially no diel variations. The constant κorg of ∼ 0. 15 is consistent with the largely uniform and high O : C value (∼ 0. 8), indicating that the aerosols under background conditions are dominated by the aged regional aerosol particles consisting of highly oxygenated organic compounds. For air masses strongly influenced by urban pollution and/or local biomass burning, lower values of κorg and organic O : C atomic ratio were observed during night, due to accumulation of freshly emitted particles, dominated by primary organic aerosol (POA) with low hygroscopicity, within a shallow nocturnal boundary layer. The O : C, κorg, and κCCN increased from the early morning hours and peaked around noon, driven by the formation and aging of secondary organic aerosol (SOA) and dilution of POA emissions into a deeper boundary layer, while the development of the boundary layer, which leads to mixing with aged particles from the residual layer aloft, likely also contributed to the increases. The hygroscopicities associated with individual organic factors, derived from PMF (positive matrix factorization) analysis of AMS (aerosol mass spectrometry) spectra, were estimated through multivariable linear regression. For the SOA factors, the variation of the κ value with O : C agrees well with the linear relationship reported from earlier laboratory studies of SOA hygroscopicity. On the other hand, the variation in O : C of ambient aerosol organics is largely driven by the variation in the volume fractions of POA and SOA factors, which have very different O : C values. As POA factors have hygroscopicity values well below the linear relationship between SOA hygroscopicity and O : C, mixtures with different POA and SOA fractions exhibit a steeper slope for the increase in κorg with O : C, as observed during this and earlier field studies. This finding helps better understand and reconcile the differences in the relationships between κorg and O : C observed in laboratory and field studies, therefore providing a basis for improved parameterization in global models, especially in a tropical context.

Thalman, Ryan↗