ALBERT

Description: We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many northern hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long time series of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.

Description: The VAMOS1 Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) was an international field program designed to make observations of poorly understood but critical components of the coupled climate system of the southeast Pacific. This region is characterized by strong coastal upwelling, the coolest SSTs in the tropical belt, and is home to the largest subtropical stratocumulus deck on Earth. The field intensive phase of VOCALS-REx took place during October and November 2008 and constitutes a critical part of a broader CLIVAR program (VOCALS) designed to develop and promote scientific activities leading to improved understanding, model simulations, and predictions of the southeastern Pacific (SEP) coupled ocean-atmosphere-land system, on diurnal to interannual timescales. The other major components of VOCALS are a modeling program with a model hierarchy ranging from the local to global scales, and a suite of extended observations from regular research cruises, instrumented moorings, and satellites. The two central themes of VOCALS-REx focus upon (a) links between aerosols, clouds and precipitation and their impacts on marine stratocumulus radiative properties, and (b) physical and chemical couplings between the upper ocean and the lower atmosphere, including the role that mesoscale ocean eddies play. A set of hypotheses designed to be tested with the combined field, monitoring and modeling work in VOCALS is presented here. A further goal of VOCALS-REx is to provide datasets for the evaluation and improvement of large-scale numerical models. VOCALS-REx involved five research aircraft, two ships and two surface sites in northern Chile. We describe the instrument payloads and key mission strategies for these platforms and give a summary of the missions conducted. 1 Variability of the American Monsoon Systems, an international CLIVAR program.

Description: Oceanic observations collected during the VOCALS-REx cruise time period, 1–30 November 2008, are assimilated into a regional ocean model (ROMS) using 4DVAR and then analyzed for their dynamics. Nonlinearities in the system prevent a complete 30-day fit, so two 15-day fits for 1–15 November and 16–30 November are executed using the available observations of hydrographic temperature and salinity, along with satellite fields of SST and sea-level height anomaly. The fits converge and reduce the cost function significantly, and the results indicated that ROMS is able to successfully reproduce both large-scale and smaller-scale features of the flows observed during the VOCALS-REx cruise. Particular attention is focused on an intensively studied eddy at 76° W, 19° S. The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signature in velocity, temperature and anomalously low salinity. The eddy has an average temperature anomaly of approximately −0.5 °C over a depth range from 50–600 m and features a cold anomaly of approximately −1 °C near 150 m depth. The eddy moves northwestward and elongates during the second 15-day fit. It exhibits a strong signature in the Okubo-Weiss parameter, which indicates significant nonlinearity in its evolution. The heat balance for the period of the cruise from the ocean state estimate reveals that the horizontal advection and the vertical mixing processes are the dominant terms that balance the temperature tendency of the upper layer of the ocean locally in time and space. Areal averages around the eddies, for a 15-day period during the cruise, suggest that vertical mixing processes generally balance the surface heating. Although, this indicates only a small role for lateral advective processes in this region during this period, this quasi-instantaneous heat budget analysis cannot be extended to interpret the seasonal or long-term upper ocean heat budget in this region.

Description: Continuous black carbon (BC) observations were conducted from 1999 through 2009 by an Aethalometer (AE10) and from 2006 through 2011 by a Multi-Angle Absorption Photometer (MAAP) at Neumayer Station (NM) under stringent contamination control. Considering the respective observation period, BC concentrations measured by the MAAP were somewhat higher (median ± standard deviation: 2.1 ± 2.0 ng m−3) compared to the AE10 results (1.6 ± 2.1 ng m−3). Neither for the AE10 nor for the MAAP data set a significant long-term trend could be detected. Consistently a pronounced seasonality was observed with both instruments showing a primary annual maximum between October and November and a minimum in April with a maximum/minimum ratio of 4.5/1.6 = 3.8 and 2.7/0.64 = 4.2 for the MAAP and AE10 data, respectively. Occasionally a secondary summer maximum in January/February was visible. With the aim to assess the impact of BC on optical properties of the aerosol at NM, we evaluated the BC data along with particle scattering coefficients measured by an integrating nephelometer. We found the mean single scattering albedo of ω550 = 0.992 ± 0.0090 (median: 0.994) at a wavelength of 550 nm with a range of values from 0.95 to 1.0.

Description: To probe the spatial extent of tropospheric ozone depletion events during Antarctic spring, a network of 10 autonomous ozone monitors was established around the Dronning Maud Land sector of Antarctica for a full calendar year. Together with manned stations in the area, the network covered a ~1200 km stretch of coast, as well as a transect ~300 km inland and to ~2000 m above sea level (a.s.l.). Here we present results from the spring period (August to October 2008). While some ozone depletion events were evident at only a single site, implying localised ozone destruction, others were evident across the network. The fact that, on occasions, ozone depletion events were observed at all coastal sites simultaneously, suggests the depleted air mass had a scale of at least 1200 km. As the ozone-poor air was advected from the Weddell Sea sea ice zone, the data imply that large areas over the Weddell Sea sea ice zone are significantly depleted in ozone on occasions during Antarctic spring.

Description: Emissions of sulphur hexafluoride (SF6), one of the strongest greenhouse gases on a per molecule basis, are targeted to be collectively reduced under the Kyoto Protocol. Because of its long atmospheric lifetime (estimated as 800 to 3200 years), the accumulation of SF6 in the atmosphere is a direct measure of its global emissions. Examination of our extended data set of globally distributed high-precision SF6 observations shows an increase in SF6 abundance from near zero in the 1970s to a global mean of 6.7 ppt by the end of 2008. In-depth evaluation of our long-term data records shows that the global source of SF6 decreased after 1995, most likely due to SF6 emission reductions in industrialised countries, but increased again after 1998. By subtracting those emissions reported by Annex I countries to the United Nations Framework Convention of Climatic Change (UNFCCC) from our observation-inferred SF6 source leaves a surprisingly large gap of more than 70–80% of non-reported SF6 emissions in the last decade. This suggests a strong under-estimation of emissions in Annex I countries and underlines the urgent need for independent atmospheric verification of greenhouse gases emissions accounting.

Description: Currently many ground-based atmospheric stations include in-situ measurements of aerosol physical and optical properties, resulting in more than 20 long-term (〉 10 yr) aerosol measurement sites in the Northern Hemisphere and Antarctica. Most of these sites are located at remote locations and monitor the aerosol particle number concentration, wavelength-dependent light scattering, backscattering, and absorption coefficients. The existence of these multi-year datasets enables the analysis of long-term trends of these aerosol parameters, and of the derived light scattering Ångström exponent and backscatter fraction. Since the aerosol variables are not normally distributed, three different methods (the seasonal Mann-Kendall test associated with the Sen's slope, the generalized least squares fit associated with an autoregressive bootstrap algorithm for confidence intervals, and the least-mean square fit applied to logarithms of the data) were applied to detect the long-term trends and their magnitudes. To allow a comparison among measurement sites, trends on the most recent 10 and 15 yr periods were calculated. No significant trends were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficients (mean slope of −2.0% yr−1) were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. The difference in the timing of emission reduction policy for the Europe and US continents is a likely explanation for the decreasing trends in aerosol optical parameters found for most American sites compared to the lack of trends observed in Europe. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient. The high altitude Pacific island station of Mauna Loa presents positive trends for both scattering and absorption coefficients.

Description: To probe the spatial extent of tropospheric ozone depletion events during Antarctic spring, a network of 10 autonomous ozone monitors was established around the Dronning Maud Land sector of Antarctica for a full calendar year. Together with manned stations in the area, the network covered a ~1200 km stretch of coast, as well as a transect ~300 km inland and to ~2000 m above sea level (a.s.l.). Here we present results from the spring period (August to October 2008). While some ozone depletion events were evident at only a single site, implying localised ozone destruction, others were evident across the network. The fact that, on occasions, ozone depletion events were observed at all coastal sites simultaneously, suggests the depleted air mass had a scale of at least 1200 km. As the ozone-poor air was advected from the Weddell Sea sea ice zone, the data imply that large areas over the Weddell Sea sea ice zone are significantly depleted in ozone on occasions during Antarctic spring.

Description: One of the major challenges to assessing the impact of ocean acidification on marine life is the need to better understand the magnitude of long-term change in the context of natural variability. This study addresses this need through a global synthesis of monthly pH and aragonite saturation state (Ωarag) climatologies for 12 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater partial pressure of CO2 and pH collected together since as early as 2010. Mooring observations suggest open ocean subtropical and subarctic sites experience present-day surface pH and Ωarag conditions outside the bounds of pre-industrial variability throughout most, if not all, of the year. In general, coastal mooring sites experience more natural variability and thus, more overlap with pre-industrial conditions; however, present day Ωarag conditions surpass biologically relevant thresholds associated with ocean acidification impacts on Mytilus californianus (Ωarag 〈 1.8) and Crassostrea gigas (Ωarag 〈 2.0) larvae in the California Current Ecosystem (CCE) and Mya arenaria larvae in the Gulf of Maine (Ωarag 〈 1.6). At the most variable mooring locations in coastal systems of the CCE, subseasonal conditions approached Ωarag = 1. Global and regional models and data syntheses of ship-based observations tended to underestimate seasonal variability compared to mooring observations. Efforts such as this to characterize all modes of pH and Ωarag variability and change at key locations are fundamental to assessing present-day biological impacts of ocean acidification, further improving experimental design to interrogate organism response under real-world conditions, and improving predictive models and vulnerability assessments seeking to quantify the broader impacts of ocean acidification.

Description: Though the environmental conditions of the Weddell Sea region and Dronning Maud Land are still relatively stable compared to the fast-changing Antarctic Peninsula, we may suspect pronounced effects of global climate change for the near future (Thompson et al., 2011). Reducing the uncertainties in climate change modeling requires a better understanding of the aerosol optical properties, and for this we need accurate data on the aerosol refractive index (RI). Due to the remoteness of Antarctica only very few RI data are available from this region (Hogan et al., 1979; Virkkula et al., 2006; Shepherd et al., 2018). We calculate the real refractive index of natural atmospheric aerosols from number size distribution measurements at the German coastal Antarctic station Neumayer III. Given the high average scattering albedo of 0.992 (Weller et al., 2013), we assumed that the imaginary part of the RI is zero. Our method uses the overlapping size range (particle diameter D between 120 and 340 nm) of a scanning mobility particle sizer (SMPS), which sizes the particles by their electrical mobility, and a laser aerosol spectrometer (LAS), which sizes the particles by their optical scattering signal at the 633 nm wavelength. Based on almost a complete year of measurement, the average effective refractive index (RIeff, as we call our retrieved RI because of the used assumptions) for the dry aerosol particles turned out to be 1.44 with a standard deviation of 0.08, in a good agreement with the RI value of 1.47, which we derived from the chemical composition of bulk aerosol sampling measurements. At Neumayer the aerosol shows a pronounced seasonal pattern in both number concentration and chemical composition. Despite this, the variability of the monthly averaged RIeff values remained between 1.40 and 1.50. Compared to the annual mean, two austral winter months (July and September) showed slightly but significantly increased values (1.50 and 1.47, respectively). The size dependency of the RIeff could be determined from time-averaged LAS and SMPS number size distributions measured between December 2017 and January 2018. Here we calculated RIeff for four different particle size ranges and observed a slight decrease from 1.47 (D range 116–168 nm) to 1.37 (D range 346–478 nm). We find no significant dependence of the derived RIeff values on the wind direction. Thus we conclude that RIeff is largely independent of the general weather situation, roughly classified as (i) advection of marine boundary layer air masses during easterly winds caused by passing cyclones in contrast to (ii) air mass transport from continental Antarctica under southern katabatic winds. Neumayer, the only relevant contamination source, is located 1.5 km north of the air chemistry observatory, where the measurements were performed. Given that northerly winds are almost absent, the potential impact of local contamination is minimized in general. Indeed our data show no impact of local contamination on RIeff. Just in one case a temporary high-contamination episode with diesel engines operating right next to the measurement site resulted in an unusual high RIeff of 1.59, probably caused by the high black carbon content of the exhaust fumes. To conclude, our study revealed largely constant RIeff values throughout the year without any sign of seasonality. Therefore, it seems reasonable to use a single, constant RIeff value of 1.44 for modeling optical properties of natural, coastal Antarctic sub-micrometer aerosol.