ALBERT

Description: 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

Description: 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

Description: 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.

Description: 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

Description: 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

Description: 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

Description: 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

Description: 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

Description: 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 Hämeri, University of Helsinki, Finland. Citation: Tellus B 2014, 66 , 23733, http://dx.doi.org/10.3402/tellusb.v66.23733

Description: Tropical aerosols of PM 2.5 and PM 10 were collected at a rural site in Morogoro, Tanzania (East Africa), and analysed for stable carbon isotopic composition (δ 13 C) of dicarboxylic acids (C 2 –C 9 ), glyoxylic acid (ωC 2 ) and glyoxal (Gly) using gas chromatography/isotope ratio mass spectrometer. PM 2.5 samples showed that δ 13 C of oxalic (C 2 ) acid are largest (mean, −18.3±1.7‰) followed by malonic (C 3 , −19.6±1.0‰) and succinic (C 4 , −21.8±2.2‰) acids, whereas those in PM 10 are a little smaller: −19.9±3.1‰ (C 2 ), −20.2±2.7‰ (C 3 ) and −23.3±3.2‰ (C 4 ). The δ 13 C of C 2 –C 4 diacids showed a decreasing trend with an increase in carbon numbers. The higher δ 13 C values of oxalic acid can be explained by isotopic enrichment of 13 C in the remaining C 2 due to the atmospheric decomposition of oxalic acid or its precursors. δ 13 C of ωC 2 and Gly that are precursors of oxalic acid also showed larger values (mean, −22.5‰ and −20.2‰, respectively) in PM 2.5 than those (−26.7‰ and −23.7‰) in PM 10 . The δ 13 C values of ωC 2 and Gly are smaller than those of C 2 in both PM 2.5 and PM 10 . On the other hand, azelaic acid (C 9 ; mean, −28.5‰) is more depleted in 13 C, which is consistent with the previous knowledge; that is, C 9 is produced by the oxidation of unsaturated fatty acids emitted from terrestrial higher plants. A significant enrichment of 13 C in oxalic acid together with its negative correlations with relative abundance of C 2 in total diacids and ratios of water-soluble organic carbon and organic carbon further support that a photochemical degradation of oxalic acid occurs during long-range transport from source regions. Keywords: PM 2.5 and PM 10 , stable carbon isotope ratios, oxalic acid, diacids, glyoxylic acid, glyoxal, organic aerosols, Tanzania, East Africa (Published: 1 October 2014) Citation: Tellus B 2014, 66 , 23702, http://dx.doi.org/10.3402/tellusb.v66.23702