Vertical profiles and storage fluxes of CO2, heat and water in a tropical rainforest at Pasoh, Peninsular Malaysia

Ambient CO₂ concentration, air temperature and relative humidity were measured intermittently for a 3-year period from the floor to the canopy top of a tropical rainforest in Pasoh, Peninsular Malaysia. Mean diurnal CO₂ storage flux ( S _c; μmol m⁻² s⁻¹) and sensible and latent heat storage fluxes ( Q _a and Q _w; W m⁻²) ranged from −12.7 to 3.2 μmol m⁻² s⁻¹, −15 to 27 W m⁻² and −10 to 20 W m⁻², respectively. Small differences in diurnal changes were observed in S _c and Q _a between the driest and wettest periods. Compared with the ranges of mean diurnal CO₂ eddy flux (−14.7 to 4.9 μmol m⁻² s⁻¹), sensible eddy flux (−12 to 169 W m⁻²) and latent eddy flux (0 to 250 W m⁻²), the contribution of CO₂ storage flux was especially large. Comparison with summertime data from a temperate Japanese cypress forest suggested a higher contribution of S _c in the tropical rainforest, probably mainly due to the difference in nighttime friction velocity at the sites. On the other hand, differences in Q _a and Q _w were smaller than the difference in S _c, probably because of the smaller nighttime sinks/sources of heat and water vapour.     

Determination of microbial versus root‐produced CO2 in an agricultural ecosystem by means of δ13CO2 measurements in soil air

The amounts of microbial and root‐respired CO₂ in a maize/winter wheat agricultural system in south western Germany were investigated by measurements of the CO₂ mixing ratio and the ¹³C/¹²C ratio in soil air. CO₂ fluxes at the soil surface for the period of investigation (1993–1995) were also determined. Root respired CO₂ shows a strong correlation with the plant mass above ground surface of the respective vegetation (R²≥0.88); the maximum CO₂ release from roots was in August for the maize (2.0±0.5 mmol m⁻² h⁻¹) and in June for winter wheat (1.5±0.5 mmol m⁻² h⁻¹). Maximum CO₂ production by roots correlate well with the maximum amount of plant root matter. Integrating the CO₂ production over the whole growing season and normalizing to the dry root matter yields, the CO₂ production per gram dry organic root matter (DORM) of maize was found to be 0.14±0.03 gC (g DORM)⁻¹. At the sites investigated, root‐produced CO₂ contributed (16±4)% for maize, and (24±4)% for winter wheat, respectively, to the total annual CO₂ production in the soil (450±50 gC m⁻² for maize, 210±30 gC m⁻² for winter wheat).     

Seasonal variation of the molecular hydrogen uptake by soils inferred from continuous atmospheric observations in Heidelberg, southwest Germany

The dominant sink of atmospheric molecular hydrogen (H₂) is its enzymatic destruction in soils. Quantitative estimates of the global sink strength, as derived from bottom-up process studies, are, however, still associated to large uncertainties. Here we present an alternative way to estimate atmosphere-to-soil flux densities, respectively deposition velocities of H₂, based on atmospheric H₂ and ²²²Rn observations in the boundary layer. Two and a half years of continuous measurements from a polluted site in the Rhine-Neckar area have been evaluated and night-time flux densities were calculated for situations of strong nocturnal boundary layer inversions using the Radon-Tracer Method. The influences from local anthropogenic combustion sources could be detected and successfully separated by parallel measurements of carbon monoxide. Inferred daily uptake fluxes in the Heidelberg catchment area range from 0.5 to 3 × 10⁻⁸ g H₂ m⁻² s⁻¹ with a mean value of (1.28 ± 0.31) × 10⁻⁸ g H₂ m⁻² s⁻¹. Uptake rates are about 25% larger during summer than during winter, when soil moisture is high, and diffusive transport of H₂ into the soil is inhibited. The mean deposition velocity is 3.0 ± 0.7 × 10⁻² cm s⁻¹_, which is very well in line with direct measurements on similar soil types in Europe and elsewhere.     

Present-day contribution of anthropogenic emissions from China to the global burden and radiative forcing of aerosol and ozone

The effect of anthropogenic emissions from China on global burdens of ozone, sulphate, organic carbon (OC) and black carbon (BC) aerosols is examined, using the three-dimensional chemistry transport model Oslo CTM2. Two model simulations were performed, the first with global present-day emissions and the second with the anthropogenic emissions from China set to their pre-industrial levels. The global radiative forcing for these species is then calculated. Industrial emissions from China are found to account for a 4–5% increase in the global burden of OC aerosol, the change in secondary organic aerosol being slightly less than that of primary organic aerosol. A 10% increase in the global sulphate aerosol burden is calculated, and the increase in BC is 23%. The global radiative forcing of aerosols from China was calculated to be −62, −3.7, −13 and 89 mW m⁻², for sulphate, secondary organic, primary organic and BC aerosols, respectively. The increase in ozone causes a forcing of 77 mW m⁻².     

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻² for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.     

Aerosol monitoring at multiple locations in China: contributions of EC and dust to aerosol light absorption

This paper reports on the analysis of 24-h aerosol data measured during 2006, at 14 monitoring sites in China. Measurements included seven-wavelength Aethalometers, thermal/optical reflectance analyses of filter samples and determination of dust aerosols. Black (elemental) carbon (BC, EC) is found to be the principal light-absorbing aerosol over many parts of China; however, the fraction of apparent light absorption attributed to dust varied from 14% in winter, to 11% in spring, to 5% in summer to 9% in autumn. Aerosol light absorption in urban areas was larger than in rural areas by factors of 2.4 in winter, 3.1 in spring and 2.5 in both summer and autumn. These differences may lead to contrasts in radiative, thermal and cloud modification effects between urban and rural areas. Absorption 'hotspots' were located in the Sichuan Basin, the provinces south of Beijing, the Pearl Delta River regions and the Guanzhong Plain. The mass absorption coefficient for aerosol BC (σ_BC) based on Aethalometer data is estimated to be 11.7 m² g⁻¹ at 880 nm wavelength (λ) with inverse (λ⁻¹) wavelength scaling, whereas the mass absorption coefficient for dust (σ_dust) is 1.3 m² g⁻¹ on average without significant wavelength dependence.     

Chemical composition of the background aerosol at two sites in southwestern and northwestern China: potential influences of regional transport

Concentrations of organic carbon (OC), elemental carbon (EC), selected trace elements and water-soluble (WS) ions were determined for samples collected from August 2004 to February 2005 to assess the aerosol background at two remote sites in China. The OC and EC concentrations in PM₁₀ from near the Tibetan Plateau at Zhuzhang (ZUZ) were comparable with other background sites, averaging 3.1 and 0.34 μg m⁻³, respectively, with no pronounced seasonality. At Akdala (AKD) on northern margin of the Zhungaer Basin, the average concentrations were similar (mean OC = 2.9 μg m⁻³ and EC = 0.35 μg m⁻³), but the concentrations were higher in winter. The aerosol mass at both sites was dominated by OC and SO₄²⁻, but a stronger contribution from soil dust was observed at AKD. At ZUZ, NO₃⁻ showed a unique weather-related fluctuation in PM₁₀ with a periodicity of ∼1 week. Anthropogenic sources in the Sichuan Basin and southeastern Yunnan Province evidently influence ZUZ in summer and autumn while pollutants from Russia and the China–Mongolia border affect AKD nearly all year. The identification of these upwind sources demonstrates that transboundary transport needs to be taken into account when assessing air quality in remote parts of China.     

Aerosol modulation of atmospheric and surface solar heating over the tropical Indian Ocean

The major finding of this study is that aerosols over the tropical Indian Ocean enhance clear sky atmospheric solar heating significantly and decrease the surface solar heating by even a larger amount. The results presented here are based on aerosol chemical, microphysical, and optical and radiometric data collected at the island of Kaashidhoo (4.97°N, 73.47°E) during February and March of 1998, as part of the first field phase of the Indian Ocean experiment (INDOEX). The aerosol optical properties were integrated with a multiple scattering Monte Carlo radiative transfer model which was validated at the surface with broadband flux measurements and at the top of the atmosphere (TOA) with the clouds and earth's radiant energy system (CERES) radiation budget measurements. We consider both externally and internally mixed aerosol models with very little difference between the two models in the estimated forcing. For the February–March period, the aerosols increase the monthly mean clear sky atmospheric solar heating by about 12 W/m²(about 15% of the total atmospheric solar heating) and decrease the sea surface clear sky solar heating by about 16 W/m² with a daily range from 5 to 23 W/m². The net aerosol forcing at the top of the atmosphere is about −4 W/m² with a daily range from −2 to −6 W/m². Although the soot contributes only about 10% to the aerosol optical thickness, it contributes more than 50% to the aerosol induced atmospheric solar heating. The fundamental conclusion of this study is that anthropogenic aerosols over the tropical Indian Ocean are altering the clear sky radiation budget of the atmosphere and surface in a major manner.     

Estimating the Asian radon flux density and its latitudinal gradient in winter using ground-based radon observations at Sado Island

Terrestrial radon-222 flux density for the Asian continent, integrated over distances of 4500 km, is estimated in two 20° latitudinal bands centred on 48.8°N and 63.2°N. The evaluation is based on three years of wintertime radon measurements at Sado Island, Japan, together with meteorological and trajectory information. A selection of 18% of observations are suitable for evaluation of an analytical expression for the continental surface flux. Various meteorological assumptions are discussed; it is found that there is a substantial effect of increased complexity of the formulation on the flux estimates obtained. The distribution of spatially integrated radon flux over the Asian landmass is reported for the first time. Expressed as geometric means and 1σ-ranges, estimated fluxes are 14.1 mBq m⁻² s⁻¹ (1σ-range: 18 mBq m⁻² s⁻¹) and 8.4 mBq m⁻² s⁻¹ (1σ-range: 10 mBq m⁻² s⁻¹) for the lower and higher latitude bands. These results constitute an annual minimum in flux densities for these regions, and are higher than previously reported. The existence of a latitudinal gradient in the continental radon source function is confirmed; the present estimate for Asia (−0.39 mBq m⁻² s⁻¹ per degree of latitude) is in agreement with the northern hemisphere terrestrial radon flux gradient proposed previously.     

Investigation of parameters controlling the soil sink of atmospheric molecular hydrogen

Enclosure measurements have been performed on a bare mineral soil at an experimental field site near Heidelberg, Germany. From observed molecular hydrogen (H₂) mixing ratio changes in the enclosure, deposition velocities were calculated ranging from  8.4 × 10⁻³  to  8.2 × 10⁻² cm s⁻¹  and with an annual mean value of  3.1 × 10⁻² cm s⁻¹  . In the studied range of  2– 27 °C  , the uptake showed a significant temperature dependence. However, this turned out not to be the primary driving mechanism of the uptake flux. Soil moisture content, co-varying with temperature, was identified as the major parameter being responsible for the diffusive permeability of H₂ in the soil and the final rate of H₂ uptake. A simple Millington–Quirk diffusion model approach could largely explain this behaviour and yielded a diffusion path length of H₂ in the studied soil of only 0.2–1.8 cm, suggesting that total H₂ consumption occurs within the first few centimetres of the soil. The diffusion model, when applied to continuous measurements of soil moisture content, atmospheric pressure, temperature and the mixing ratio of H₂ in the atmosphere, could largely reproduce the measured deposition flux densities, assuming a mean thickness of the diffusion path length of 0.7 cm.     

Spectral absorption coefficients and imaginary parts of refractive indices of Saharan dust during SAMUM-1

During the SAMUM-1 experiment, absorption coefficients and imaginary parts of refractive indices of mineral dust particles were investigated in southern Morocco. Main absorbing constituents of airborne samples were identified to be iron oxide and soot. Spectral absorption coefficients were measured using a spectral optical absorption photometer (SOAP) in the wavelength range from 300 to 800 nm with a resolution of 50 nm. A new method that accounts for a loading-dependent correction of fibre filter based absorption photometers, was developed. The imaginary part of the refractive index was determined using Mie calculations from 350 to 800 nm. The spectral absorption coefficient allowed a separation between dust and soot absorption. A correlation analysis showed that the dust absorption coefficient is correlated ( R ² up to 0.55) with the particle number concentration for particle diameters larger than 0.5 μm, whereas the coefficient of determination R ² for smaller particles is below 0.1. Refractive indices were derived for both the total aerosol and a dust aerosol that was corrected for soot absorption. Average imaginary parts of refractive indices of the entire aerosol are 7.4 × 10⁻³, 3.4 × 10⁻³ and 2.0 × 10⁻³ at wavelengths of 450, 550 and 650 nm. After a correction for the soot absorption, imaginary parts of refractive indices are 5.1 × 10⁻³, 1.6 × 10⁻³ and 4.5 × 10⁻⁴.     

Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM

Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May–10 June 2006) at the site Porte au Sahara (30.237°N, 5.607°W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.     

Broadband extinction method to determine atmospheric aerosol optical properties

The equivalent wavelength ( λ _E), at which the aerosol optical depth (AOD) is equal to broadband AOD (BAOD), can change in a wide range from 0.619 μm to 1.575 μm in the usual aerosol conditions. By using the least squares technique and some empirical corrections, a parameterized relationship of λ _E with BAOD, Ångström wavelength exponent ( α ), solar zenith angle ( θ ₀) and H₂O amount is developed. Using this relationship, and based on the strong sensitivity of BAOD on θ ₀ when θ ₀>70°, the broadband extinction method to derive the spectral AOD and α is further proposed. As shown in comparative simulations to retrieve AOD by the present, Molineaux et al. and Gueymard methods, the present method has the best accuracy in most simulations using Junge, MODTRAN, log‐normal and Deirmendjian aerosol models. A key question of the pyrheliometer method to determine wavelength-dependent AODs is the effect of uncertainty in the aerosol size istribution. It is found that the AOD solution around λ _E is less sensitive to the uncertainty. The wavelength exponent α is derived using an assumption of the stable atmospheric turbidity. If the pyrheliometer data from θ ₀=85° to 70° are used and the change of the turbidity is ±10%, the error of solution α is usually within ±0.32. If the variation of the turbidity is random, the mean value of a lot of the measurements of α would be very reasonable.     

Evolution of boundary‐layer aerosol particles due to in‐cloud chemical reactions during the 2nd Lagrangian experiment of ACE‐2

The second Aerosol Characterisation Experiment (ACE‐2) was aimed at investigating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment, an air mass was tracked over a 30‐h period during conditions of extensive stratocumulus cover. Boundary‐layer measurements of the aerosol size distribution obtained with a passive cavity aerosol spectrometer probe (PCASP) during the experiment show a gradual growth in size of particles in the 0.1–0.2 μm diameter mode. Simultaneously, SO₂ concentrations were found to decrease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ionic mass increased from 0.68±0.07 to 0.82±0.09 for small particles (diameter below 1.7 μm) and from 0.21±0.04 to 0.34±0.03 for large particles (diameter above 1.7 μm). The measurements were compared with a multicyclic parcel model of gas phase diffusion into cloud droplets and aqueous phase chemical reactions. The model was able to broadly reproduce the observed transformation in the aerosol spectra and the timescale for the transformation of SO₂ to sulphate aerosol. The modelled SO₂ concentration in the boundary layer fell to below half its initial value over a 6.5‐h time period due to a combination of the entrainment of cleaner tropospheric air and cloud chemical reactions. NH₃ and HCl gas were also found to play an important rôle in cloud processing in the model.     

Aircraft observation of carbon dioxide at 8–13 km altitude over the western Pacific from 1993 to 1999

Abstract The spatial and temporal variations of atmospheric CO₂ at 8–13 km from April 1993 to April 1999 were observed by measuring CO₂ mixing ratios in samples collected biweekly from a commercial airliner between Australia and Japan. The CO₂ growth rate showed a considerable interannual variation, with a maximum of about 3 ppm yr⁻¹ during late 1997. This variation is related to the El Niño/Southern Oscillation (ENSO) events. A year-to-year change related to the ENSO events was also found in the latitudinal distribution pattern of the CO₂ annual mean between 30°N and 30°S. The averaged CO₂ seasonal cycle in the Northern Hemisphere gradually decayed toward the equator, and a relatively complicated variation with a double seasonal maximum appeared in the Southern Hemisphere. A significant yearly change of the seasonal cycle pattern was observed in the Southern Hemisphere. The impact of a tropical biomass-burning injection on the upper tropospheric CO₂ was estimated on the basis of the CO data from the same airliner observation.     

Aircraft study of aerosol vertical distributions over Beijing and their optical properties

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration ( N _a) reveals a high aerosol number density in this region with average surface level number concentration ( N ₀) of about 6600 cm⁻³ (0.12–3.0 μm). The average vertical profile of N _a approximately satisfies an exponential decline function with a scale height of 1419 m. The N _a profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of N _a profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.     

Properties of dust aerosol particles transported to Portugal from the Sahara desert

Aerosol properties of mineral particles in the far field of an African desert dust outbreak were investigated that brought Saharan dust over the Mediterranean in different layers to Portugal. The measurements were performed inside the project Desert Aerosols over Portugal (DARPO) which was linked to the Saharan Mineral Dust Experiment (SAMUM). The maximum particle mass concentration was about 150 μg m⁻³ and the corresponding scattering coefficient was 130 M m⁻¹ which results in a mass scattering efficiency of 0.87 m² g⁻¹. The aerosol optical depth reached values up to 0.53 and the lidar ratio was between 45 and 50 in the whole dust loaded column. A comparison between particle size distributions and refractive indices derived from different instruments and models showed a general good agreement but some minor differences could also be observed. Measurements as well as calculations with a particle transport model suggest that there is a relatively higher concentration of very large particles in the upper region of the dust layer than on the surface which is likely connected with meteorological conditions at the observational site (Évora, Portugal).     

Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10⁻³ to 5.2 × 10⁻³ at 450 nm and from 0.3 × 10⁻³ to 2.5 × 10⁻³ at 700 nm. Groupings of k values could be attributed to different source regions.     

中国区域MODIS陆上气溶胶光学厚度产品检验

以我国MODIS共享网站积累的MODIS L1B数据和美国威斯康辛大学提供的IMAPP软件包气溶胶产品软件为基础, 经过产品运行本地化改进处理, 在国家卫星气象中心建立了气溶胶产品业务化生成和发布机制。为支持气溶胶遥感产品算法改进以及潜在用户对产品的合理应用, 给出对国家卫星气象中心运行的MODIS气溶胶遥感产品质量检验分析结果。利用2005年1月— 2007年5月AERONET地基气溶胶监测网的L2.0级气溶胶光学厚度产品作为真值, 用它匹配MODIS陆上气溶胶光学厚度产品开展检验。检验结果表明:以卫星过境前后30min地基观测时间平均值匹配地基站点位置10 km半径范围内的卫星反演结果空间平均值开展检验, 总体样本的气溶胶光学厚度均方根误差约为0.25;满足产品误差要求 (±0.05±0.20τ) 的样本占总样本数的44%; 气溶胶光学厚度反演结果精度具有季节和地域差异, 干季(秋、冬、春)的气溶胶光学厚度误差较小, 而雨季气溶胶光学厚度误差较大, 云是雨季气溶胶光学厚度反演结果误差较大的主要影响因素。     

Atmospheric concentrations and deposition of oxidised sulfur and nitrogen species at Petaling Jaya, Malaysia, 1993–1998

Wet‐only rainwater composition, acid‐precursor gas mixing ratios and aerosol loading were determined from weekly‐averaged samples at Petaling Jaya, Malaysia, over the five year period from March 1993 to March 1998. Annual deposition fluxes of acidic sulfur and nitrogen species estimated from these data show this site to be heavily impacted by acidic deposition, with total oxidised sulfur plus nitrogen deposition in the range 277–480 meq m⁻² yr⁻¹. Average contributions were 56% as sulfur species, 44% as nitrogen species, with wet deposition in this region of high rainfall accounting for 67% of total deposition. Thus total acid deposition fluxes were equivalent to levels that provided motivation for emissions reduction programs in both Europe and North America. The possibility of adverse environmental effects in Malaysia caused by acid deposition therefore merits serious consideration and assessment.