Spectroscopic Studies of Oxygen and Iodine Using Cavity Enhanced Extinction Spectroscopy

This work uses cavity enhanced extinction spectroscopy (CEES) to develop molecular spectroscopy of oxygen, a natural green-house gas, and in conjunction with other instrumentation studies the role of reactive trace gases (iodine and glyoxal) that modify aerosols which are relevant for the public health and climate. Oxygen-oxygen collision-induced absorption (O2-O2 CIA) of solar radiation heats the atmosphere, affects radiative transfer, and needs to be considered in spectroscopy applications in Earth's atmosphere. The lack of O2 - O2 CIA spectra in the gas phase below 335\,nm wavelength is limiting remote sensing applications in this spectral range. This study reports measurements of the O2 - O2&nbsp;CIA cross-section in the gas-phase at high signal-to-noise ratio and spectral resolution, sufficient to fully resolve spectral band shapes (0.31--0.42 nm FWHM, full width at half maximum) using CEES at atmospheric pressure, and variable temperature (293, 263, and 223 K). Excellent agreement with literature spectra is observed for selected strong bands. Several heretofore unmeasured weak absorption bands are characterized for the first time in the gas phase under controlled laboratory conditions, i.e., the bands centered at 315 nm, 328 nm, and 420 nm. The oscillator strengths, and the band shape at 344 nm deviate from the available literature, in part owing to the overlap of neighboring bands that give rise to non-zero continuum absorption. A complete O2 - O2CIA cross section spectrum is presented between 297--500 nm, that is optimized for use in hyperspectral (and other) remote sensing applications, provides opportunities to develop theory, and warrants application in radiative transfer calculations to re-assess O2 - O2&nbsp;heating rates in the atmosphere. Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950, and are projected to increase in the future. While iodic acid (HIO3) grows particles more efficiently than sulphuric acid, and is widespread, its gas-phase formation mechanism is essentially unknown. Rapid and efficient HIO3&nbsp;formation was measured following iodine photolysis at atmospherically relevant iodine atom production rates within the CERN CLOUD chamber, and are explained by (R1) IOIO + O3 &rarr; IOIO4&nbsp;followed by (R2) IOIO4 + H2O &rarr; HIO3 + HOI. The laboratory derived reaction rates are corroborated by theory, and shown to explain field measurements of daytime HIO3&nbsp;in the remote lower free troposphere. These results suggest a catalytic role of iodine in particle formation, and provide a missing link in atmospheric models between iodine sources and particle formation. Glyoxal is a near-ubiquitous trace gas that is emitted into the atmosphere in the combustion of fuels, and formed in the atmosphere as oxidation product of hydrocarbons. The largest global source of glyoxal is the oxidation of isoprene, but it can also be formed by hetereogeneous chemistry over oceans. Despite its low mass and associated high volatility, glyoxal contributes to particle growth via reactive uptake. Glyoxal sources in the remote free troposphere, and its fate within particles are not well understood. In this work the glyoxal abundance at the Ma&iuml;do mountaintop research station was determined over two months using CEES. While glyoxal was frequently detected in concentrations from 5 to 50 pptv, no firm conclusions can be drawn about the diurnal cycle in the free troposphere, because of frequent island influences on the sampled air masses during the day. Exploratory experiments at the CERN CLOUD chamber demonstrate that glyoxal can be handled at the facility, and that it is efficiently lost to the walls; but significant contamination leading to a uncontrolled baseline interfered in quantifying its effects on aerosol formation, and further studies are warranted.</p

Spectral variations of AeBe Herbig stars in the Mon R1 association

We present the change in the Halpha emission-line profile of the spectra of some AeBe Herbig stars. In the spectrum of VY Mon, Halpha may have one of three profile types: P Cyg, P Cyg III or single line in accordance with the brightness variations of the star. HD259431 now shows a double Halpha profile with the red component stronger than the blue component, while in the earlier observations the blue peak was higher than the red peak. Finally, the last Halpha profile of LkHalpha215 is very similar to that obtained by Finkenzeller et al.Comment: 4pages, 3figure

New methods for the calibration of optical resonators : integrated calibration by means of optical modulation (ICOM) and narrow-band cavity ring-down (NB-CRD)

Optical resonators are used in spectroscopic measurements of atmospheric trace gases to establish long optical path lengths L with enhanced absorption in compact in-struments. In cavity-enhanced broad-band methods, the ex-act knowledge of both the magnitude of L and its spectral dependency on the wavelength lambda is fundamental for the correct retrieval of trace gas concentrations. L(lambda) is connected to the spectral mirror reflectivity R (lambda), which is often referred to instead. L(lambda) is also influenced by other quantities like broad-band absorbers or alignment of the optical resonator. The established calibration techniques to determine L(lambda), e.g. introducing gases with known optical properties or measuring the ring-down time, all have limitations: limited spectral resolution, insufficient absolute accuracy and precision, inconvenience for field deployment, or high cost of implementation. Here, we present two new methods that aim to overcome these limitations: (1) the narrow-band cavity ring-down (NB-CRD) method uses cavity ring-down spectroscopy and a tunable filter to retrieve spectrally resolved path lengths L(lambda); (2) integrated calibration by means of op-tical modulation (ICOM) allows the determination of the op-tical path length at the spectrometer resolution with high ac-curacy in a relatively simple setup. In a prototype setup we demonstrate the high accuracy and precision of the new approaches. The methods facilitate and improve the determination of L(lambda), thereby simplifying the use of cavity-enhanced absorption spectroscopy.Peer reviewe

Inter-comparison of MAX-DOAS measurements of tropospheric HONO slant column densities and vertical profiles during the CINDI-2 campaign

We present the inter-comparison of delta slant column densities (SCDs) and vertical profiles of nitrous acid (HONO) derived from measurements of different multi-axis differential optical absorption spectroscopy (MAX-DOAS) instruments and using different inversion algorithms during the Second Cabauw Inter-comparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2) in September&nbsp;2016 at Cabauw, the Netherlands (51.97∘&thinsp;N, 4.93∘&thinsp;E). The HONO vertical profiles, vertical column densities (VCDs), and near-surface volume mixing ratios are compared between different MAX-DOAS instruments and profile inversion algorithms for the first time. Systematic and random discrepancies of the HONO results are derived from the comparisons of all data sets against their median values. Systematic discrepancies of HONO delta SCDs are observed in the range of &plusmn;0.3&times;1015&thinsp;molec.&thinsp;cm&minus;2, which is half of the typical random discrepancy of 0.6&times;1015&thinsp;molec.&thinsp;cm&minus;2. For a typical high HONO delta SCD of 2&times;1015&thinsp;molec.&thinsp;cm&minus;2, the relative systematic and random discrepancies are about 15&thinsp;% and 30&thinsp;%, respectively. The inter-comparison of HONO profiles shows that both systematic and random discrepancies of HONO VCDs and near-surface volume mixing ratios (VMRs) are mostly in the range of &sim;&plusmn;0.5&times;1014&thinsp;molec.&thinsp;cm&minus;2 and &sim;&plusmn;0.1&thinsp;ppb (typically &sim;20&thinsp;%). Further we find that the discrepancies of the retrieved HONO profiles are dominated by discrepancies of the HONO delta SCDs. The profile retrievals only contribute to the discrepancies of the HONO profiles by &sim;5&thinsp;%. However, some data sets with substantially larger discrepancies than the typical values indicate that inappropriate implementations of profile inversion algorithms and configurations of radiative transfer models in the profile retrievals can also be an important uncertainty source. In addition, estimations of measurement uncertainties of HONO dSCDs, which can significantly impact profile retrievals using the optimal estimation method, need to consider not only DOAS fit errors, but also atmospheric variability, especially for an instrument with a DOAS fit error lower than &sim;3&times;1014&thinsp;molec.&thinsp;cm&minus;2. The MAX-DOAS results during the CINDI-2 campaign indicate that the peak HONO levels (e.g. near-surface VMRs of &sim;0.4&thinsp;ppb) often appeared in the early morning and below 0.2&thinsp;km. The near-surface VMRs retrieved from the MAX-DOAS observations are compared with those measured using a co-located long-path DOAS instrument. The systematic differences are smaller than 0.15 and 0.07&thinsp;ppb during early morning and around noon, respectively. Since true HONO values at high altitudes are not known in the absence of real measurements, in order to evaluate the abilities of profile inversion algorithms to respond to different HONO profile shapes, we performed sensitivity studies using synthetic HONO delta SCDs simulated by a radiative transfer model with assumed HONO profiles. The tests indicate that the profile inversion algorithms based on the optimal estimation method with proper configurations can reproduce the different HONO profile shapes well. Therefore we conclude that the features of HONO accumulated near the surface derived from MAX-DOAS measurements are expected to represent the ambient HONO profiles well. Full List of Authors: Yang Wang1, Arnoud Apituley2, Alkiviadis Bais3, Steffen Beirle1, Nuria Benavent4, Alexander Borovski5, Ilya Bruchkouski6, Ka Lok Chan7,8, Sebastian Donner1, Theano Drosoglou3, Henning Finkenzeller9,10, Martina M. Friedrich11, Udo Frie&szlig;12, David Garcia-Nieto4, Laura G&oacute;mez-Mart&iacute;n13, Fran&ccedil;ois Hendrick11, Andreas Hilboll14, Junli Jin15, Paul Johnston16, Theodore K. Koenig9,10, Karin Kreher17, Vinod Kumar1, Aleksandra Kyuberis18, Johannes Lampel12,19, Cheng Liu20, Haoran Liu20, Jianzhong Ma21, Oleg L. Polyansky18,22, Oleg Postylyakov5, Richard Querel16, Alfonso Saiz-Lopez4, Stefan Schmitt12, Xin Tian23,24, Jan-Lukas Tirpitz12, Michel Van Roozendael11, Rainer Volkamer9,10, Zhuoru Wang8, Pinhua Xie24, Chengzhi Xing25, Jin Xu24, Margarita Yela13, Chengxin Zhang25, and Thomas Wagner11Max Planck Institute for Chemistry, Mainz, Germany 2Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands 3Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece 4Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano (CSIC), Madrid, Spain 5A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia 6National Ozone Monitoring Research and Education Center BSU (NOMREC BSU), Belarusian State University, Minsk, Belarus 7Meteorologisches Institut, Ludwig-Maximilians-Universit&auml;t M&uuml;nchen, Munich, Germany 8Remote Sensing Technology Institute, German Aerospace Center (DLR), Oberpfaffenhofen, Germany 9Department of Chemistry, University of Colorado Boulder, Boulder, CO, USA 10Cooperative Institute for Research in Environmental Sciences, Boulder, CO, USA 11Royal Belgian Institute for Space Aeronomy, Brussels, Belgium 12Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany 13National Institute of Aerospatial Technology, Madrid, Spain 14Institute of Environmental Physics, University of Bremen, Bremen, Germany 15Meteorological Observation Center, China Meteorological Administration, Beijing, China 16National Institute of Water &amp; Atmospheric Research (NIWA), Lauder, New Zealand 17BK Scientific, Mainz, Germany 18Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia 19Airyx GmbH, Justus-von-Liebig-Str. 14, 69214 Eppelheim, Germany 20Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei, Anhui, China 21Chinese Academy of Meteorology Science, China Meteorological Administration, Beijing, China 22Department of Physics and Astronomy, University College London, Gower St, London, WC1E 6BT, UK 23Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, China 24Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, China 25School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, China </ul

The driving factors of new particle formation and growth in the polluted boundary layer

New particle formation (NPF) is a significant source of atmospheric particles, affecting climate and air quality. Understanding the mechanisms involved in urban aerosols is important to develop effective mitigation strategies. However, NPF rates reported in the polluted boundary layer span more than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have been postulated to participate in NPF, including sulfuric acid, ammonia, amines and organics, but their relative roles remain unclear. We investigated NPF in the CLOUD chamber using mixtures of anthropogenic vapours that simulate polluted boundary layer conditions. We demonstrate that NPF in polluted environments is largely driven by the formation of sulfuric acid&ndash;base clusters, stabilized by the presence of amines, high ammonia concentrations and lower temperatures. Aromatic oxidation products, despite their extremely low volatility, play a minor role in NPF in the chosen urban environment but can be important for particle growth and hence for the survival of newly formed particles. Our measurements quantitatively account for NPF in highly diverse urban environments and explain its large observed variability. Such quantitative information obtained under controlled laboratory conditions will help the interpretation of future ambient observations of NPF rates in polluted atmospheres. Full List of Authors: Mao Xiao1, Christopher R. Hoyle1,2, Lubna Dada3, Dominik Stolzenburg4, Andreas K&uuml;rten5, Mingyi Wang6, Houssni Lamkaddam1, Olga Garmash3, Bernhard Mentler7, Ugo Molteni1, Andrea Baccarini1, Mario Simon5, Xu-Cheng He3, Katrianne Lehtipalo3,8, Lauri R. Ahonen3, Rima Baalbaki3, Paulus S. Bauer4, Lisa Beck3, David Bell1, Federico Bianchi3, Sophia Brilke4, Dexian Chen6, Randall Chiu9, Ant&oacute;nio Dias10, Jonathan Duplissy3,11, Henning Finkenzeller9, Hamish Gordon6, Victoria Hofbauer6, Changhyuk Kim13,14, Theodore K. Koenig9,a, Janne Lampilahti3, Chuan Ping Lee1, Zijun Li15, Huajun Mai13, Vladimir Makhmutov16, Hanna E. Manninen17, Ruby Marten1, Serge Mathot17, Roy L. Mauldin18,19, Wei Nie20, Antti Onnela17, Eva Partoll7, Tuukka Pet&auml;j&auml;3, Joschka Pfeifer5,17, Veronika Pospisilova1, Lauriane L. J. Qu&eacute;l&eacute;ver3, Matti Rissanen3,b, Siegfried Schobesberger15, Simone Schuchmann17,c, Yuri Stozhkov16, Christian Tauber4, Yee Jun Tham3, Ant&oacute;nio Tom&eacute;21, Miguel Vazquez-Pufleau4, Andrea C. Wagner5,9,d, Robert Wagner3, Yonghong Wang3, Lena Weitz5, Daniela Wimmer3,4, Yusheng Wu3, Chao Yan3, Penglin Ye6,22, Qing Ye6, Qiaozhi Zha3, Xueqin Zhou5, Antonio Amorim10, Ken Carslaw12, Joachim Curtius5, Armin Hansel7, Rainer Volkamer9,19, Paul M. Winkler4, Richard C. Flagan13, Markku Kulmala3,11,20,23, Douglas R. Worsnop3,22, Jasper Kirkby5,17, Neil M. Donahue6, Urs Baltensperger1, Imad El Haddad1, and Josef Dommen1 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland 2Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland 3Institute for Atmospheric and Earth System Research (INAR)/Physics, University of Helsinki, 00014 Helsinki, Finland 4Faculty of Physics, University of Vienna, 1090 Vienna, Austria 5Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany 6Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA 15213, USA 7Institute of Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria 8Atmospheric Composition Research Unit, Finnish Meteorological Institute, 00560 Helsinki, Finland 9Department of Chemistry &amp; CIRES, University of Colorado Boulder, Boulder, CO 80309, USA 10CENTRA and FCUL, University of Lisbon, 1749-016 Lisbon, Portugal 11Helsinki Institute of Physics, University of Helsinki, 00014 Helsinki, Finland 12School of Earth and Environment, University of Leeds, LS2 9JT Leeds, United Kingdom 13Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA 14School of Civil and Environmental Engineering, Pusan National University, 46241 Busan, Republic of Korea 15Department of Applied Physics, University of Eastern Finland, 70211 Kuopio, Finland 16Solar and Cosmic Ray Physics Laboratory, P.N. Lebedev Physical Institute of the Russian Academy of Sciences, 119991 Moscow, Russian Federation 17CERN, 1211 Geneva, Switzerland 18Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA 19Department of Oceanic and Atmospheric Sciences, University of Colorado Boulder, Boulder, CO 80309, USA 20Joint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, Nanjing, Jiangsu Province, China 21IDL-Universidade da Beira Interior, Covilh&atilde;, Portugal 22Aerodyne Research Inc., Billerica, MA 01821-3976, USA 23Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, China anow at: College of Environmental Sciences and Engineering, Peking University, 100871 Beijing, China bnow at: Aerosol Physics Laboratory, Physics Unit, Faculty of Engineering and Natural Sciences, Tampere University, 33720 Tampere, Finland cnow at: Experimentelle Teilchen- und Astroteilchenphysik, Johannes Gutenberg University Mainz, 55128 Mainz, Germany dnow at: Department of Chemistry &amp; CIRES, University of Colorado Boulder, Boulder, CO 80305, USA Correspondence: Urs Baltensperger ([email protected]) and Imad El Haddad ([email protected])</p

The Molecular Gas Environment around Two Herbig Ae/Be Stars: Resolving the Outflows of LkHa 198 and LkHa 225S

Observations of outflows associated with pre-main-sequence stars reveal details about morphology, binarity and evolutionary states of young stellar objects. We present molecular line data from the Berkeley-Illinois-Maryland Association array and Five Colleges Radio Astronomical Observatory toward the regions containing the Herbig Ae/Be stars LkHa 198 and LkHa 225S. Single dish observations of 12CO 1-0, 13CO 1-0, N2H+ 1-0 and CS 2-1 were made over a field of 4.3' x 4.3' for each species. 12CO data from FCRAO were combined with high resolution BIMA array data to achieve a naturally-weighted synthesized beam of 6.75'' x 5.5'' toward LkHa 198 and 5.7'' x 3.95'' toward LkHa 225S, representing resolution improvements of factors of approximately 10 and 5 over existing data. By using uniform weighting, we achieved another factor of two improvement. The outflow around LkHa 198 resolves into at least four outflows, none of which are centered on LkHa 198-IR, but even at our resolution, we cannot exclude the possibility of an outflow associated with this source. In the LkHa 225S region, we find evidence for two outflows associated with LkHa 225S itself and a third outflow is likely driven by this source. Identification of the driving sources is still resolution-limited and is also complicated by the presence of three clouds along the line of sight toward the Cygnus molecular cloud. 13CO is present in the environments of both stars along with cold, dense gas as traced by CS and (in LkHa 225S) N2H+. No 2.6 mm continuum is detected in either region in relatively shallow maps compared to existing continuum observations.Comment: 14 pages, 10 figures (5 color), accepted for publication in Ap

HD 144432: a young triple system

We present new imaging and spectroscopic data of the young Herbig star HD 144432 A, which was known to be a binary star with a separation of 1.47 arcsec. High-resolution NIR imaging data obtained with NACO at the VLT reveal that HD 144432 B itself is a close binary pair with a separation of 0.1 arcsec. High-resolution optical spectra, acquired with FEROS at the 2.2m MPG/ESO telescope in La Silla, of the primary star and its co-moving companions were used to determine their main stellar parameters such as effective temperature, surface gravity, radial velocity, and projected rotational velocity by fitting synthetic spectra to the observed stellar spectra. The two companions, HD 144432 B and HD 144432 C, are identified as low-mass T Tauri stars of spectral type K7V and M1V, respectively. From the position in the HRD the triple system appears to be co-eval with a system age of 6+/-3 Myr.Comment: Accepted for publication in Astronomy & Astrophysics, 4 pages, 4 figure

Measuring the mass accretion rates of Herbig Ae/Be stars with X-shooter

We present the results of our observations of eight magnetic Herbig Ae/Be stars obtained with the X-shooter spectrograph mounted on UT2 at the VLT. X-shooter provides a simultaneous, medium-resolution and high-sensitivity spectrum over the entire wavelength range from 300 to 2500 nm. We estimate the mass accretion rates M_acc of the targets from 13 different spectral diagnostics using empiric calibrations derived previously for T Tauri-type stars and brown dwarfs. We have estimated the mass accretion rates of our targets, which range from 2x10^-9 to 2x10^-7 M_sun/yr. Furthermore, we have found accretion rate variability with amplitudes of 0.10-0.40 dex taking place on time scales from one day to tens of days. Additional future night-to-night observations need to be carried out to investigate the character of M_acc variability in details. Our study shows that the majority of the calibrational relations can be applied to Herbig Ae/Be stars, but several of them need to be re-calibrated on the basis of new spectral data for a larger number of Herbig Ae/Be stars.Comment: 19 pages, 13 figures, nine tables, accepted for publication in Astronomische Nachrichte

New Herbig Ae/Be stars confirmed via high-resolution optical spectroscopy

We present FEROS high-resolution (R~45000) optical spectroscopy of 34 Herbig Ae/Be star candidates with previously unknown or poorly constrained spectral types. Within the sample, 16 sources are positionally coincident with nearby (d<250 pc) star-forming regions (SFRs). All the candidates have IR excess. We determine the spectral type and luminosity class of the sources, derive their radial and rotational velocities, and constrain their distances employing spectroscopic parallaxes. We confirm 13 sources as Herbig Ae/Be stars and find one classical T Tauri star. Three sources are emission line early-type giants and may be Herbig Ae/Be stars. One source is a main-sequence A-type star. Fourteen sources are post-main-sequence giant and supergiant stars. Two sources are extreme emission-line stars. Most of the sources appear to be background stars at distances over 700 pc. We show that high-resolution optical spectroscopy is a crucial tool for distinguishing young stars from post-main sequence stars in samples taken from emission-line star catalogs based on low-resolution spectroscopy. Within the sample, 3 young stars (CD-38 4380, Hen 3-1145, and HD 145718) and one early-type luminosity class III giant with emission lines (Hen 3-416) are at distances closer than 300 pc and are positionally coincident with a nearby SFR. These 4 sources are likely to be nearby young stars and are interesting for follow-up observations at high-angular resolution. Furthermore, seven confirmed Herbig Ae/Be stars at d>700 pc (Hen 2-80, Hen 3-1121 N&S, HD 313571, MWC 953, WRAY 15-1435, and Th 17-35) are inside or close (<5') to regions with extended 8 micron continuum emission and in their 20' vicinity have astronomical sources characteristic of SFRs. These 7 sources are likely to be members of SFRs. These regions are attractive for future studies of their stellar content.Comment: 24 pages, 6 Figures, accepted by Astronomy and Astrophysics, in press

The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

To trace the accretion and outflow processes around YSOs, diagnostic spectral lines such as the BrG 2.166 micron line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the BrG line emission with mass infall or mass outflow processes. Using the VLTI/AMBER instrument, we spatially and spectrally (R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the adjacent continuum. All objects (except MWC297) show an increase of visibility within the BrG emission line, indicating that the BrG-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the most likely line-emitting mechanism. We search for general trends and find that the size of the BrG-emitting region does not seem to depend on the basic stellar parameters, but correlates with the H-alpha line profile shape. We find evidence for at least two distinct BrG line-formation mechanisms. Stars with a P-Cygni H-alpha line profile and a high mass-accretion rate seem to show particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with a double-peaked or single-peaked H-alpha-line profile show a significantly more extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar wind or a disk wind.Comment: 20 pages; 11 figures; Accepted by A&A; a high quality version of the paper can be obtained at http://www.skraus.eu/papers/kraus.HAeBe-BrGsurvey.pd