Marine Boundary Layer Research Articles

Physicochemical characterization of free troposphere and marine boundary layer ice-nucleating particles collected by aircraft in the eastern North Atlantic

Abstract. Atmospheric ice nucleation impacts the hydrological cycle and climate by modifying the radiative properties of clouds. To improve our predictive understanding of ice formation, ambient ice-nucleating particles (INPs) need to be collected and characterized. Measurements of INPs at lower latitudes in a remote marine region are scarce. The Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign, in the region of the Azores islands, provided the opportunity to collect particles in the marine boundary layer (MBL) and free troposphere (FT) by aircraft during the campaign's summer and winter intensive operation period. The particle population in samples collected was examined by scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy. The identified INPs were analyzed by scanning electron microscopy with energy-dispersive X-ray analysis. We observed differences in the particle population characteristics in terms of particle diversity, mixing state, and organic volume fraction between seasons, mostly due to dry intrusion events during winter, as well as between the sampling locations of the MBL and FT. These differences are also reflected in the temperature and humidity conditions under which water uptake, immersion freezing (IMF), and deposition ice nucleation (DIN) proceed. Identified INPs reflect typical particle types within the particle population on the samples and include sea salt, sea salt with sulfates, and mineral dust, all associated with organic matter, as well as carbonaceous particles. IMF and DIN kinetics are analyzed with respect to heterogeneous ice nucleation rate coefficients, Jhet, and ice nucleation active site density, ns, as a function of the water criterion Δaw. DIN is also analyzed in terms of contact angles following classical nucleation theory. Derived MBL IMF kinetics agree with previous ACE-ENA ground-site INP measurements. FT particle samples show greater ice nucleation propensity compared to MBL particle samples. This study emphasizes that the types of INPs can vary seasonally and with altitude depending on sampling location, thereby showing different ice nucleation propensities, which is crucial information when representing mixed-phase cloud and cirrus cloud microphysics in models.

Insights of warm-cloud biases in Community Atmospheric Model 5 and 6 from the single-column modeling framework and Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) observations

Abstract. There has been a growing concern that most climate models predict precipitation that is too frequent, likely due to lack of reliable subgrid variability and vertical variations in microphysical processes in low-level warm clouds. In this study, the warm-cloud physics parameterizations in the singe-column configurations of NCAR Community Atmospheric Model version 6 and 5 (SCAM6 and SCAM5, respectively) are evaluated using ground-based and airborne observations from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign near the Azores islands during 2017–2018. The 8-month single-column model (SCM) simulations show that both SCAM6 and SCAM5 can generally reproduce marine boundary layer cloud structure, major macrophysical properties, and their transition. The improvement in warm-cloud properties from the Community Atmospheric Model 5 and 6 (CAM5 to CAM6) physics can be found through comparison with the observations. Meanwhile, both physical schemes underestimate cloud liquid water content, cloud droplet size, and rain liquid water content but overestimate surface rainfall. Modeled cloud condensation nuclei (CCN) concentrations are comparable with aircraft-observed ones in the summer but are overestimated by a factor of 2 in winter, largely due to the biases in the long-range transport of anthropogenic aerosols like sulfate. We also test the newly recalibrated autoconversion and accretion parameterizations that account for vertical variations in droplet size. Compared to the observations, more significant improvement is found in SCAM5 than in SCAM6. This result is likely explained by the introduction of subgrid variations in cloud properties in CAM6 cloud microphysics, which further suppresses the scheme's sensitivity to individual warm-rain microphysical parameters. The predicted cloud susceptibilities to CCN perturbations in CAM6 are within a reasonable range, indicating significant progress since CAM5 which produces an aerosol indirect effect that is too strong. The present study emphasizes the importance of understanding biases in cloud physics parameterizations by combining SCM with in situ observations.

Sensitivity of tropospheric ozone to halogen chemistry in the chemistry–climate model LMDZ-INCA vNMHC

Abstract. The atmospheric chemistry of halogenated species (Cl, Br, I) participates in the global chemical sink of tropospheric ozone and perturbs the oxidising capacity of the troposphere, notably by influencing the atmospheric lifetime of methane. Global chemistry–climate models are commonly used to assess the global budget of ozone and its sensitivity to emissions of its precursors, as well as to project its long-term evolution. Here, we report on the implementation of tropospheric sources and chemistry of halogens in the chemistry–climate model LMDZ-INCA (Laboratoire de Météorologie Dynamique general circulation model, LMDZ, and Interactions with Chemistry and Aerosols, INCA, version with Non-Methane HydroCarbon chemistry, vNMHC) and evaluate halogen effects on the tropospheric ozone budget. Overall, the results show that the model simulates satisfactorily the impact of halogens on the photo-oxidising system in the troposphere, in particular in the marine boundary layer. To quantify the effects of halogen chemistry in LMDZ-INCA, standard metrics representative of the behaviour of the tropospheric chemical system (Ox, HOx, NOx, CH4 and non-methane volatile organic compounds – NMVOCs) are computed with and without halogens. The addition of tropospheric halogens in the LMDZ-INCA model leads to a decrease of 22 % in the ozone burden, 8 % in OH and 33 % in NOx. Sensitivity simulations show for the first time that the inclusion of halogen chemistry makes ozone more sensitive to perturbations in CH4, NOx and NMVOCs. Consistent with other global model studies, the sensitivity of the tropospheric ozone burden to changes from pre-industrial to present-day emissions is found to be ∼20 % lower when tropospheric halogens are taken into account.

Changes in Relative Humidity Profiles over Earth’s Oceans in a Warming Climate: A Satellite-Data-Based Inference

Abstract Recently, we presented a classification of “primitive” relative humidity (RH) profiles into eight distinct clusters over Earth’s oceans, based on about 18 years (2003–20) of observations from the AIRS on NASA’s Aqua satellite. Here we investigate the seasonal variability and decadal trends, both in the vertical structure of these RH profiles, and in their associated area of occurrence. Since vertical structures (except in the marine boundary layer) of each RH class are generally robust across all seasons and change only weakly in a warming climate, seasonal or decadal changes to their occurrence areas shift patterns of global moisture distribution. Globally, the marine boundary layer exhibits nonlinear moistening effects after about 2010, the end of the warming hiatus. Annual time series of ocean areas dominated by RH classes have linear trends, which are positive only for the most moist and driest RH classes (in terms of the free troposphere) associated with deep convection and large-scale subsidence favoring conditions for low-level stratocumulus clouds, respectively. Based on estimated linear trends of RH-class occurrences and sea surface temperatures, we infer projected linear responses of RH in a warming climate. Ocean areas dominated by most moist and driest RH classes (in terms of the free atmosphere) are estimated to increase by about 1% and 2%, respectively (corresponding to about 2.5% K−1 and 4.5% K−1, respectively). The averaged global and tropical RH structure remain almost constant in a warming climate. While this is consistent with other studies, our results show how increases in most moist and dry areas compensate each other, indicating possible increases in the frequency or persistence of future extreme events.

Vertical Structure of Clouds and Precipitation During Arctic Cold‐Air Outbreaks and Warm‐Air Intrusions: Observations From COMBLE

AbstractThe Arctic is marked by deep intrusions of warm, moist air, alternating with outbreaks of cold air down to lower latitudes. The typical vertical structure of clouds and precipitation during these two synoptic weather extremes is examined at a coastal site at 69°N in Norway. The Norwegian Sea is a corridor for warm‐air intrusions (WAIs) and frequently witnesses cold‐air outbreaks (CAOs). This study uses data from profiling radar, lidar, and microwave radiometer, radiosondes and other probes that were collected during the CAOs in the Marine Boundary Layer Experiment (COMBLE) between 1 December 2019 and 31 May 2020. Marine CAOs are defined in terms of thermal instability relative to the sea surface temperature, and WAIs in terms of equivalent potential temperature stratification between the surface and 850 hPa. Cloud structures in CAOs are convective, driven by strong surface heat fluxes over a long fetch of open water, with cloud tops rarely exceeding 6 km above sea level. The mostly open‐cellular convection produces intermittent moderately‐heavy precipitation at the observational site, notwithstanding the low precipitable water vapor (PWV). In contrast, WAIs are marked by high values of PWV and integrated vapor transport. WAI clouds are synoptically driven, stratiform, with cloud tops often exceeding 5 km, sometimes layered, and generally producing persistent precipitation that can be heavier than in CAOs.

Sensitivity of Nocturnal Warm Sector Rainfall Simulation to the Configuration of Initial and Lateral Boundary Conditions: A Case Study in Southern China Based on the Operational TRAMS Model

AbstractA heavy nocturnal warm sector rainfall occurred over the western coastal region of South China in June 2020, of which the precipitation magnitude was seriously underestimated by the operational TRAMS (Tropical Regional Atmosphere Model System) model. In this study, by improving the configuration of initial conditions (ICs) and lateral boundary conditions (LBCs) in offline nesting, the simulation of the convection initiation (CI) process can be well improved. The CI simulation is found to be sensitive to the vertical resolution in ICs in this case. When the low‐level vertical resolution of ICs is increased, the nocturnal near‐surface cold layer caused by inland mountains can be resolved better, which is necessary to form the convergence line along the coastline. Another important impact of increasing the vertical resolution of ICs is the successful simulation of horizontal convective rolls (HCRs) over the northern South China Sea. The HCRs apparently increase the depth of the warm‐moist marine boundary layer jet (MBLJ), and finally lead to the CI along the coastline. The increased vertical resolutions of LBCs can reduce the discontinuity of simulated moist tongue across the southern lateral boundary of the TRAMS model, which provides more moisture for the warm sector rainfall through MBLJ. LBCs with Higher temporal frequency help to alleviate the western position bias of rain belt by reducing the temporal interpolation error. Our study highlights the importance of finer ICs and LBCs in offline nesting for regional operational NWP systems in the South China region.

Insight into NOx and HONO chemsitry in the tropical marine boundary layer at Cape Verde during MarParCloud campaign

insight into NOx and HONO chemsitry in the tropical marine boundary layer at Cape Verde during MarParCloud campaign

Continuous Measurement and Molecular Compositions of Atmospheric Water‐Soluble Brown Carbon in the Nearshore Marine Boundary Layer of Northern China: Secondary Formation and Influencing Factors

AbstractUnderstanding the evolution of brown carbon (BrC) in the atmosphere is essential for investigating its climate effects. This study deployed a novel in‐situ BrC continuous observation system to firstly measure water‐soluble BrC absorption (AbsWS‐BrC) in an offshore island over the Bohai Sea in the winter of 2020. The AbsWS‐BrC abundance before the cold wave (BCW) was more than twice higher than that after the cold wave (ACW). This was mainly ascribed to the substantially suppressed formation of secondary WS‐BrC (WS‐BrCsec). Diurnal patterns of AbsWS‐BrC exhibited nighttime peaks, which derived from enhanced primary emissions and strong aqueous‐phase processes. Photochemical processes bleached daytime WS‐BrC during BCW, while daytime peaks of WS‐BrCsec emerged during ACW due to the weakened photobleaching effect. Statistical analysis indicated ambient temperature and relative humidity as well as total oxidized nitrogen (NO2 + NO3−) and reduced nitrogen (NH4+ + NH3) were the dominant factors promoting WS‐BrCsec. Moderate aerosol pH (>2.5) also facilitated the formation of WS‐BrCsec while no obvious dependences of WS‐BrCsec on gas‐particle partitioning of ammonia, O3, and sulfur precursors were found. At the molecular level, BrC chromophores with the identified compositions, carbon oxidation state, O/C, H/C, and absorption spectra were compared between the two periods. Characteristics of water‐soluble CHO‐ and CHON‐BrC were more of secondary origin during BCW, while primary emissions contributed significantly to BrC chromophores during ACW. This study highlights the advantage of high resolution BrC measurement in probing its dynamic evolution and influencing factors.

Determining sources of reactive mercury compounds in Reno, Nevada, United States

There is much uncertainty regarding the sources of reactive mercury (RM) compounds and atmospheric chemistry driving their formation. This work focused on assessing the chemistry and potential sources of reactive mercury measured in Reno, Nevada, United States, using 1 year of data collected using Reactive Mercury Active System. In addition, ancillary meteorology and criteria air pollutant data, Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) analyses, and a generalized linear model were applied to better understand reactive mercury observations. During the year of sampling, a fire event impacted the sampling site, and gaseous elemental Hg and particulate-bound mercury concentrations increased, as did HgII-S compounds. Data collected on a peak above Reno showed that reactive mercury concentrations were higher at higher elevation, and compounds found in Reno were the same as those measured on the peak. HYSPLIT results demonstrated RM compounds were generated inside and outside of the basin housing Reno. Compounds were sourced from San Francisco, Sacramento, and Reno in the fall and winter, and from long-range transport and the marine boundary layer during the spring and summer. The generalized linear model produced correlations that could be explained; however, when applying the model to similar data collected at two other locations, the Reno model did not predict the observations, suggesting that sampling location chemistry and concentration cannot be generalized.

Enhanced daytime atmospheric mercury in the marine boundary layer in the South Oceans

Understanding the spatial and temporal variations of atmospheric mercury (Hg) in the marine boundary layer could advance our knowledge on ocean evasion of Hg. Here, we conducted continuous measurements of total gaseous mercury (TGM) in the marine boundary layer during a round-the-world cruise from August 2017 to May 2018. We observed the highest and lowest TGM concentrations in Southern Indian Ocean (1.29 ± 0.22 ng m−3) and Southern Atlantic Ocean (0.61 ± 0.28 ng m−3), respectively. During the daytime, enhanced TGM was observed with the diurnal amplitude difference reaching its maximum in the range of 0.30–0.37 ng m−3 in Southern Indian Ocean and Southern Ocean. The positive correlation between TGM (R2 = 0.68–0.92) and hourly solar radiation in each ocean suggested that the daytime enhanced TGM was likely driven by Hg photoreduction in seawater, after excluding the influence of other meteorological factors. The diurnal amplitude of TGM in the marine boundary layer might be impacted by the microbial productivity and the ratio of ultraviolet radiation. Our study highlights that ocean acts as a net TGM source during the daytime in the Southern Hemisphere and aqueous photoreduction process may play an important role in the biogeochemical cycling of Hg.

A seasonal analysis of aerosol NO3− sources and NOx oxidation pathways in the Southern Ocean marine boundary layer

Abstract. Nitrogen oxides, collectively referred to as NOx (NO + NO2), are an important component of atmospheric chemistry involved in the production and destruction of various oxidants that contribute to the oxidative capacity of the troposphere. The primary sink for NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx-to-NO3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean, particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15N, δ18O, and Δ17O) of atmospheric nitrate in coarse-mode (> 1 µm) aerosols, collected between South Africa and the sea ice edge in summer, winter, and spring. Similar latitudinal trends in δ15N–NO3- were observed in summer and spring, suggesting similar NOx sources. Based on δ15N–NO3-, the main NOx sources were likely a combination of lightning, biomass burning, and/or soil emissions at the low latitudes, as well as oceanic alkyl nitrates and snowpack emissions from continental Antarctica or the sea ice at the mid-latitudes and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack and snow on sea ice. A combination of natural NOx sources, likely transported from the lower-latitude Atlantic, contribute to the background-level NO3- observed in winter, with the potential for a stratospheric NO3- source evidenced by one sample of Antarctic origin. Greater values of δ18O–NO3- in spring and winter compared to summer suggest an increased influence of oxidation pathways that incorporate oxygen atoms from O3 into the end product NO3- (i.e. N2O5, DMS, and halogen oxides (XO)). Significant linear relationships between δ18O and Δ17O suggest isotopic mixing between H2O(v) and O3 in winter and isotopic mixing between H2O(v) and O3/XO in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer. As a result, isotopic mixing with an additional third end-member (atmospheric O2) occurs in spring.

Observational Investigation of the Effect of Turbulence on Microphysics and Precipitation in Warm Marine Boundary Layer Clouds

AbstractTurbulence is omnipresent in the marine boundary layer (MBL) and several mechanisms have been proposed regarding its role on MBL cloud microphysics. In particular, past modeling studies suggested that increasing turbulence broaden the droplet size distribution (DSD) and leads to precipitation initiation, this suggested relationship has not been evaluated yet using long‐term observations. Here, comprehensive retrievals from long‐term ground‐based observations are used to explore the impact of turbulence on microphysics and precipitation in drizzling MBL clouds. Our analysis supports that broader DSD and larger droplets are observed with strong turbulence, but interestingly, the precipitation is reduced instead of being enhanced. This suppression mechanism is mainly due to the entrainment and mixing process in which the entrained dry air evaporates cloud water. This study provides observational evidence to show the complex effect of turbulence on MBL cloud properties and implies the potential influence of turbulence on the clouds lifetime.

Cover Feature: Theoretical Study of the Hydroxyl‐Radical‐Initiated Degradation Mechanism, Kinetics, and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere (ChemPhysChem 10/2023)

The Cover Feature shows that hydroxyl-radical-initiated degradation and transformation of methyl and ethyl iodides are crucial in the iodine chemical cycle in the marine boundary layer. Their atmospheric degradation would produce atomic iodine, which leads to ozone layer depletion. Cover design by Feng-Yang Bai, Jia-Ming Sun and Co-workers. More information can be found in the Research Article by Feng-Yang Bai, Shuang Ni, Zhen Zhao and co-workers.

Aerosol first indirect effect of African smoke at the cloud base of marine cumulus clouds over Ascension Island, southern Atlantic Ocean

Abstract. The interactions between aerosols and clouds are among the least understood climatic processes and were studied over Ascension Island. A ground-based UV polarization lidar was deployed on Ascension Island, which is located in the stratocumulus-to-cumulus transition zone of the southeastern Atlantic Ocean, to infer cloud droplet sizes and droplet number density near the cloud base of marine boundary layer cumulus clouds. The aerosol–cloud interaction (ACI) due to the presence of smoke from the African continent was determined during the monsoonal dry season. In September 2016, a cloud droplet number density ACIN of 0.3 ± 0.21 and a cloud effective radius ACIr of 0.18 ± 0.06 were found, due to the presence of smoke in and under the clouds. Smaller droplets near the cloud base makes them more susceptible to evaporation, and smoke in the marine boundary layer over the southeastern Atlantic Ocean will likely accelerate the stratocumulus-to-cumulus transition. The lidar retrievals were tested against more traditional radar–radiometer measurements and shown to be robust and at least as accurate as the lidar–radiometer measurements. The lidar estimates of the cloud effective radius are consistent with previous studies of cloud base droplet sizes. The lidar has the large advantage of retrieving both cloud and aerosol properties using a single instrument.

The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities.

Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1-100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Potential Implications of the Sesquiterpene Presence over the Remote Marine Boundary Layer in the Arctic Region

We present reactive VOC observations over the North Pacific and the Arctic Ocean from airborne and shipborne measurements, investigating, in particular, distributions of biogenic volatile organic compounds that may be emitted from phytoplankton. In contrast to terrestrial observations, isoprene (C5H8), the most dominant BVOC emission from the terrestrial ecosystem, was mostly present under the lower detection limit along with monoterpenes (C10H16), the second largest emission from the terrestrial ecosystem. However, we consistently detected sesquiterpenes (C15H24) over the Arctic Ocean for the two Arctic cruises. The results of the analysis of sorbent cartridge samplings conducted over the Arctic Ocean on Korean icebreaker R/V Araon in 2016 and 2017 illustrate that few tens ppt levels of sesquiterpenes were present over the Arctic Ocean. Moreover, the concentration variation was positively correlated with the quantitative indicators of ocean biological activities, such as chlorophyll-a, dissolved DMS, and the ratio of dissolved O2 and Ar. This suggests that further investigations on sesquiterpene’s emission and atmospheric transformation processes over the marine boundary layer are required.

Volatile organic compounds measured by proton transfer reaction mass spectrometry over the complex terrain of Quintero Bay, Central Chile

This research provides new evidence regarding the different kinds of air quality episodes, and their underlying mechanisms, that frequently impact the urban area of Quintero Bay in Central Chile, which is located along complex coastal terrain and is surrounded by industries. The monitoring campaign was carried out in January 2022 and encompassed two distinctive meteorological regimes. The first part of the month was dominated by a coastal low centered to the south of Quintero, which resulted in prevailing northerly flow (or weak southerlies) and a deep cloud-topped marine boundary layer. After a 2–3-day transition, the latter collapsed, and a clear-sky regime ensued, which was characterized by a shallow boundary layer and strong southerly winds during the daytime that lasted until the end of the campaign.By using proton transfer reaction time of flight mass spectrometry (PTR–TOF–MS) at a high temporal resolution (1 s), we measured high levels of volatile organic compounds (VOCs) during air quality episodes in real time. The episodes detected were associated with different prevailing meteorological regimes, suggesting that different point sources were involved. In the first episode, propene/cyclopropane, butenes, benzene, toluene and ethylbenzene/xylenes were associated with north and northwesterly weak winds. Complaints associated with hydrocarbon odor were reported. The pollution originated from industrial and petrochemical units located to the north of Quintero, which transport and store natural gas, liquified petroleum gas and oil. The second episode was linked to an oil refinery located south of our measurement site. In this case, high levels of phenol, furan and cresols occurred under strong southwesterly winds. During this event, headaches and dizziness were reported. By contrast, the levels of other aromatic compounds (benzene, toluene, ethylbenzene/xylenes) were lower than in the first air pollution episode.

Vertical Structure and Ice Production Processes of Shallow Convective Postfrontal Clouds over the Southern Ocean in MARCUS. Part I: Observational Study

Abstract A study of the vertical structure of postfrontal shallow clouds in the marine boundary layer over the Southern Ocean is presented. The central question of this two-part study regards cloud phase (liquid/ice) of precipitation, and the associated growth mechanisms. In this first part, data from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign are analyzed, starting with a 75-h case with continuous sea surface-based thermal instability, modest surface heat fluxes, an open-cellular mesoscale organization, and very few ice nucleating particles (INPs). The clouds are mostly precipitating and shallow (tops mostly around 2 km above sea level), with weak up- and downdrafts, and with cloud-top temperatures generally around −18° to −10°C. The case study is extended to three other periods of postfrontal shallow clouds in MARCUS. While abundant supercooled liquid water is commonly present, an experimental cloud-phase algorithm classifies nearly two-thirds of clouds in the 0° to −5°C layer as containing ice (cloud ice, snow, or mixed phase), implying that much of the precipitation grows through cold-cloud processes. The best predictors of ice presence are cloud-top temperature, cloud depth, and INP concentration. Measures of convective activity and turbulence are found to be poor indicators of ice presence in the studied environment. The water-phase distribution in this cloud regime is explored through numerical simulations in Part II. Significance Statement Climate models generally predict a lower albedo than observed over the Southern Ocean, and this is largely attributed to a lack of cloudiness, especially in the postfrontal cold sector of midlatitude cyclones. This in turn may be due to an excess of ice in these simulated clouds, resulting in rapid precipitation fallout and an overly brief cloud lifespan. The objective of this study is to examine whether shallow postfrontal clouds over the Southern Ocean are dominated by supercooled drops, or by snow and ice, using data collected by a U.S. Department of Energy Atmospheric Radiation Measurement Mobile Facility deployed aboard an Australian Antarctic supply vessel. We find that these clouds contain much supercooled liquid, even though cloud-top temperatures generally are around −18° to −8°C, and that about two-thirds of the clouds just above the freezing level contain ice. Much of the precipitation appears to grow through cold-cloud processes above the freezing level, rather than drizzle/rain. Updrafts and/or turbulence in convection or in cloud-top generating cells do not initiate much ice, compared to observations elsewhere in a similar temperature range. This may be attributable to the extremely low concentration of ice nucleating particles in this environment. Ultimately, the deepest clouds with the coldest cloud tops are most likely to be ice dominated.

Measurement report: Summertime fluorescence characteristics of atmospheric water-soluble organic carbon in the marine boundary layer of the western Arctic Ocean

Abstract. Accelerated warming and a decline in sea ice coverage in the summertime Arctic Ocean can significantly affect the emissions of marine organic aerosols and biogenic volatile organic compounds. However, how these changes affect the characteristics of atmospheric water-soluble organic carbon (WSOC), which plays an important role in the climate system, remains unclear. Thus, to improve our understanding of WSOC characteristics in the rapidly changing Arctic Ocean, including its summertime fluorescence characteristics, we simultaneously measured atmospheric concentrations of ionic species and WSOC, a fluorescence excitation–emission matrix coupled with parallel factor (EEM-PARAFAC) analysis of WSOC, and marine biological parameters in surface seawaters of the western Arctic Ocean during the summer of 2016. WSOC was predominantly present as fine-mode aerosols (diameter <2.5 µm, median =92 %), with the mean concentration being higher in the coastal water areas (462±130 ngC m−3) than in the sea-ice-covered areas (242±88.4 ngC m−3). Moreover, the WSOC in the fine-mode aerosols was positively correlated with the methanesulfonic acid in the fine-mode aerosol samples collected over the sea-ice-covered areas (r=0.88, p<0.01, n=10), suggesting high rates of sea–air gas exchange and emissions of aerosol precursor gases due to sea ice retreat and increasingly available solar radiation during the Arctic summer. Two fluorescent components, humic-like C1 and protein-like C2, were identified by the PARAFAC modeling of fine-mode atmospheric WSOC. The two components varied regionally between coastal and sea-ice-covered areas, with low and high fluorescence intensities observed over the coastal areas and the sea-ice-covered areas, respectively. Further, the humification index of WSOC was correlated with the fluorescence intensity ratio of the humic-like C1 / protein-like C2 (r=0.89, p<0.01) and the WSOC concentration in the fine-mode aerosols (r=0.66, p<0.05), with the highest values observed in the coastal areas. Additionally, the WSOC concentration in the fine-mode aerosols was positively correlated with the fluorescence intensity ratio of the humic-like C1 / protein-like C2 (r = 0.77, p<0.01) but was negatively correlated with the biological index (r=-0.69, p<0.01). Overall, these results suggested that the WSOC in the fine-mode aerosols in the coastal areas showed a higher degree of polycondensation and higher aromaticity compared to that in the sea-ice-covered areas, where WSOC in the fine-mode aerosols was associated with relatively new, less oxygenated, and biologically derived secondary organic components. These findings can improve our understanding of the chemical and biological linkages of WSOC at the ocean–sea-ice–atmosphere interface.

On Saharan Air Layer Stability and Suppression of Convection over the Northern Tropical Atlantic: Case Study Analysis of a 2007 Dust Outflow Event

A prominent Saharan Air Layer (SAL) was detected over the Northern Atlantic from the West African Coast to the Caribbean Sea in 2007. Data was collected from the Aerosols and Ocean Science Expedition (AEROSE), which encountered a major dust outflow on 13 and 14 May 2007. These observational measurements came from onboard instrumentation and radiosondes that captured the dust-front event from 13 to 14 May 2007. Aerosol backscatter was confined within the Marine Boundary Layer (MBL), with layers detected up to 3 km. Aerosol Optical Depth (AOD) increased by one order of magnitude during the dust front, from 0.1 to 1. Downward solar radiation was also attenuated by 200 W/m2 and 100 W/m2 on the first and second days, respectively. A weaker gradient at and above 500 m from potential temperature profiles indicates a less-defined MBL, and an ambient air temperature of 26 °C on 14 May and 28 °C on 15 May were observed above 500 m, reinforcing the temperature inversion and static stability of the SAL. Subsequent days, clear and boundary-layer cloudy days were observed after the dust front. From 14 to 18 May, a Convective Inhibition (CIN) layer started to form at the top of the MBL, developing into a negative buoyancy from 17 to 23 May, and reinforcing the large-scale anticyclonic atmospheric conditions. These results show that the SAL acts as positive feedback on suppressing deep convection over the tropical Atlantic during this dust outflow and several days after its passage.