Abstract: During the mid-1980s, there were 31 stations in China that successfully participated in the Global Network of Isotopes in Precipitation. However, most observations were suspended after the mid-1990s. The discontinuous data hindered the application of precipitation isotopes, which are strongly affected in China by the Asian monsoon and are thus of intrinsic interest for palaeoclimatologists. Therefore, to continuously observe precipitation isotopes nationwide, the Chinese Network of Isotopes in Precipitation was established in 2004. The current study reviewed the major characteristics of the 928 samples that were collected from 2005 to 2010. The ranges of δD and δ 18 O values generally followed the pattern NE>NW>TP>NC>SC, and the amount-weighted δ-values followed the pattern SC>NW>NC>TP>NE. Temporal variations presented a ‘V’-shaped pattern at the SC region and reverse ‘V’-shaped patterns at the NE and NW regions. Decreasing trends with the advent of the rainy season were found at the NC and TP regions. The Chinese Meteoric Water Line has been established as δD=7.48δ 18 O+1.01. The distributions of scattering along the line demonstrated different water vapour origins and characteristics. The values of δ 18 O showed strong temperature dependence at the NE (0.27‰/°C) and NW stations (0.37‰/°C), and this dependent variable switched to water vapour pressure and vapour pressure at the SC stations. The geographical controls of δ 18 O were −0.22‰/° and −0.13‰/100 m for latitude and altitude, respectively. The δ 18 O/ Latitude gradient increased from south to north at the Eastern Monsoon Region, and the δ 18 O/ Altitude gradient (−0.30‰/100 m) was especially significant for the TP region. The results of this study could provide basic isotopic information for on-going investigations in hydrology, meteorology, palaeoclimatology and ecology at different regions of China. Keywords: δD, δ 18 O, CHNIP, GNIP, environmental controls, multiple non-linear stepwise regressions (Published: 17 March 2014) Citation: Tellus B 2014, 66 , 22567, http://dx.doi.org/10.3402/tellusb.v66.22567

Abstract: Lakes and other inland waters contribute significantly to regional and global carbon budgets. Emissions from lakes are often computed as the product of a gas transfer coefficient, k 600 , and the difference in concentration across the diffusive boundary layer at the air–water interface. Eddy covariance (EC) techniques are increasingly being used in lacustrine gas flux studies and tend to report higher values for derived k 600 than other approaches. Using results from an EC study of a small, boreal lake, we modelled k 600 using a boundary-layer approach that included wind shear and cooling. During stratification, fluxes estimated by EC occasionally were higher than those obtained by our models. The high fluxes co-occurred with winds strong enough to induce deflections of the thermocline. We attribute the higher measured fluxes to upwelling-induced spatial variability in surface concentrations of CO 2 within the EC footprint. We modelled the increased gas concentrations due to the upwelling and corrected our k 600 values using these higher CO 2 concentrations. This approach led to greater congruence between measured and modelled k values during the stratified period. k 600 has a well-resolved and ~cubic relationship with wind speed when the water column is unstratified and the dissolved gases well mixed. During stratification and using the corrected k 600 , the same pattern is evident at higher winds, but k 600 has a median value of ~7 cm h −1 when winds are less than 6 m s −1 , similar to observations in recent oceanographic studies. Our models for k 600 provide estimates of gas evasion at least 200% higher than earlier wind-based models. Our improved k 600 estimates emphasize the need for integrating within lake physics into models of greenhouse gas evasion. Keywords: carbon dioxide, gas transfer, upwelling, eddy covariance, heat flux, thermocline tilting (Published: 19 May 2014) Citation: Tellus B 2014, 66 , 22827, http://dx.doi.org/10.3402/tellusb.v66.22827

Abstract: The influence of aerosol water uptake on the aerosol particle light scattering was examined at the regional continental research site Melpitz, Germany. The scattering enhancement factor f (RH), defined as the aerosol particle scattering coefficient at a certain relative humidity (RH) divided by its dry value, was measured using a humidified nephelometer. The chemical composition and other microphysical properties were measured in parallel. f (RH) showed a strong variation, e.g. with values between 1.2 and 3.6 at RH=85% and λ=550 nm. The chemical composition was found to be the main factor determining the magnitude of f (RH), since the magnitude of f (RH) clearly correlated with the inorganic mass fraction measured by an aerosol mass spectrometer (AMS). Hysteresis within the recorded humidograms was observed and explained by long-range transported sea salt. A closure study using Mie theory showed the consistency of the measured parameters. Keywords: aerosol particle light scattering, hygroscopic growth, scattering enhancement, aerosol mass spectrometer, field measurements, optical closure study (Published: 14 March 2014) Citation: Tellus B 2014, 66 , 22716, http://dx.doi.org/10.3402/tellusb.v66.22716

Abstract: Biomass burning produces aerosols and air pollutants during springtime in Southeast Asia. At the Lulin Atmospheric Background Station (LABS) (elevation 2862 m) in central Taiwan, the concentrations of carbon monoxide (CO), ozone (O 3 ) and particulate matter with a diameter less than 10 µm (PM 10 ) were found to be 135–200 ppb, 40–56 ppb and 13–26 µg/m 3 , respectively, in the springtime (February–April) between 2006 and 2009, which are 2–3 times higher than those in other seasons. Simulation results indicate that higher concentrations during springtime are related to biomass-burning plumes transported from the Indochinese peninsula of Southeast Asia. The spatial distribution of high aerosol optical depth (AOD) was identified by satellite measurement and Aerosol Robotic Network (AERONET) ground observation, and could be reasonably captured by the WRF-Chem model during the study period of 15–18 March 2008. Simulated AOD reached as high as 0.8–1.2 in Indochina situated between 10–22°N and 95–107°E. According to the simulation results, 34% of the AOD was attributed to organic carbon over Indochina, while the contribution of black carbon to AOD was about 4%. During the study period, biomass-burning aerosols over Indochina have a net negative effect (−26.85 W·m −2 ) at ground surface, a positive effect (22.11 W·m −2 ) in the atmosphere and a negative forcing (−4.74 W·m −2 ) at the top of atmosphere. Under the influence of biomass-burning aerosol plume transported by strong wind, there is a NE−SW zone stretching from southern China to Taiwan with reduction in shortwave radiation of about 20 W·m −2 at ground surface. Such significant reduction in radiation attributed to biomass-burning aerosols and their impact on the regional climate in East Asia merit attention. Keywords: biomass burning, aerosol optical depth, radiative forcing, modelling Responsible Editor: Kaarle Hameri, University of Helsinki, Finland. Citation: Tellus B 2014, 66 , 23733, http://dx.doi.org/10.3402/tellusb.v66.23733

Abstract: We estimate regional long-term surface ocean pCO 2 growth rates using all available underway and bottled biogeochemistry data collected over the past four decades. These observed regional trends are compared with those simulated by five state-of-the-art Earth system models over the historical period. Oceanic pCO 2 growth rates faster than the atmospheric growth rates indicate decreasing atmospheric CO 2 uptake, while ocean pCO 2 growth rates slower than the atmospheric growth rates indicate increasing atmospheric CO 2 uptake. Aside from the western subpolar North Pacific and the subtropical North Atlantic, our analysis indicates that the current observation-based basin-scale trends may be underestimated, indicating that more observations are needed to determine the trends in these regions. Encouragingly, good agreement between the simulated and observed pCO 2 trends is found when the simulated fields are subsampled with the observational coverage. In agreement with observations, we see that the simulated pCO 2 trends are primarily associated with the increase in surface dissolved inorganic carbon (DIC) associated with atmospheric carbon uptake, and in part by warming of the sea surface. Under the RCP8.5 future scenario, DIC continues to be the dominant driver of pCO 2 trends, with little change in the relative contribution of SST. However, the changes in the hydrological cycle play an increasingly important role. For the contemporary (1970–2011) period, the simulated regional pCO 2 trends are lower than the atmospheric growth rate over 90% of the ocean. However, by year 2100 more than 40% of the surface ocean area has a higher oceanic pCO 2 trend than the atmosphere, implying a reduction in the atmospheric CO 2 uptake rate. The fastest pCO 2 growth rates are projected for the subpolar North Atlantic, while the high-latitude Southern Ocean and eastern equatorial Pacific have the weakest growth rates, remaining below the atmospheric pCO 2 growth rate. Our work also highlights the importance and need for a sustained long-term observing strategy to continue monitoring the change in the ocean anthropogenic CO 2 sink and to better understand the potential carbon cycle feedbacks to climate that could arise from it. Keywords: surface pCO 2 , ocean CO 2 sinks, Earth system models, CMIP5 projections, ocean biogeochemistry (Published: 19 May 2014) Citation: Tellus B 2014, 66 , 23083, http://dx.doi.org/10.3402/tellusb.v66.23083 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Abstract: Anhydrosugars including levoglucosan and mannosan are the most effective organic tracers for biomass burning aerosol in the atmosphere. In this study, to investigate the contribution of biomass burning emissions to the aerosol burden in the Pearl River Delta (PRD) region, China, 24-hour integrated PM 2.5 samples were collected simultaneously at four locations, (i) Guangzhou (GZ), (ii) Zhaoqing (ZQ) in Guangdong province, (iii) Hok Tsui (HT) and (iv) Hong Kong Polytechnic University (PU) in Hong Kong, in four seasons between 2006 and 2007. Levoglucosan and mannosan, together with water-soluble inorganic ions and water-soluble organic carbon (WSOC), were determined to elucidate the seasonal and spatial variations in biomass burning contributions. The concentrations of levoglucosan and mannosan were on average 82.4±123 and 5.8±8.6ng m −3 , respectively. The WSOC concentrations ranged from 0.2 to 9.4µg m −3 , with an average of 2.1±1.6µg m −3 . The relative contributions of biomass burning emissions to OC were 33% in QZ, 12% in GZ, 4% at PU and 5% at HT, respectively, estimated by the measured levoglucosan to organic carbon ratio (LG/OC) relative to literature-derived LG/OC values. The contributions from biomass burning emissions were in general 1.7–2.8 times higher in winter than those in other seasons. Further, it was inferred from diagnostic tracer ratios that a significant fraction of biomass burning emissions was derived from burning of hard wood and likely also from field burning of agricultural residues, such as rice straw, in the PRD region. Our results highlight the contributions from biomass/biofuel burning activities on the regional aerosol budget in South China. Keywords : fine aerosol, biomass burning, WSOC, levoglucosan, Pearl River Delta (Published: 6 June 2014) Citation : Tellus B 2014, 66 , 22577, http://dx.doi.org/10.3402/tellusb.v66.22577

Abstract: Wind-blown dust derived from the arid and semi-arid regions is an important atmospheric component affecting the Earth’s radiation budget. Dust storms are prevailing in central Asia. Deposition of atmospheric dust in snow was measured on Glacier No.12 (5040 ;m a.s.l.) in the Laohugou Basin and Shiyi Glacier (4510 ;m a.s.l.) in the Yeniugou Basin, of western Qilian Mountains, China, mainly focusing on dust concentration and size distribution, chemistry, SEM–EDX analysis and Nd–Sr isotopic composition. An analysis of spatial distribution of dust concentration and size distribution in the snow cover at two sites suggests that deposition of dust in western Qilian Mountains is different between sites as the environment changes from west to east. Mean mass concentration of dust with 0.57 < d <40 ;μm in the snow is 3461 ;μg kg −1 on Glacier No.12 and 2876 μg kg −1 on Shiyi Glacier, respectively. Annual flux of dust deposition to western Qilian Mountains has a range of 143.8–207.6 ;μg cm −2 yr −1 for particles with 0.57 < d <40 ;μm. Dust layers in the snowpit contain Ca- and Na-rich materials typically found in Asian dust particles. Number–size distribution indicated that most of the dust diameter is <2.0 ;μm, implying the significant influences of finer particles to alpine glacier regions of central Asia. Volume size distributions of dust particles showed single-modal structures having volume median diameters from 3 to 22 ;μm, which is comparable to the results of dust deposition on glaciers of the adjacent Tianshan Mountains in western China. SEM–EDX analysis suggested that dust particles were mainly composed of mineral particles, besides some fly ash particles and soot. EDX shows that mineral particles contain Si-, Al-, Ca-, K-, and Fe-rich materials, such as quartz, albite, aluminate, etc. The Nd–Sr isotopic composition of insoluble particles in two glaciers showed that Badain Jaran Desert and Qaidam Basin were the most possible source regions of dust in the western Qilian Mountains. Physicochemical constituents of dust indicated that the snowpack was influenced by both local atmospheric environment, e.g. local dust, and anthropogenic activities in central Asia. Keywords: atmospheric dust, physicochemical characteristics, SEM–EDX, dust source, glacier melting, western Qilian Mountains (Published: 21 January 2013) Citation: Tellus B 2014, 66 , 20956, http://dx.doi.org/10.3402/tellusb.v66.20956

Abstract: This investigation focuses on the characterisation of the aerosol particle hygroscopicity. Aerosol particle optical properties were measured at Granada, Spain, during winter and spring seasons in 2013. Measured optical properties included particle light-absorption coefficient (σ ap ) and particle light-scattering coefficient (σ sp ) at dry conditions and at relative humidity (RH) of 85±10%. The scattering enhancement factor, f(RH=85%), had a mean value of 1.5±0.2 and 1.6±0.3 for winter and spring campaigns, respectively. Cases of high scattering enhancement were more frequent during the spring campaign with 27% of the f(RH=85%) values above 1.8, while during the winter campaign only 8% of the data were above 1.8. A Saharan dust event (SDE), which occurred during the spring campaign, was characterised by a predominance of large particles with low hygroscopicity. For the day when the SDE was more intense, a mean daily value of f(RH=85%)=1.3±0.2 was calculated. f(RH=85%) diurnal cycle showed two minima during the morning and afternoon traffic rush hours due to the increase in non-hygroscopic particles such as black carbon and road dust. This was confirmed by small values of the single-scattering albedo and the scattering Angstrom exponent. A significant correlation between f(RH=85%) and the fraction of particulate organic matter and sulphate was obtained. Finally, the impact of ambient RH in the aerosol radiative forcing was found to be very small due to the low ambient RH. For high RH values, the hygroscopic effect should be taken into account since the aerosol forcing efficiency changed from −13 W/m 2 at dry conditions to −17 W/m 2 at RH=85%. Keywords: Aerosol particle light scattering, hygroscopic growth, scattering enhancement, radiative forcing, field measurements (Published: 8 September 2014) Citation: Tellus B 2014, 66 , 24536, http://dx.doi.org/10.3402/tellusb.v66.24536

Abstract: Aerosol vertical profile significantly affects the aerosol direct radiative forcing at the TOA level. The degree to which the aerosol profile impacts the aerosol forcing depends on many factors such as presence of cloud, surface albedo and aerosol single scattering albedo (SSA). Using a radiation model, we show that for absorbing aerosols (with an SSA of 0.7–0.8) whether aerosols are located above cloud or below induces at least one order of magnitude larger changes of the aerosol forcing than how aerosols are vertically distributed in clear skies, above cloud or below cloud. To see if this finding also holds for the global average aerosol direct radiative effect, we use realistic AOD distribution by integrating MODIS, MISR and AERONET observations, SSA from AERONET and cloud data from various satellite observations. It is found that whether aerosols are above cloud or below controls about 70–80% of the effect of aerosol vertical profile on the global aerosol radiative effect. Aerosols below cloud contribute as much to the global aerosol radiative effect as aerosols above cloud. Keywords: Aerosol, radiative forcing, vertical profile, cloud (Published: 23 May 2014) Citation: Tellus B 2014, 66 , 24376, http://dx.doi.org/10.3402/tellusb.v66.24376

Abstract: An integrated terrestrial ecosystem model and an atmospheric radiative transfer module are developed and applied to evaluate aerosol direct radiative effects on carbon dynamics of global terrestrial ecosystems during 2003–2010. The Moderate-Resolution Imaging Spectroradiometer measurements of key atmosphere parameters have been used to quantify aerosol effects on downward solar radiation. Simulations with and without considering the aerosol loadings show that aerosol affects terrestrial ecosystem carbon dynamics through the effects on plant phenology, thermal and hydrological conditions as well as solar radiation. The simulations also show that aerosol enhances the terrestrial gross primary production by 4.9 Pg C yr −1 , the net primary production by 3.8 Pg C yr −1 , the net ecosystem production by 3.9 Pg C yr −1 , and the plant respiration by 1.1 Pg C yr −1 during the period. The aerosol loading at a magnitude of 0.1 Pg C yr − 1 reduces ecosystem heterotrophic respiration. These results support previous findings of the positive effects of aerosol light scattering on plant production, but suggest there is a strong spatial variation due to cloud cover. This study suggests that both direct and indirect aerosol radiative effects through aerosol–cloud interactions should be considered to quantify the global carbon cycle. Keywords: aerosol, carbon dynamics, terrestrial ecosystem model (Published: 22 May 2014) Citation: Tellus B 2014, 66 , 21808, http://dx.doi.org/10.3402/tellusb.v66.21808

Abstract: Based on the MEGAN (Model of Emissions of Gases and Aerosols from Nature) module embedded within the global chemical transport model (GEOS-Chem), we estimate the changes in emissions of biogenic volatile organic compounds (BVOCs) and their impacts on surface-layer O 3 and secondary organic aerosols (SOA) in China between the late 1980s and the mid-2000s by using the land cover dataset derived from remote sensing images and land use survey. The land cover change in China from the late 1980s to the mid-2000s can be characterised by an expansion of urban areas (the total urban area in the mid-2000s was four times that in the late 1980s) and a reduction in total vegetation coverage by 4%. Regionally, the fractions of land covered by forests exhibited increases in southeastern and northeastern China by 10–30 and 5–15%, respectively, those covered by cropland decreased in most regions except that the farming–pastoral zone in northern China increased by 5–20%, and the factions of grassland in northern China showed a large reduction of 5–30%. With changes in both land cover and meteorological fields, annual BVOC emission in China is estimated to increase by 11.4% in the mid-2000s relative to the late 1980s. With anthropogenic emissions of O 3 precursors, aerosol precursors and aerosols fixed at year 2005 levels, the changes in land cover and meteorological parameters from the late 1980s to the mid-2000s are simulated to change the seasonal mean surface-layer O 3 concentrations by −4 to +6 ppbv (−10 to +20%) and to change the seasonal mean surface-layer SOA concentrations by −0.4 to +0.6 µg m −3 (−20 to +30%) over China. We find that the decadal changes in meteorological parameters had larger collective effects on BVOC emissions and surface-layer concentrations of O 3 and SOA than those in land cover and land use alone. We also perform a sensitivity simulation to compare the impacts of changes in anthropogenic emissions on concentrations of O 3 and SOA with those of the changes in meteorological parameters and land cover. Keywords: biogenic emissions, land cover and land use, tropospheric O 3 , secondary organic aerosol (Published: 18 November 2014) Citation: Tellus B 2014, 66 , 24987, http://dx.doi.org/10.3402/tellusb.v66.24987

Abstract: Two diel field campaigns under different weather patterns were carried out in the summer and autumn of 2013 to measure CO 2 and CH 4 fluxes and to probe the rates of gas exchange across the air–water interface in a subtropical eutrophic pond in China. Bubble emissions of CH 4 accounted for 99.7 and 91.67% of the total CH 4 emission measured at two sites in the summer; however, no bubble was observed in the autumn. The pond was supersaturated with CO 2 and CH 4 during the monitoring period, and the saturation ratios (i.e. observed concentration/equilibrium concentration) of CH 4 were much higher than that of CO 2 . Although the concentration of dissolved CO 2 in the surface water collected in the autumn was 1.24 times of that in the summer, the mean diffusive CO 2 flux across the water–air interface measured in the summer is almost twice compared with that in the autumn. The mean concentration of dissolved CH 4 in the surface water in the autumn was around half of that in the summer, but the mean diffusive CH 4 flux in the summer is 4–5 times of that in the autumn. Our data showed that the variation in gas exchange rate was dominated by differences in weather patterns and primary production. Averaged k 600 -CO 2 and k 600 -CH 4 (the gas transfer velocity normalised to a Schmidt number of 600) were 0.65 and 0.55 cm/h in the autumn, and 2.83 and 1.64 cm/h in the summer, respectively. No statistically significant correlation was found between k 600 and U 10 (wind speed at 10 m height) in the summer at low wind speeds in clear weather. Diffusive gas fluxes increased during the nights, which resulted from the nighttime cooling effect of water surface and stronger turbulent mixing in the water column. The chemical enhancements for CO 2 were estimated up to 1.94-fold in the hot and clear summer with low wind speeds, which might have been resulted from the increasing hydration reactions in water due to the high water temperature and active metabolism in planktonic algae. However, both the air and surface water temperatures decreased continually, and relatively lower temperature and overcast weather with occasionally light rain dominated the second campaign in the autumn. The concentration of dissolved oxygen in the surface water and U 10 controlled gas transfer velocities of CO 2 and CH 4 , respectively, in the cool autumn. When the surface water temperature was higher than the air temperature, higher CO 2 flux was observed because the water body was unstable and overturned quickly, inducing quick CO 2 emitted from plankton algae in surface water to the atmosphere. Keywords: gas transfer velocity, the chemical enhancement, convective cooling, wind speed, pond, subtropical, primary productivity (Published: 9 December 2014) Citation: Tellus B 2014, 66 , 23795, http://dx.doi.org/10.3402/tellusb.v66.23795

Abstract: This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.08–2 µm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase. Keywords: lowermost stratosphere, elemental composition, volcanic aerosol, sulphurous aerosol, carbonaceous aerosol (Published: 27 March 2014) Citation: Tellus B 2014, 66 , 23428, http://dx.doi.org/10.3402/tellusb.v66.23428

Abstract: During summer, large amounts of mineral dust are emitted and transported from North Africa over the tropical North Atlantic towards the Caribbean with the exact quantity varying greatly from year to year. Much effort has been made to explain the variability of summer season mineral dust load, for example, by relating dust variability to teleconnection indices such as ENSO and the NAO. However, only weak relationships between such climate indices and the abundance of mineral dust have been found. In this work, we demonstrate the role of the near-surface convergence zone over West Africa in controlling dust load and transport of mineral dust. We apply the ‘Center of Action’ approach to obtain indices that quantify the movement and strength of the convergence zone using NCEP/NCAR Reanalysis data. The latitudinal position of the convergence zone is significantly correlated with the quantity of mineral dust at Barbados over the period 1965–2003 ( r= −0.47). A southward displacement of the convergence zone is associated with both increased near-surface flow and decreased precipitation over the dust source regions of the southern Saharan desert, Sahel and Lake Chad. This in turn reduces soil moisture and vegetation, furthering the potential for dust emission. In contrast, the intensity of the convergence zone is not correlated with dust concentration at Barbados. We conclude that the coupling of changes in near-surface winds with changes in precipitation in source regions driven by a southward movement of the convergence zone most directly influence dust load at Barbados and over the tropical North Atlantic during summer. Keywords: Saharan mineral dust, ITCZ, West Africa, interannual variability, climate, Centres of Action (Published: 23 July 2014) Citation: Tellus B 2014, 66 , 23191, http://dx.doi.org/10.3402/tellusb.v66.23191

Abstract: Global warming will bring about changes in surface energy balance of Arctic ecosystems, which will have implications for ecosystem structure and functioning, as well as for climate system feedback mechanisms. In this study, we present a unique, long-term (2000–2010) record of summer-time energy balance components (net radiation, R n ; sensible heat flux, H ; latent heat flux, LE ; and soil heat flux, G ) from a high Arctic tundra heath in Zackenberg, Northeast Greenland. This area has been subjected to strong summer-time warming with increasing active layer depths (ALD) during the last decades. We observe high energy partitioning into H , low partitioning into LE and high Bowen ratio ( β = H / LE ) compared with other Arctic sites, associated with local climatic conditions dominated by onshore winds, slender vegetation with low transpiration activity and relatively dry soils. Surface saturation vapour pressure deficit ( D s ) was found to be an important variable controlling within-year surface energy partitioning. Throughout the study period, we observe increasing H / R n and LE / R n and decreasing G / R n and β , related to increasing ALD and decreasing soil wetness. Thus, changes in summer-time surface energy balance partitioning in Arctic ecosystems may be of importance for the climate system. Keywords: energy budget, energy balance, Arctic, sensible heat, latent heat, ground heat, net radiation, climate change, global warming (Published: 21 July 2014) Citation: Tellus B 2014, 66 , 21631, http://dx.doi.org/10.3402/tellusb.v66.21631

Abstract: Seasonal variations of photosynthetic capacity parameters, notably the maximum carboxylation rate, V cmax , play an important role in accurate estimation of CO 2 assimilation in gas-exchange models. Satellite-derived normalised difference vegetation index (NDVI), enhanced vegetation index (EVI) and model-data fusion can provide means to predict seasonal variation in V cmax . In this study, V cmax was obtained from a process-based model inversion, based on an ensemble Kalman filter (EnKF), and gross primary productivity, and sensible and latent heat fluxes measured using eddy covariance technique at two deciduous broadleaf forest sites and a mixed forest site. Optimised V cmax showed considerable seasonal and inter-annual variations in both mixed and deciduous forest ecosystems. There was noticeable seasonal hysteresis in V cmax in relation to EVI and NDVI from 8 d composites of satellite data during the growing period. When the growing period was phenologically divided into two phases (increasing VIs and decreasing VIs phases), significant seasonal correlations were found between V cmax and VIs, mostly showing R 2 >0.95. V cmax varied exponentially with increasing VIs during the first phase (increasing VIs), but second and third-order polynomials provided the best fits of V cmax to VIs in the second phase (decreasing VIs). The relationships between NDVI and EVI with V cmax were different. Further efforts are needed to investigate V cmax –VIs relationships at more ecosystem sites to the use of satellite-based VIs for estimating V cmax . Keywords: V cmax , spectral vegetation indices, ensemble Kalman filter, BEPS model (Published: 15 April 2014) Citation: Tellus B 2014, 66, 23279, http://dx.doi.org/10.3402/tellusb.v66.23279

Abstract: In this study, the concentrations of total mercury (THg) and ions deposited in the surface snow and snow pits in the eastern Antarctic along the 29th inland route of the Chinese National Antarctic Research Expedition were analysed. The THg concentrations in the surface snow ranged from 0.22 to 8.29 ng/L and elevated concentrations were detected in the inland regions of higher altitudes (3000–4000 m). The spatial distribution of the THg in the snow pits showed greater inland concentrations with mean concentrations of <0.2–1.33 ng/L. The THg concentrations in the coastal snow pit (29-A) showed higher concentrations in the summer snow layers than in the winter snow layers. The THg records from the two inland snow pits (29-K and 29-L) spanned decades and indicated elevated THg concentrations between the late 1970s and early 1980s and during the mid-1990s. The temporal variations of THg in the Antarctic snow layers were consistent with anthropogenic emissions around the world. In addition, the Pinatubo volcanic eruption was the primary contributor to the 1992 THg peak that was observed in the inland snow pits. Keywords: total mercury, spatio-temporal variation, Dome A, eastern Antarctica (Published: 3 December 2014) Citation: Tellus B 2014, 66 , 25152, http://dx.doi.org/10.3402/tellusb.v66.25152 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Abstract: We present observations of CH 4 concentrations from the lower to upper troposphere (LT and UT) over Japan during 1988–2010 based on aircraft measurements from the Tohoku University (TU) The analysis is aided by simulation results using an atmospheric chemistry transport model (ie ACTM) Tropospheric CH 4 over Japan shows interannual and seasonal variations that are dependent on altitudes, primarily reflecting differences in air mass origins at different altitudes The long-term trend and interannual variation of CH 4 in the LT are consistent with previous reports of measurements at surface baseline stations in the northern hemisphere However, those in the UT show slightly different features from those in the LT In the UT, CH 4 concentrations show a seasonal maximum in August due to efficient transport of air masses influenced by continental CH 4 sources, while LT CH 4 reaches its seasonal minimum during summer due to enhanced chemical loss Vertical profiles of the CH 4 concentrations also vary with season, reflecting the seasonal cycles at the respective altitudes In summer, transport of CH 4 -rich air from Asian regions elevates UT CH 4 levels, forming a uniform vertical profile above the mid-troposphere On the other hand, CH 4 decreases nearly monotonically with altitude in winter–spring The ACTM simulations with different emission scenarios reproduce general features of the tropospheric CH 4 variations over Japan Tagged tracer simulations using the ACTM indicate substantial contributions of CH 4 sources in South Asia and East Asia to the summertime high CH 4 values observed in the UT This suggests that our observations over Japan are highly sensitive to CH 4 emission signals particularly from Asia Keywords: methane, aircraft observation, long-term variation, seasonality, vertical profile, Asian outflow, atmospheric chemistry transport model (Published: 22 May 2014) Citation: Tellus B 2014, 66 , 23837, http://dxdoiorg/103402/tellusbv6623837 To access the supplementary material to this article, please see Supplementary files under Article Tools online

Abstract: Ecosystem carbon (C) storage strongly regulates climate-C cycle feedback and is largely determined by both C residence time and C input from net primary productivity (NPP). However, spatial patterns of ecosystem C storage and its variation have not been well quantified in earth system models (ESMs), which is essential to predict future climate change and guide model development. We intended to evaluate spatial patterns of ecosystem C storage capacity simulated by ESMs as part of the 5th Climate Model Intercomparison Project (CMIP5) and explore the sources of multi-model variation from mean residence time (MRT) and/or C inputs. Five ESMs were evaluated, including C inputs (NPP and [gross primary productivity] GPP), outputs (autotrophic/heterotrophic respiration) and pools (vegetation, litter and soil C). ESMs reasonably simulated the NPP and NPP/GPP ratio compared with Moderate Resolution Imaging Spectroradiometer (MODIS) estimates except NorESM. However, all of the models significantly underestimated ecosystem MRT, resulting in underestimation of ecosystem C storage capacity. CCSM predicted the lowest ecosystem C storage capacity (~10 kg C m −2 ) with the lowest MRT values (14 yr), while MIROC-ESM estimated the highest ecosystem C storage capacity (~36 kg C m −2 ) with the longest MRT (44 yr). Ecosystem C storage capacity varied considerably among models, with larger variation at high latitudes and in Australia, mainly resulting from the differences in the MRTs across models. Our results indicate that additional research is needed to improve post-photosynthesis C-cycle modelling, especially at high latitudes, so that ecosystem C residence time and storage capacity can be appropriately simulated. Keywords: ecosystem C storage capacity, ecosystem residence time, C input, model intercomparison, uncertainty, CMIP5, net primary productivity (Published: 22 May 2014) Citation: Tellus B 2014, 66 , 22568, http://dx.doi.org/10.3402/tellusb.v66.22568

Abstract: Non-dispersive infrared CO 2 /H 2 O gas analysers produce erroneous CO 2 outputs when CO 2 is measured in humid air, unless a correction for water vapour cross-sensitivity is applied. Spectroscopic cross-sensitivities arising from direct absorption interference and from the pressure broadening effect are significant in CO 2 flux measurements by the eddy covariance technique using open-path gas analysers over the ocean, as opposed to land-surface measurements, where CO 2 fluxes are orders of magnitude larger. In this study, a widely used analyser with manufacturer-determined correction coefficients for both cross-sensitivities was tested by laboratory experiments. Our results showed that the correction coefficient for direct absorption interference was not optimised to calculate CO 2 flux accurately, and that the correction coefficient for the pressure broadening caused overestimation of the CO 2 mixing ratio flux in the same direction as the water vapour flux. Overestimations of open-path eddy covariance measurements of upward CO 2 fluxes in previous ocean observations probably resulted from inaccuracies in both of these correction coefficients. We also found that slight changes in spectroscopic cross-sensitivities due to contamination of the analyser’s optical windows by sea salt caused a low bias in CO 2 outputs with increasing H 2 O; however, this contamination effect was not always observed in repeated tests under different contamination conditions. We suggest that previously proposed methods for correcting the effect of optical window contamination is of limited value and that measurement of small CO 2 fluxes by the open-path eddy covariance technique over the ocean should be performed after confirming the spectroscopic cross-sensitivity and ensuring that the optical windows are as clean as possible. Keywords: open-path CO 2 /H 2 O gas analyser, cross-sensitivity, non-dispersive infrared gas analyser, eddy covariance technique, CO 2 flux (Published: 14 October 2014) Citation: Tellus B 2014, 66 , 23803, http://dx.doi.org/10.3402/tellusb.v66.23803

Abstract: This paper describes the NO y plumes originating from lightning emissions based on 4 yr (2001–2005) of MOZAIC measurements in the upper troposphere of the northern mid-latitudes, together with ground- and space-based observations of lightning flashes and clouds. This analysis is primarily for the North Atlantic region where the MOZAIC flights are the most frequent and for which the measurements are well representative in space and time. The study investigates the influence of lightning NO x (LNO x ) emissions on large-scale (300–2000 km) plumes (LSPs) of NO y . One hundred and twenty seven LSPs (6% of the total MOZAIC NO y dataset) have been attributed to LNO x emissions. Most of these LSPs were recorded over North America and the Atlantic mainly in spring and summer during the maximum lightning activity occurrence. The majority of the LSPs (74%) is related to warm conveyor belts and extra-tropical cyclones originating from North America and entering the intercontinental transport pathway between North America and Europe, leading to a negative (positive) west to east NO y (O 3 ) zonal gradient with −0.4 (+18) ppbv difference during spring and −0.6 (+14) ppbv difference in summer. The NO y zonal gradient can correspond to the mixing of the plume with the background air. On the other hand, the O 3 gradient is associated with both mixing of background air and with photochemical production during transport. Such transatlantic LSPs may have a potential impact on the European pollution. The remaining sampled LSPs are related to mesoscale convection over Western Europe and the Mediterranean Sea (18%) and to tropical convection (8%). Keywords: lightning NO x emissions, nitrogen species, ozone, plumes, the MOZAIC programme (Published: 18 November 2014) Citation: Tellus B 2014, 66 , 25544, http://dx.doi.org/10.3402/tellusb.v66.25544

Abstract: We use data from simulations performed with the global aerosol-climate model ECHAM5-HAM to test the proposition that shipping emissions do not have a statistically significant effect on water clouds over tropical oceans on climate scales put forward in earlier satellite based work. We analyse a total of four sensitivity experiments, three of which employ global shipping emissions and one simulation which only employs shipping emissions in the mid-Atlantic Ocean. To ensure comparability to earlier results from observations, we sample the model data using a method previously applied to satellite data aimed at separating ‘clean’ from ‘polluted’ oceanic regions based on i) the location of main shipping routes and ii) wind direction at 10 m above sea level. The model simulations run with realistic present-day shipping emissions show changes in the lower tropospheric aerosol population attributable to shipping emissions across major shipping corridors over tropical oceans. However, we find the resulting effect on cloud properties to be non-distinguishable from natural gradients and variability, that is, gradients of cloud properties sampled across major shipping corridors over tropical oceans are very similar among those simulations. Our results therefore compare well to the earlier findings from satellite observations. Substantial changes of the aerosol population and cloud properties only occur when shipping emissions are increased 10-fold. We find that aerosol advection and rapid aerosol removal from the atmosphere play an important role in determining the non-significant response in i) column integrated aerosol properties and ii) cloud microphysical properties in the realistic simulations. Additionally, high variability and infrequent occurrence of simulated low-level clouds over tropical oceans in ECHAM5-HAM limit the development of aerosol indirect effects because i) in-cloud production of sulphate from ship-emitted sulphuric species via aqueous oxidation pathways is very low and ii) a possible observational signal is blurred out by high variability in simulated clouds. Our results highlight i) the importance of adequately accounting for atmospheric background conditions when determining climate forcings from observations and ii) the effectiveness of buffering mechanisms on micro- and macroscopic scales which limit the emergence of such climate forcings. Keywords: aerosol indirect effects, climate modelling, aerosol microphysics, parameterization, cloud parameterization, remote sensing, geoengineering (Published: 9 May 2014) Citation: Tellus B 2014, 66 , 24054, http://dx.doi.org/10.3402/tellusb.v66.24054 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Abstract: We carried out simulations with predefined and simulated aerosol distributions in order to investigate the improvement in the forecasting capabilities of an operational weather forecast model by the use of an improved aerosol representation. This study focuses on convective cumulus clouds developing after the passage of a cold front on 25 April 2008 over Germany. The northerly flow after the cold front leads to increased sea salt aerosol concentrations compared to prefrontal conditions. High aerosol number concentrations are simulated in the interactive scenario representing typically polluted conditions. Nevertheless, due to the presence of sea salt particles, effective radii of cloud droplets reach values typical of pristine clouds (between 7 µm and 13 µm) at the same time. Compared to the predefined continental and maritime aerosol scenarios, the simulated aerosol distribution leads to a significant change in cloud properties such as cloud droplet radii and number concentrations. Averaged over the domain covered by the convective cumuli clouds, we found a systematic decrease in precipitation with increasing aerosol number concentrations. Differences in cloud cover, short wave radiation and cloud top heights are buffered by systematic differences in precipitation and the related diabatic effects. Comparisons with measured precipitation show good agreement for the interactive aerosol scenario as well as for the extreme maritime aerosol scenario. Keywords : aerosol–cloud interaction, precipitation, natural aerosol, anthropogenic aerosol, regional modelling Responsible Editor: Annica Ekman, Stockholm University, Sweden. (Published: 2 July 2014) Citation : Tellus B 2014, 66 , 22528, http://dx.doi.org/10.3402/tellusb.v66.22528

Abstract: The observed variability of shortwave (SW) irradiance, clouds and temperature and the potential connections between them is studied for the subarctic site Bergen (60.4°N, 5.3°E), located on the Norwegian west coast. Focusing on the quality and spatial representativity of the data, we compare observations from independent instruments and neighbouring measurement sites. The observations indicate that the decrease of sunshine duration and SW irradiance during the 1970s and 80s in Bergen is associated with the increasing frequency of clouds, in particular clouds of low base heights. We argue that the observed cloud changes are indicative of increased frequencies of storms in northern Europe. The annual mean observational time series show an increase in SW irradiance since 1990, which is not accompanied by a cloud cover (NN) decrease. This implies the influence of factors other than clouds, for example, decreasing aerosol emissions. Calculations of the aerosol optical depth (AOD) based on irradiance observations for hours when the sun is unobscured by clouds confirm a decreasing aerosol load after 1990, from 0.15 to 0.10 AOD which corresponds to 2–6 Wm −2 of brightening. At the same time, a seasonal analysis reveals opposite changes in SW irradiance and NN during the months of strongest changes – March, April and August – also during the recent period of increasing SW irradiance. We conclude that the seasonally decreasing NN also contributes to the recent changes in SW irradiance. Finally, we address the relationship between temperature, SW irradiance and clouds. In winter (December–February), the surface air temperature in Bergen is statistically linked to the warming influence of clouds. In all other seasons, the North Atlantic sea surface temperature variability has a more dominant influence on the air temperature in Bergen compared to local cloud and SW irradiance variability. Keywords: clouds, solar irradiance, global dimming and brightening (Published: 22 December 2014) Citation: Tellus B 2014, 66 , 25897, http://dx.doi.org/10.3402/tellusb.v66.25897

Abstract: Carbon-containing particles are associated with adverse health effects, and their light-absorbing fractions were recently estimated to be the second largest contributor to global warming after carbon dioxide. Knowledge on the spatiotemporal variability of light-absorbing carbon (LAC) particles in urban areas is relevant for air quality management and to better diagnose the population exposure to these particles. This work reports on the first mobile LAC mass concentrations (M LAC ) measured on-board four taxis in the Stockholm metropolitan area in November 2011. On average, concentrations were higher and more variable during daytime (median of 1.9 µg m −3 and median absolute deviation of 2.3 µg m −3 ). Night-time (21:00–05:00) measurements were very similar for all road types and also compared to levels monitored at an urban background fixed site (median of 0.9 µg m −3 ). We observed a large intra-urban variability in concentrations, with maxima levels inside road tunnels (median and 95th percentile of 7.5 and 40.1 µg m −3 , respectively). Highways presented the second ranked concentrations (median and 95th percentile of 3.2 and 9.7 µg m −3 , respectively) associated with highest vehicle speed (median of 65 km h −1 ), traffic rates (median of 62 000 vehicles day −1 and 1500 vehicles h −1 ) and diesel vehicles share (7–10%) when compared to main roads, canyon streets, and local roads. Multiple regression modelling identified hourly traffic rate and M LAC concentration measured at an urban background site as the best predictors of on-road concentrations, but explained only 25% of the observed variability. This feasibility study proved to be a time- and cost-effective approach to map out ambient M LAC concentrations in Stockholm and more research is required to represent the distribution in other periods of the year. Simultaneous monitoring of other pollutants, closely correlated to M LAC levels in traffic-polluted environments, and including video recording of road and traffic changes would be an asset. Keywords: black carbon, elemental carbon, mobile measurements, urban aerosols, air quality, multiple regression modelling, Sweden (Published: 9 April 2014) Citation: Tellus B 2014, 66 , 23533, http://dx.doi.org/10.3402/tellusb.v66.23533 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Abstract: In this paper, we pose the question where the source regions of the aerosol, which occurs in the European Arctic, are located. Long-term aerosol optical depth (AOD) data from Ny-Alesund and Sodankyla as well as short-term data from a campaign on a Russian drifting station were analysed by air backtrajectories, analysis of the general circulation pattern and a correlation to chemical composition from in-situ measurements. Surprisingly, our data clearly shows that direct transport of pollutants from Europe does not play an important role. Instead, Arctic haze in Ny-Alesund has been found for air masses from the Eastern Arctic, while events with increased AOD but chemically more diverse composition have been found for air from Siberia or the central Arctic. Moreover, the AOD in Ny-Alesund does not depend on the North Atlantic Oscillation (NAO). Hence, either the pollution pathways of aerosol are more complex or aerosol is significantly altered by clouds. Keywords: Arctic haze, photometer, AOD, pollution pathways, EOF analysis (Published: 11 March 2014) Citation: Tellus B 2014, 66 , 21450, http://dx.doi.org/10.3402/tellusb.v66.21450

Abstract: The ocean's sinks and sources determine the concentration of methane in the water column and by that regulating the emission of methane to the atmosphere. In this study, we investigate how sensitive the sea–air exchange of methane is to increasing/decreasing sinks and sources as well as changes of different drivers with a time-dependent biogeochemical budget model for one of the shallow shelf sea in the Siberian Arctic, the Laptev Sea. The applied changes are: increased air temperature, river discharge, wind, atmospheric methane, concentration of nutrients in the river runoff or flux of methane from the sediment. Furthermore, simulations are performed to examine how the large range in observations for methane concentration in the Lena River as well as the rate of oxidation affects the net sea–air exchange. In addition, a simulation with five of these changes applied together was carried out to simulate expected climate change at the end of this century. The result indicates that none of the simulations changed the seawater to becoming a net sink for atmospheric methane and all simulations except three increased the outgassing to the atmosphere. The three exceptions were: doubling the atmospheric methane, decreasing the rivers’ concentration of methane and increasing the oxidation rate where the latter is one of the key mechanisms controlling emission of methane to the atmosphere. Keywords: Arctic Ocean, Laptev Sea, methane, carbon, sea–air exchange, modelling (Published: 13 October 2014) Citation: Tellus B 2014, 66 , 24174, http://dx.doi.org/10.3402/tellusb.v66.24174

Abstract: On seasonal and interannual time scales, weather is highly influential in aerosol variability. In this study, we investigate the relationship between fine-mode AOD (fAOD) and precipitation on these scales, in order to unravel the effect of wet weather on aerosol amount. We find with integrated satellite and ground observations that biomass burning related fAOD has a relatively greater seasonal variation than fossil fuel combustion related fAOD. It is also found that wet weather reduces biomass burning fAOD and increases fossil fuel combustion fAOD. Aerosol simulation models forced by reanalyses consistently simulate the biomass burning fAOD reduced during wet weather but only in the tropics and furthermore do not consistently increase fossil fuel combustion fAOD during wet conditions. The identified relationship between fAOD and precipitation in observations allows for seasonal predictability of fAOD, since average precipitation can be predicted a few to several months in advance due to the well-established predictability of El Nino-Southern Oscillation (ENSO). We reveal ENSO-covariant fAOD using a rotated component principal analysis of combined interannual variation of sea surface temperature, precipitation and fAOD. During the warm phase of ENSO, we find that fAOD increases over Indonesia and the eastern coastal area of China, and decreases over South Asia, the Amazon and the continental parts of China. Keywords: aerosol, AOD, aerosol simulation, weather, meteorology, prediction (Published: 29 January 2014) Citation: Tellus B 2014, 66 , 23037, http://dx.doi.org/10.3402/tellusb.v66.23037

Abstract: To investigate the air quality during 2010 World Expo, continuous measurements of particle number size distributions and optical parameters were performed at urban Shanghai from April to June 2010. Total particle number and volume concentrations in the size range 16–600 nm were 12 700±6200 cm −3 and 16±8 µm 3 /cm 3 , respectively. Meanwhile, the optical parameters, particle light-scattering coefficient b sp,532nm and absorption coefficient b ap,532nm were 210±140 Mm −1 and 26±20 Mm −1 , respectively. Strong correlation ( R =0.69) was observed between b sp,532nm and the number concentration of accumulation mode particles, especially in the specific size range 300–500 nm ( R =0.87). Cluster analysis on air mass history was performed to explore the relationship between air mass origins and the optical as well as microphysical parameters. Back trajectories were classified into four major clusters. Air masses came from the west direction, in which continental aerosols were predominant, associating with the polluted cases during the summertime in Shanghai. In addition, the northern air mass showed the lower values of aerosol single scattering albedo ω 0,532nm , indicating the presence of smaller light-absorbing particles originated from the North China Plain. Keywords: particle number size distribution, aerosol optical properties, air mass history, cluster analysis, 2010 World Expo (Published: 29 October 2014) Citation: Tellus B 2014, 66 , 22319, http://dx.doi.org/10.3402/tellusb.v66.22319