bluuebunny/crossref_metadata_2025 huggingface dataset:
Ship emissions measurement in the Arctic from plume intercepts of the Canadian Coast Guard Amundsen icebreaker from the Polar 6 aircraft platform
Authors: Aliabadi A. A. , Thomas J. L. , Herber A. , Staebler R. M. , Leaitch W. R. , Law K. S. , Marelle L. , Burkart J. , Willis M. , Abbatt J. P. D. , Bozem H. , Hoor P. , Köllner F. , Schneider J. , Levasseur M.
Published: January 2016

Abstract. Decreasing sea ice and increasing marine navigability in northern latitudes have changed Arctic ship traffic patterns in recent years and are predicted to increase annual ship traffic in the Arctic in the future. Development of effective regulations to manage environmental impacts of shipping requires an understanding of ship emissions and atmospheric processing in the Arctic environment. As part of the summer 2014 NETCARE (Network on Climate and Aerosols) campaign, the plume dispersion and gas and particle emission factors of emissions originating from the Canadian Coast Guard Amundsen icebreaker operating near Resolute Bay, NU, Canada have been investigated. The Amundsen burnt distillate fuel with 1.5 wt % sulfur. Emissions were studied via plume intercepts using aircraft measurements, an analytical plume dispersion model, and using the FLEXPART-WRF Lagrangian particle dispersion model. The first plume intercepts by research aircraft were carried out on 19 July 2014 during the operation of the Amundsen in the open water. The second and third plume intercept measurements were carried out on 20 and 21 July 2014 when the Amundsen had reached the ice edge and operated under icebreaking conditions. Typical of Arctic marine navigation, the engine load was low compared to cruising conditions for all of the plume intercepts. The measured species included mixing ratios of CO2, NOx, CO, SO2, particle number concentration (CN), refractory Black Carbon (rBC), and Cloud Condensation Nuclei (CCN). The results were compared to similar experimental studies in mid latitudes. Plume expansion rates (γ) were calculated using the analytical model and found to be γ = 0.75 ± 0.80, 0.93 ± 0.37, and 1.19 ± 0.39 for plumes 1, 2, and 3, respectively. These rates are smaller than prior studies conducted at mid latitudes, likely due to polar boundary layer dynamics, including reduced turbulent mixing compared to mid latitudes. All emission factors were in agreement with prior observations at low engine loads in mid latitudes. Icebreaking increased the NOx emission factor from EFNOx = 22.3 ± 8.0 to 57.8 ± 11.0 and 65.8 ± 4.0 g kg–diesel−1 for plumes 1, 2, and 3, likely due to change in combustion temperatures. The CO emission factor was EFCO = 6.4 ± 11.7, 6.8 ± 2.2 and 5.0 ± 1.0 g kg–diesel−1 for plumes 1, 2, and 3. The rBC emission factor was EFrBC = 0.20 ± 0.04 and 0.25 ± 0.12 g kg–diesel−1 for plumes 1 and 2. The CN emission factor was reduced while icebreaking from EFCPC = 1.96 ± 0.41 to 0.43 ± 0.11 and 0.47 ± 0.04 × 1016 kg–diesel−1 for plumes 1, 2, and 3. At 0.6 % supersaturation, the CCN emission factor was lower than observations in mid latitudes at low engine loads with EFCCN = 1.63 ± 0.41 to 1.06 ± 0.32 and 0.28 ± 0.07 × 1014 kg–diesel−1 for plumes 1, 2, and 3.
Are BVOC exchanges in agricultural ecosystems overestimated? Insights from fluxes measured in a maize field over a whole growing season
Authors: Bachy A. , Aubinet M. , Schoon N. , Amelynck C. , Bodson B. , Moureaux C. , Heinesch B.
Published: January 2016

Abstract. Although maize is the second most important crop worldwide, and the most important C4 crop, no study on biogenic volatile organic compounds (BVOCs) has yet been conducted on this crop at ecosystem scale and over a whole growing season. This has led to large uncertainties in cropland BVOC emission estimations. This paper seeks to fill this gap by presenting, for the first time, BVOC fluxes measured in a maize field at ecosystem scale (using the disjunct eddy covariance by mass scanning technique) over a whole growing season in Belgium. The maize field emitted mainly methanol, although exchanges were bi-directional. The second most exchanged compound was acetic acid, which was taken up mainly in the growing season. Bi-directional exchanges of acetaldehyde, acetone and other oxygenated VOCs also occurred, whereas the terpenes, benzene and toluene exchanges were small, albeit significant. Surprinsingly, BVOC exchanges were as same order of magnitude on bare soil and on well developped vegetation, suggesting that soil is a major BVOC reservoir in agricultural ecosystems. Quantitatively, the maize BVOC emissions observed were lower than those reported in other maize, crops and grasses studies. The standard emission factors (SEFs) estimated in this study (231 ± 19 µgm−2h−1 for methanol, 8 ± 5 µgm−2h−1 for isoprene and 4 ± 6 µgm−2h−1 for monoterpenes) were also much lower than those currently used by models for C4 crops, particularly for terpenes. These results suggest that maize fields are small BVOC exchangers in north-western Europe, with a lower BVOC emission impact than that modelled for growing C4 crops in this in this part of the world. They also reveal the high variability in BVOC exchanges across world regions for maize and suggest that SEFs should be estimated for each region separately.
Air-Sea Fluxes of CO2 and CH4 from the Penlee Point Atmospheric Observatory on the South West Coast of the UK
Authors: Yang M. , Bell T. G. , Hopkins F. E. , Kitidis V. , Cazenave P. W. , Nightingale P. D. , Yelland M. J. , Pascal R. W. , Prytherch J. , Brooks I. M. , Smyth T. J.
Published: January 2016

Abstract. We present air-sea fluxes of carbon dioxide (CO2), methane (CH4), momentum, and sensible heat measured by the eddy covariance method from the recently established Penlee Point Atmospheric Observatory (PPAO) on the South West coast of the United Kingdom. Measurements from the southwest direction (background marine air) at three different sampling heights (approximately 15, 18, 27 m above mean sea level, AMSL) in three different periods during 2014–2015 are shown. At sampling heights ≥ 18 m AMSL, measured fluxes of momentum and sensible heat demonstrate reasonable agreement with their expected transfer rates over the open ocean. This confirms the suitability of PPAO for air-sea exchange measurements. We observed reductions in the air-to-sea fluxes of CO2 from spring to summer in both years, which coincided with the breakdown of the spring phytoplankton bloom. At all sampling heights, mean CH4 fluxes were positive, suggesting marine emissions. Higher CH4 fluxes were observed during rising tides (20±3; 29±6; 38±3 μmole m−2 d−1 at 15, 27, 18 m AMSL) than during falling tides (14±2; 21±5; 22±2 μmole m−2 d−1, respectively), consistent with an elevated CH4 source from an estuarine outflow driven by local tidal circulation. Based on observations at PPAO, we also estimate the detection limit of the eddy covariance CH4 flux measurement to be ~20 μmole m−2 d−1 over hourly timescales (~4 μmole m−2 d−1 over 24 hours).
Investigation of processes controlling GEM oxidation at mid-latitudinal marine, coastal, and inland sites
Authors: Ye Z. , Mao H. , Lin C.-J. , Kim S. Y.
Published: January 2016

Abstract. A box model incorporating a state-of-the-art chemical mechanism for atmospheric mercury (Hg) cycling was developed to investigate oxidation of gaseous elemental mercury (GEM) at three locations in the northeastern United States: Appledore Island (marine), Thompson Farm (coastal, rural), and Pack Monadnock (inland, rural, elevated). The chemical mechanism improved model's ability to simulate the formation of gaseous oxidized mercury (GOM) at the study sites. At the coastal and inland sites, GEM oxidation was predominated by O3 and OH, contributing 80–99 % of total GOM production during daytime. H2O2 initiated GEM oxidation was significant (~ 33 % of the total GOM) at the inland site during nighttime. In the marine boundary layer (MBL), Br and BrO were dominant GEM oxidants contributing ~ 70 % of the total GOM production during mid-day, while O3 dominated GEM oxidation (50–90 % of GOM production) over the remaining day. Following the production of HgBr from GEM + Br, HgBr was oxidized by BrO, HO2, OH, ClO, and IO to form Hg(II) brominated GOM species. However, under atmospheric conditions, the prevalent GEM oxidants in the MBL could be Br / BrO or O3 / OH depending on Br and BrO mixing ratios. Relative humidity and products of the CH3O2 + BrO reaction possibly affected significantly the mixing ratios of Br or BrO radicals and subsequently GOM formation. Gas-particle partitioning could be potentially important in the production of GOM as well as Br and BrO at the marine site.
The representation of solar cycle signals in stratospheric ozone – Part 1: A comparison of satellite observations
Authors: Maycock A. , Matthes K. , Tegtmeier S. , Thiéblemont R. , Hood L.
Published: January 2016

Abstract. The impact of changes in incoming solar ultraviolet irradiance on stratospheric ozone forms an important part of the climate response to solar variability. To realistically simulate the climate response to solar variability using climate models, a minimum requirement is that they should include a solar cycle ozone component that has a realistic amplitude and structure, and which varies with season. For climate models that do not include interactive ozone chemistry, this component must be derived from observations and/or chemistry–climate model simulations and included in an externally prescribed ozone database that also includes the effects of all major external forcings. Part 1 of this two part study presents the solar-ozone responses in a number of updated satellite datasets for the period 1984–2004, including the Stratospheric Aerosol and Gas Experiment (SAGE) II version 6.2 and version 7.0 data, and the Solar Backscatter Ultraviolet Instrument (SBUV) version 8.0 and version 8.6 data. A number of combined datasets, which have extended SAGE II using more recent satellite measurements, are also analysed for the period 1984–2011. It is shown that SAGE II derived solar-ozone signals are sensitive to the independent temperature measurements used to convert ozone number density to mixing ratio units. A change in these temperature measurements in the recent SAGE II v7.0 data leads to substantial differences in the mixing ratio solar-ozone response compared to the previous v6.2, particularly in the tropical upper stratosphere. We also show that alternate satellite ozone datasets have issues (e.g., sparse spatial and temporal sampling, low vertical resolution, and shortness of measurement record), and that the methods of accounting for instrument offsets and drifts in merged satellite datasets can have a substantial impact on the solar cycle signal in ozone. For example, the magnitude of the solar-ozone response varies by around a factor of two across different versions of the SBUV VN8.6 record, which appears to be due to the methods used to combine the separate SBUV timeseries. These factors make it difficult to extract more than an annual-mean solar-ozone response from the available satellite observations. It is therefore unlikely that satellite ozone measurements alone can be applied to estimate the necessary solar cycle ozone component of the prescribed ozone database for future coupled model intercomparison projects (e.g., CMIP6).
Detecting long-term changes in point source fossil CO2 emissions with tree ring archives
Authors: Keller E. D. , Turnbull J. C. , Norris M. W.
Published: January 2016

Abstract. We examine the utility of tree ring 14C archives for detecting long term changes in fossil CO2 emissions from a point source. Trees assimilate carbon from the atmosphere during photosynthesis, in the process faithfully recording the average atmospheric 14C content in each new annual tree ring. Using 14C as a proxy for fossil CO2, we examine interannual variability over six years of fossil CO2 observations between 2004-05 and 2011-12 from two trees growing near the Kapuni Natural Gas Plant in rural Taranaki, New Zealand. We quantify the amount of variability that can be attributed to transport and meteorology by simulating constant point source fossil CO2 emissions over the observation period with the atmospheric transport model WindTrax. We compare model simulation results to observations and calculate the amount of change in emissions that we can detect with new observations over annual or multi-year time periods given both measurement uncertainty of 1ppm and the modelled variation in transport. In particular, we ask, what is the minimum amount of change in emissions that we can detect using this method, given a reference period of six years? We find that changes of 42% or more could be detected in a new sample from one year at the same observation location, or 22% in the case of four years of new samples. This threshold lowers and the method becomes more practical the more the size of the signal increases. For point sources 10 times larger than the Kapuni plant (a more typical size for power plants worldwide), it would be possible to detect sustaine d emissions changes on the order of 10% given suitable meteorology and observations.
Investigating the Source, Transport, and Isotope Composition of Water Vapor in the Planetary Boundary Layer
Authors: Griffis T. J. , Wood J. D. , Baker J. M. , Lee X. , Xiao K. , Chen Z. , Welp L. R. , Schultz N. M. , Gorski G. , Chen M. , Nieber J.
Published: January 2016

Abstract. Increasing atmospheric humidity and convective precipitation over land provide evidence of intensification of the hydrologic cycle – an expected response to surface warming. The extent to which terrestrial ecosystems modulate these hydrologic factors is important to understanding feedbacks in the climate system. We measured the oxygen and hydrogen isotope composition of water vapor from a very tall tower (185 m) in the Upper Midwest, United States to help diagnose the sources, transport, and fractionation of water vapor in the planetary boundary layer (PBL) over a 3-year period (2010 to 2012). These measurements represent the first set of annual water vapor isotope observations for the region. Models and cross wavelet analyses were used to assess the importance of Rayleigh, evapotranspiration (ET), and PBL entrainment processes on the isotope composition of water vapor. The vapor isotope composition at this tall tower site showed a very large seasonal amplitude (mean monthly δ18Ov ranged from −40.1 to −15.5 ‰ and δ2Hv ranged from −278.7 to −109.1 ‰) and followed the familiar Rayleigh distillation relation with water vapor mixing ratio at the annual time-scale. However, this relation was strongly modulated by ET and PBL entrainment processes at time-scales ranging from hours to several days. The wavelet coherence spectra indicate that the oxygen isotope ratio and the deuterium excess (dx) of water vapor are sensitive to synoptic and PBL processes. According to the phase of the coherence analyses, we show that ET often leads changes in dx, confirming that it is a potential tracer of regional ET. Isotope mixing models indicate that on average about 31 % of the growing season PBL water vapor is derived from regional ET. However, isoforcing calculations and mixing model analyses for high PBL water vapor mixing ratios events (> 25 mmol mol−1) indicate that regional ET can account for 40 % to 60 % of the PBL water vapor. These estimates are in relatively good agreement with that derived from numerical weather model simulations. This relatively large fraction of ET-derived water vapor implies that ET has an important impact on the precipitation recycling ratio within the region. Based on multiple constraints, we estimate that the summer season recycling fraction is about 30 %, indicating a potentially important link with convective precipitation.
Characteristics of total gaseous mercury (TGM) concentrations in an industrial complex in southern Korea: Impacts from local sources
Authors: Seo Yong-Seok , Jeong Seung-Pyo , Holsen Thomas M. , Han Young-Ji , Choi Eunhwa , Park Eun Ha , Kim Tae Young , Eum Hee-Sang , Park Dae Gun , Kim Eunhye , Kim Soontae , Kim Jeong-Hun , Choi Jaewon , Yi Seung-Muk
Published: January 2016

Abstract. Total gaseous mercury (TGM) concentrations were measured every 5 min in Pohang, Gyeongsangbuk-do, Korea during summer (17 August–23 August 2012), fall (9 October–17 October 2012), winter (22 January–29 January 2013), and spring (26 March–3 April 2013) to: 1) characterize the hourly and seasonal variations of atmospheric TGM concentrations, 2) identify the relationships between TGM and co-pollutants, and 3) identify likely source directions and locations of TGM using conditional probability function (CPF), conditional bivariate probability function (CBPF) and total potential source contribution function (TPSCF). The TGM concentration was statistically significantly highest in fall (6.7 ± 6.4 ng m−3), followed by spring (4.8 ± 4.0 ng m−3), winter (4.5 ± 3.2 ng m−3) and summer (3.8 ± 3.9 ng m−3). There was a statistically significant negative correlation between the TGM concentration and ambient air temperature (r = −0.08) (p < 0.05). Although the daytime temperature (14.7 ± 10.0 °C) was statistically significantly higher than that in the nighttime (13.0 ± 9.8 °C) (p < 0.05), the daytime TGM concentration (5.3 ± 4.7 ng m−3) was statistically significantly higher than those in the night time (4.7 ± 4.7 ng m−3) (p < 0.01), possibly due to local emissions related to industrial activities and activation of local surface emission sources. The observed ΔTGM/ΔCO was significantly lower than that of Asian long-range transport, but similar to that of local sources in Korea and in US industrial events suggesting that local sources are more important than that of long-range transport. CPF, CBPF and TPSCF indicated that the main sources of TGM were iron and manufacturing facilities, the hazardous waste incinerators and the coastal areas.
Stratosphere-troposphere exchange in the vicinity of a tropopause fold
Authors: Hofmann Christiane , Kerkweg Astrid , Hoor Peter , Jöckel Patrick
Published: January 2016

Abstract. Transport of air masses from the stratosphere to the troposphere along tropopause folds can lead to peaked ozone concentrations at ground level and hereby influence the long-term trend of tropospheric ozone. To improve the understanding of responsible processes and preferred regions of exchange, transient and reversible exchange processes in the vicinity of a tropopause fold are analysed on the basis of a case study. The global and regional atmospheric chemistry model system MECO(n), which couples the limited-area atmospheric chemistry and climate model COSMO-CLM/MESSy to the global model ECHAM5/MESSy for Atmospheric Chemistry (EMAC) is applied. Using similar process parametrisations in both model instances, the system allows for very consistent, simultaneous simulations at different spatial resolutions. Simulated ozone enhancements at ground level, caused by descending stratospheric air masses, are evaluated with observational data. Because of the coarse resolution of the global model, the observed ozone enhancements are not captured by the global model instance. However, the results of the finer resolved, regional model instance coincide well with the measurements. Based on the combination of Eulerian and Lagrangian analysis methods it is shown that stratosphere-troposphere-exchange (STE) in the vicintity of the tropopause fold occurs in regions of turbulence and diabatic processes. Within the framework of a Lagrangian study the efficiency of mixing along a tropopause fold is quantified, showing that almost all (97 %) of the air masses originating in the tropopause fold are transported into the troposphere during the following two days.
The impact of human activity on anthropogenic dust emission over global semi-arid regions
Authors: Guan X. , Huang J. , Zhang Y. , Xie Y. , Liu J.
Published: January 2016

Abstract. Anthropogenic dust is acknowledged as a product of human activities on disturbed soil, and is generated mainly from sensitive and fragile regions including croplands, pastures, and urbanized regions. In this study, we analyzed the behaviour of anthropogenic dust in semi-arid region of globe, and its relationship to human activities. An obvious peak in the total anthropogenic dust column, much higher magnitude than those of wet regions, was observed in semi-arid regions with population growth rates of more than 11.46 %. Four typical semi-arid regions, East China, India, North America and North Africa were selected to explore the local difference in anthropogenic dust production. The population growth rates in these areas were approximately 6.16 %, 17.71 %, 11.21 %, and 29.26 %, and the anthropogenic dust levels were 0.17 g m−2, 0.38 g m−2, 0.10 g m−2 and 0.21 g m−2, which are higher than the natural dust column burden. The anthropogenic dust column burden is positively correlated with the population and population change, indicating a contribution from human activities to the anthropogenic dust production. Based on the fact that anthropogenic dust can act as warming aerosol, the radiative effect of anthropogenic dust in semi-arid regions can not be ignored and requires further investigation.
Tropospheric column ozone response to ENSO in GEOS-5 assimilation of OMI and MLS ozone data
Authors: Olsen M. A. , Wargan K. , Pawson S.
Published: January 2016

Abstract. We use GEOS-5 analyses of Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) ozone observations to investigate the magnitude and spatial distribution of the El Niño Southern Oscillation (ENSO) influence on tropospheric column ozone (TCO) into the middle latitudes. This study provides the first explicit spatially resolved characterization of the ENSO influence and demonstrates coherent patterns and teleconnections impacting the TCO in the extratropics. The response is evaluated and characterized by both the variance explained and sensitivity of TCO to the Niño 3.4 index. The tropospheric response in the tropics agrees well with previous studies and verifies the analyses. However, we show a newly identified two-lobed response symmetric about the Equator in the western Pacific/Indonesian region consistent with the large-scale vertical transport. We also find that the large-scale transport in the tropics dominates the response compared to the small-scale convective transport. The ozone response is weaker in the middle latitudes, but significant explained variance of the TCO is found over several small regions, including the central United States. However, the sensitivity of TCO to the Niño 3.4 index is statistically significant over a large area of the middle latitudes. The sensitivity maxima and minima coincide with anomalous anti-cyclonic and cyclonic circulations where the associated vertical transport is consistent with the sign of the sensitivity. Also, ENSO related changes to the mean tropopause height can contribute significantly to the midlatitude response. Comparisons to a 22-year chemical transport model simulation demonstrate that these results from the nine-year assimilation are representative of the longer-term. This investigation brings insight to several seemingly disparate prior studies of the El Niño influence on tropospheric ozone in the middle latitudes.
The Importance of Plume Rise on the Concentrations and Atmospheric Impacts of Biomass Burning Aerosol
Authors: Walter Carolin , Freitas Saulo R. , Kottmeier Christoph , Kraut Isabel , Rieger Daniel , Vogel Heike , Vogel Bernhard
Published: January 2016

Abstract. We quantiﬁed the effects of the plume rise of biomass burning aerosol and gases for the forest ﬁres that occurred in Saskatchewan, Canada, in July 2010. For this purpose, simulations with different assumptions regarding the plume rise and the vertical distribution of the emissions were conducted. Based on comparisons with observations, applying a one-dimensional plume rise model to predict the injection layer in combination with a parametrisation of the vertical distribution of the emissions outperforms approaches in which the plume heights are initially predeﬁned. Approximately 30 % of the ﬁres exceed the height of 2 km and the maximum height is 8.6 km. Using this plume rise model, comparisons with satellite images in the visible spectral range show a very good agreement between the simulated and observed spatial distributions of the biomass burning plume. The simulated AOD with data of an AERONET station is in good agreement with respect to the absolute values and the timing of the maximum. Comparison of the vertical distribution of the biomass burning aerosol with CALIPSO retrievals also showed the best agreement when the plume rise model was applied. We found that down-welling surface short-wave radiation below the forest ﬁre plume is reduced by up to 50 % and that the 2 m temperature is decreased by up to 6 K. In addition, we simulated a strong change in atmospheric stability within the biomass burning plume.
Analysis of particulate emissions from tropical biomass burning using a global aerosol model and long-term surface observations
Authors: Reddington C. L. , Spracklen D. V. , Artaxo P. , Ridley D. , Rizzo L. V. , Arana A.
Published: January 2016

Abstract. We use the GLOMAP global aerosol model evaluated against observations of surface particulate matter (PM2.5) and aerosol optical depth (AOD) to better understand the impacts of biomass burning on tropical aerosol. To explore the uncertainty in emissions we use three satellite-derived fire emission datasets (GFED3, GFAS1 and FINN1) in the model, in which tropical fires account for 66–84 % of global particulate emissions from fire. The model underestimates PM2.5 concentrations where observations are available over South America and AOD over South America, Africa and Southeast Asia. Underestimation of AOD over tropical regions impacted by biomass burning is consistent with previous studies. Where coincident observations of surface PM2.5 and AOD are available we find a greater model underestimation of AOD than PM2.5 Increasing particulate emissions to improve simulation of AOD can therefore lead to overestimation of surface PM2.5 concentrations. With FINN1 emissions increased by a factor of 1.5 the model reasonably simulates PM2.5 concentrations in South America and AOD over Southeast Asia, but underestimates AOD over South America and Africa. The model with GFAS1 emissions better matches observed PM2.5 and AOD when emissions are increased by a factor of 3.4. The model with GFED3 emissions scaled by a factor of 1.5 reasonably simulates PM2.5 concentrations in South America, but requires a larger scaling factor to capture observed AOD in all regions. The model with GFED3 emissions poorly simulates observed seasonal variability of surface PM2.5 and AOD in regions where small fires dominate, providing independent evidence that GFED3 omits emissions from small fires. Seasonal variability of both PM2.5 and AOD is better simulated by the model using FINN1 and GFAS1 emissions. Detailed observations of the vertical profile of aerosol over biomass burning regions are required to better constrain emissions and modelled AOD.
Classical nucleation theory of immersion freezing: Sensitivity of contact angle schemes to thermodynamic and kinetic parameters
Authors: Ickes L. , Welti A. , Lohmann U.
Published: January 2016

Abstract. Heterogeneous ice formation by immersion freezing in mixed-phase clouds can be parameterized in general circulation models (GCMs) by Classical Nucleation Theory (CNT). CNT parameterization schemes describe freezing as a stochastic process including the properties of insoluble aerosol particles, so called ice nuclei, in the droplets. There are different ways how to describe the properties of aerosol particles (i.e. contact angle schemes), which are compiled and tested in this paper. The goal of this study is to ﬁnd a parameterization scheme for GCMs to describe immersion freezing with the ability to shift and adjust the slope of the freezing curve compared to homogeneous freezing to match experimental data. The results of using CNT are very sensitive to unconstrained kinetic and thermodynamic parameters in the case of homogeneous freezing leading to uncertainties in calculated nucleation rates Jhom of several orders of magnitude. Here we investigate how sensitive the outcome of a parameter estimation for contact angle schemes from experimental data is to kinetic and thermodynamic parameters. We show that additional free parameter can mask the uncertainty of Jimm due to thermodynamic and kinetic parameters. Different CNT formulations are ﬁtted to an extensive immersion freezing dataset as a function of particle diameter (d), temperature T and time t for different mineral dust types, namely kaolinite, illite, montmorillonite, microcline (K-feldspar) and Arizona test dust. It is investigated how accurate different CNT formulations (with the estimated ﬁt parameters) reproduce the measured freezing curves, especially the time and particle size dependence of the freezing process. The results are compared to a simpliﬁed deterministic freezing scheme. It is evaluated in this context which CNT based parameterization scheme to represent particle properties is a good choice to describe immersion freezing in a GCM.
Future Arctic ozone recovery: the importance of chemistry and dynamics
Authors: Bednarz E. M. , Maycock A. C. , Abraham N. L. , Braesicke P. , Dessens O. , Pyle J. A.
Published: January 2016

Abstract. Future trends in Arctic springtime total column ozone, and its chemical and dynamical drivers, are assessed using a 7 member ensemble from the Met Office Unified Model with United Kingdom Chemistry and Aerosols (UM-UKCA) simulating the period 1960-2100. The Arctic mean March total column ozone increases throughout the 21st century at a rate of ~11.5 DU decade-1, and is projected to return to the 1980 level in the late 2030s. However, the integrations show that even past 2060 springtime Arctic ozone can episodically drop by ~50-100 DU below the long-term mean to near present day values. Consistent with the global decline in inorganic chlorine (Cly) over the century, the estimated mean halogen induced chemical ozone loss in the Arctic lower atmosphere in spring decreases by around a factor of two between 1981-2000 and 2061-2080. However, in the presence of a cold and strong polar vortex elevated halogen losses well above the long-term mean continue to occur in the simulations into the second part of the century. The ensemble shows a radiatively-driven cooling trend modelled in the Arctic winter mid- and upper stratosphere, but there is less consistency across the seven ensemble members in the lower stratosphere (100-50 hPa). This is partly due to an increase in downwelling over the Arctic polar cap in winter, which increases transport of ozone into the polar region as well as drives adiabatic warming that partly offsets the radiatively-driven stratospheric cooling. However, individual years characterised by significantly suppressed downwelling, reduced transport and low temperatures continue into the future. We conclude that despite the future long-term recovery of Arctic ozone, the large interannual dynamical variability is expected to continue thereby facilitating episodic reductions in springtime ozone columns. Whilst our results suggest that the relative role of dynamical processes for determining Arctic springtime ozone will increase in the future, halogen chemistry will remain a smaller but non-negligible contributor for many decades.
Sources of organic ice nucleating particles in soils
Authors: Hill T. C. J. , DeMott P. J. , Tobo Yutaka , Fröhlich-Nowoisky J. , Moffett B. F. , Franc G. D. , Kreidenweis S. M.
Published: January 2016

Abstract. Soil organic matter (SOM) may be a significant source of atmospheric ice nucleating particles (INPs), especially of those active >-15 °C. However, due to both a lack of investigations and the complexity of the SOM itself, the principal sources of these INPs remain unknown. To more comprehensively characterize organic INPs we tested locally representative soils in Wyoming and Colorado for total organic INPs, INPs in the heat-labile fraction, ice nucleating (IN) bacteria, IN fungi, IN fulvic and humic acids, IN plant tissue, and ice nucleation by monolayers of aliphatic alcohols. All soils contained ≈106 to ≈5 ×107 INPs g-1 dry soil active at -10 °C. Removal of SOM with H2O2 effectively removed all INPs active >-18 °C (the limit of testing), while heating of soil suspensions to 105 °C showed that labile INPs increasingly predominated >-12 °C and comprised ≥90% of INPs active >-9 °C. Papain protease, which inactivates IN proteins produced by the fungus Mortierella alpina, common in the region’s soils, lowered INPs active at ≥-11 °C by ≥75% in two arable soil and sagebrush shrubland soil. By contrast, lysozyme, which digests bacterial cell walls, only reduced INPs active at ≥-7.5 or ≥-6 °C, depending on the soil. The known IN bacteria were not detected in any soil, using PCR for the ina gene that codes for the active protein. We directly isolated and photographed two individual INPs from soil, using repeated cycles of freeze-testing and subdivision of droplets of dilute soil sus pensions: They were complex and apparently organic entities. Ice nucleation activity was not affected by digestion of Proteinase K-susceptible proteins or the removal of entities composed of fulvic and humic acids, sterols or aliphatic alcohol monolayers. Organic INPs active at temperatures colder than -10° to -12 °C were resistant to all investigations other than heat, oxidation with H2O2 and, for some, digestion with papain. They may originate from decomposing plant material, microbial biomass and/or the humin component of the SOM. If the latter then they are most likely to be a carbohydrate. Reflecting the diversity of the SOM itself, soil INPs have a range of sources, occur with differing relative abundances, and may be protected by different mechanisms.
Fennec dust forecast intercomparison over the Sahara in June 2011
Authors: Chaboureau J.-P. , Flamant C. , Dauhut T. , Kocha C. , Lafore J.-P. , Lavaysse C. , Marnas F. , Mokhtari M. , Pelon J. , Reinares Martínez I. , Schepanski K. , Tulet P.
Published: January 2016

Abstract. In the framework of the Fennec international programme, a ﬁeld campaign was conducted in June 2011 over the western Sahara. It led to the ﬁrst observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the inﬂuence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection- permitting models, respectively. The unique airborne and ground-based data sets allowed the ﬁrst ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large Aerosol Optical Depths (AODs) were forecast over the Sahara, a feature observed by some satellite retrievals but mislocated by others over the Sahel. The AOD intensity was correctly predicted by the highresolution models while being underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield to emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80 % of dust emission, all the models were successful in forecasting the deep well-mixed SABL. Some of them, however, missed the large near-surface dust extinction generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only.
Characterization of Organosulfates in Secondary Organic Aerosol Derived from the Photooxidation of Long-Chain Alkanes
Authors: Riva M. , Da Silva Barbosa T. , Lin Y.-H. , Stone E. A. , Gold A. , Surratt J. D.
Published: January 2016

Abstract. We report the formation of aliphatic organosulfates (OSs) in secondary organic aerosol (SOA) from the photooxidation of C10 – C12 alkanes. The results complement those from our laboratories reporting the formation of OSs and sulfonates from gas-phase oxidation of polycyclic aromatic hydrocarbons (PAHs). Both studies strongly support formation of OSs from gas-phase oxidation of anthropogenic precursors, hypothesized on the basis of recent field studies in which aromatic and aliphatic OSs were detected in fine aerosol collected from several major urban locations. In this study, dodecane, cyclodecane and decalin, considered to be important SOA precursors in urban areas, were photochemically oxidized in an outdoor smog chamber in the presence of either non-acidified or acidified ammonium sulfate seed aerosol. Effects of chemical structure, acidity and relative humidity on OS formation were examined. Aerosols collected from all experiments were characterized by ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of flight mass spectrometry (UPLC/ESI-HR-QTOFMS). Most of the OSs identified could be explained by formation of gaseous epoxide precursors with subsequent acid-catalyzed reactive uptake onto sulfate aerosol. The OSs identified here were also observed and quantified in fine urban aerosol samples collected in Lahore, Pakistan, and Pasadena, USA. Many of the OSs identified from the photooxidation of decalin and cyclodecane are isobars of known monoterpene organosulfates, and thus care must be taken in the analysis of alkane-derived organosulfates in urban aerosol.
Variation of CCN activity during new particle formation events in the North China Plain
Authors: Ma N. , Zhao C. S. , Tao J. C. , Wu Z. J. , Kecorius S. , Wang Z. B. , Größ J. , Liu H. J. , Bian Y. X. , Kuang Y. , Teich M. , Spindler G. , Müller K. , van Pinxteren D. , Herrmann H. , Hu M. , Wiedensohler A.
Published: January 2016

Abstract. The aim of this investigation was to obtain a better understanding of the variability of the cloud condensation nuclei (CCN) activity during new particle formation (NPF) events in an anthropogenically polluted atmosphere of the North China Plain (NCP). We investigated the size-resolved activation ratio as well as particle number size distribution, hygroscopicity and chemical composition during an intensive field experiment at a regional atmospheric observatory at Xianghe. Interestingly, two types of NPF events were found, in which the growth of the newly formed particles is dominated by either sulfate or organic matters. The particle CCN activity therefore significantly differs in those NPF events, indicating that it might be difficult to find a simple parameterization of particle CCN activity during NPF events over the NCP. For an accurate estimation of the potential CCN number concentration (NCCN) during NPF events, the variation of CCN activity has to be taken into account. Considering that a fixed activation ratio curve or critical diameter are usually used to calculate NCCN, the influence of the variation of particle CCN activity on the calculation of NCCN during NPF events was evaluated based on these two parameterizations. It was found that NCCN might be underestimated by up to 30 % if a fix activation ratio curve (representative of the region and season) is used in the calculation; and might be underestimated by up to 50 % if a fix critical diameter (representative of the region and season) is used. Therefore, we suggest not using a fixed critical diameter in the prediction of NCCN in NPF seasons. If real-time CCN activity data is not available, using a proper fixed activation ratio curve can be a compromising choice.
Relation between ice and liquid water mass in mixed-phase cloud layers measured with Cloudnet
Authors: Bühl Johannes , Seifert Patric , Myagkov Alexander , Ansmann Albert
Published: January 2016

Abstract. An analysis of the Cloudnet dataset collected at Leipzig, Germany, with special focus on mixed-phase layered clouds is presented. We derive liquid and ice water content together with vertical motions of ice particles falling through cloud base. The ice mass flux is calculated by combining measurements of ice water content and particle fall velocity. The efficiency of heterogeneous ice formation and its impact on cloud lifetime is estimated for different cloud-top temperatures by relating the ice mass flux and the liquid water content at cloud top. Cloud radar measurements of polarization and fall velocity yield, that ice crystals formed in cloud layers with a geometrical thickness of less than 350 m are mostly pristine when they fall out of the cloud. It is also found that current and future spaceborne cloud radars might miss a large portion of that primary ice formation, especially for cloud layers with top temperatures warmer than −15 °C.
The Lifetime of Nitrogen Oxides in an Isoprene Dominated Forest
Authors: Romer Paul S. , Duffey Kaitlin C. , Wooldridge Paul J. , Allen Hannah M. , Ayres Benjamin R. , Brown Steven S. , Brune William H. , Crounse John D. , de Gouw Joost , Draper Danielle C. , Feiner Philip A. , Fry Juliane L. , Goldstein Allen H. , Koss Abigail , Misztal Pawel K. , Nguyen Tran B. , Olson Kevin , Teng Alex P. , Wennberg Paul O. , Wild Robert J. , Zhang Li , Cohen Ronald C.
Published: January 2016

Abstract. The lifetime of NOx (NOx ≡ NO + NO2) affects the concentration and distribution of NOx and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NOx is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NOx and NOx reservoirs. We find that the lifetime of NOx during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (ΣANs). During SOAS, ΣAN production was rapid, averaging 90 ppt hr−1 during the day, and occurred predominantly during isoprene oxidation. Analysis of the ΣAN and HNO3 budgets indicate that ΣANs have an average lifetime of under 2 hours, and that approximately 45 % of the ΣANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that ΣAN hydrolysis is the largest source of HNO3 and the primary pathway to permanent removal of NOx from the boundary layer in this location. Using these new constraints on the fate of ΣANs, we find that the NOx lifetime is 11 ± 5 hours under typical midday conditions. The lifetime is extended by storage of NOx in temporary reservoirs, including acyl peroxy nitrates and ΣANs.
A method for the direct measurement of surface tension of atmospherically relevant aerosol particles using atomic force microscopy
Authors: Hritz A. D. , Raymond T. M. , Dutcher D. D.
Published: January 2016

Abstract. Accurate estimates of particle surface tension are required for models concerning atmospheric aerosol nucleation and activation. However, it is difficult to collect sufficiently large volumes of atmospheric aerosol for use in typical instruments that measure surface tension, such as goniometers or Wilhelmy plates. In this work, a method that measures the surface tension of collected liquid nanoparticles using atomic force microscopy is presented. A film of particles is collected via impaction and is probed using nanoneedle tips with the atomic force microscope. This micro-Wilhelmy method allows for direct measurements of surface tension of small amounts of sample. This method was verified using liquids whose surface tensions were known. Particles of oxidized α-pinene were then produced, collected, and analyzed using this method. Preliminary results show that oxidized α-pinene particles formed in dry conditions have a surface tension similar to that of pure α-pinene, and particles formed in wet conditions have a surface tension that is significantly higher.
Linking variations in sea spray aerosol particle hygroscopicity to composition during two microcosm experiments
Authors: Forestieri Sara D. , Cornwell Gavin C. , Helgestad Taylor M. , Moore Kathryn A. , Lee Christopher , Novak Gordon A. , Sultana Camille M. , Wang Xiaofei , Bertram Timothy H. , Prather Kimberly A. , Cappa Christopher D.
Published: January 2016

Abstract. The extent to which water uptake influences the light scattering ability of marine sea spray aerosol (SSA) particles depends critically on SSA chemical composition. The organic fraction of SSA can increase during phytoplankton blooms, decreasing the salt content and therefore the hygroscopicity of the particles. In this study, subsaturated hygroscopic growth factors at 85 % relative humidity (GF(85 %)) of SSA particles were quantified during two induced phytoplankton blooms in marine aerosol reference tanks (MARTs). One MART was illuminated with fluorescent lights and the other was illuminated with sunlight, referred to as the "indoor" and "outdoor" MARTs, respectively. GF(85 %) values for SSA particles were derived from measurements of light scattering and particle size distributions, concurrently with online single particle and bulk aerosol composition measurements. During both microcosm experiments, the observed bulk average GF(85 %) values were depressed substantially relative to pure, inorganic sea salt, by 10 to 19 %, with a one (indoor MART) and six (outdoor MART) day lag between GF(85 %) depression and the peak chlorophyll-a concentrations. The fraction of organiccontaining SSA particles generally increased after the peak of the phytoplankton blooms. The GF(85 %) values were inversely correlated with the fraction of particles containing organic or other biological markers. This indicates these particles were less hygroscopic than the particles identified as predominately sea salt containing and demonstrates a clear relationship between SSA particle composition and the sensitivity of light scattering to variations in relative humidity. The implications of these observations to the direct climate effects of SSA particles are discussed.
Attribution of Atmospheric Sulfur Dioxide over the English Channel to Dimethylsulfide and Changing Ship Emissions
Authors: Yang Mingxi , Bell Thomas G. , Hopkins Frances E. , Smyth Timothy J.
Published: January 2016

Abstract. Atmospheric sulfur dioxide (SO2) was measured continuously from the Penlee Point Atmospheric Observatory(PPAO) near Plymouth, United Kingdom between May 2014 and November 2015. This coastal site is exposed to marine air across a wide wind sector. The predominant southwesterly winds carry relatively clean background Atlantic air. In contrast, air from the southeast is heavily influenced by exhaust plumes from ships in the English Channel as well as near the Plymouth Sound. New International Maritime Organization (IMO) regulation came into force in January 2015 to reduce sulfur emissions tenfold in Sulfur Emission Control Areas such as the English Channel. Our observations suggest a three-fold reduction from 2014 to 2015 in ship-emitted SO2 from that direction. Apparent fuel sulfur content calculated from coincidental SO2 and carbon dioxide (CO2) peaks from local ship plum es show a high level of compliance to the IMO regulation (> 95 %) in both years. Dimethylsulfide (DMS) is an important source of atmospheric SO2 even in this semi-polluted region. The relative contribution of DMS oxidation to the SO2 burden over the English Channel increased from ~ 1/3 in 2014 to ~ 1/2 in 2015 due to the reduction in ship sulfur emissions. Our diel analysis suggests that SO2 is removed from the marine atmospheric boundary layer in about half a day, with dry deposition to the ocean accounting for a quarter of the total loss.