Observations of the evolution of the aerosol, cloud and boundary‐layer characteristics during the 1st ACE‐2 Lagrangian experiment

During the 1st Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE‐2), a parcel of air was tagged by releasing a smart, constant level balloon into it from the Research Vessel Vodyanitskiy . The Meteorological Research Flight's C‐130 aircraft then followed this parcel over a period of 30 h characterising the marine boundary layer (MBL), the cloud and the physical and chemical aerosol evolution. The air mass had originated over the northern North Atlantic and thus was clean and had low aerosol concentrations. At the beginning of the experiment the MBL was over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the free troposphere caused the depth of the MBL to almost halve during the experiment and, after 26 h, the MBL became well mixed throughout its whole depth. Salt particle mass in the MBL increased as the surface wind speed increased from 8 m s⁻¹ to 16 m s⁻¹ and the accumulation mode (0.1μm to 3.0 μm) aerosol concentrations quadrupled from 50 cm⁻³ to 200 cm⁻³. However, at the same time the total condensation nuclei (>3 nm) decreased from over 1000 cm⁻³ to 750 cm⁻³. The changes in the accumulation mode aerosol concentrations had a significant effect on the observed cloud microphysics. Observational evidence suggests that the important processes in controlling the Aitken mode concentration which, dominated the total CN concentration, included, scavenging of interstitial aerosol by cloud droplets, enhanced coagulation of Aitken mode aerosol and accumulation mode aerosol due to the increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air.     

Observation of regional new particle formation in the urban atmosphere

Long-term measurements of fine particle number-size distributions were carried out over 9.5 yr (May 1997–December 2006), in the urban background atmosphere of Helsinki. The total number of days was 3528 with about 91.9% valid data. A new particle formation event (NPF) is defined if a distinct nucleation mode of aerosol particles is observed below 25 nm for several hours, and it shows a growth pattern. We observed 185 NPF events, 111 d were clear non-events and most of the days (around 83.5%) were undefined. The observed events were regional because they were observed at Hyytiälä (250 km north of Helsinki). The events occurred most frequently during spring and autumn. The observed formation rate was maximum during the spring and summer (monthly median 2.87 cm⁻³ s⁻¹) and the modal growth rate was maximum during late summer and Autumn (monthly median 6.55 nm h⁻¹). The events were observed around noon, and the growth pattern often continued on the following day. The observation of weak NPF events was hindered due to pre-existing particles from both local sources. It is clear that regional NPF events have a clear influence on the dynamic behaviour of aerosol particles in the urban atmosphere.     

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.     

Latitudinal distribution of radon-222 flux from continents

Global atmospheric transport models are frequently tested by using ²²²Rn as a tracer. Generally this tracer is assumed to be emitted at a uniform rate (1 atom cm⁻² s⁻¹) from all ice-free land surfaces. The analysis of published data suggests a strong decrease from 30°N northwards to 0.2 atom cm⁻² s⁻¹ at 70°N. This could be a result of increasing water tables and proportions of organic soils as indicated by larger proportions of wetlands in northern latitudes.     

Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE‐2). Very high cloud droplet number concentrations (almost 3000 cm⁻³) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150–3000 cm⁻³. The updraught velocity during the cloud formation was estimated to 2.2 m s⁻¹ by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.     

On small particles in the Arctic summer boundary layer: observations at two different heights near Ny-Ålesund, Svalbard

Concurrent observations of particle number densities and size distributions observed at two different heights (near ocean level and 475 m above sea level) in Ny-Ålesund, Svalbard were studied with respect to the diurnal variation during a summer period in June 2004. The results show that observed variation in particle number density in the Arctic boundary layer may be strongly modulated by vertical mixing and dilution. The particles appeared to be formed in the early morning when solar intensity reached about 30% of the mid-day intensity. Based on differences in the observed number densities at the two heights it appears as if particles are formed in the lower part of the boundary layer. The formation rate of 10 nm diameter particles is estimated to be 0.11 cm⁻³ s⁻¹ and the growth rate is in a range between 1 and 2 nm h⁻¹.     

Precipitable water evaluations from infrared sun‐photometric measurements analyzed using the atmospheric hygrometry technique

Measurements of direct solar irradiance were taken employing 4 different sun‐photometers at near infrared wavelengths, suitable for use in atmospheric hygrometry. This technique utilising a set of spectral ratios, in and out of selected water vapour absorption bands, was applied to the measurements to obtain accurate evaluations of precipitable water. For all the hygrometric ratios given by the 4 sun‐photometers used at the 3 stations of Sagres, Monchique and Mt. Foia, during the CLEARCOLUMN experiment, we determined the calibration curves by correcting them for the Rayleigh scattering effects and, then, plotting the natural logarithms of such corrected ratios versus the square root of the water vapour mass present along the atmospheric slant path. The regression lines drawn for the various scatter diagrams were estimated to give evaluations of precipitable water with an uncertainty of less than 5%, 3% and 10% at the 3 stations, respectively. The calibration curves of the sun‐photometer located at the Sagres station were determined using the precipitable water evaluations obtained from the local radiosounding measurements taken on 5 clear‐sky days. Those of the sun‐photometers used at the Monchique and Mt. Foia stations were instead determined through intercomparison between subsets of measurements simultaneously taken with various instruments at Sagres and Mt. Foia. Using these calibration curves, we examined all the field measurements determining the time‐patterns of precipitable water at the 3 stations. During the period from 16 June to 25 July 1997, precipitable water was found to vary between 1.1 and 3.7 g cm⁻² at the Sagres station (with an accuracy within ±13%), between 1.0 and 2.8 g cm⁻² at Monchique (±11%) and between 0.8 and 3.0 g cm⁻² at the top of Mt. Foia (±26%).     

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.     

Physical characterization of aerosol particles during nucleation events

Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR campaigns (14 April–22 May 1998, 27 July–21 August 1998 and 20 March–24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm⁻³ and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm⁻³. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm⁻³, volume mean diameter of 2.0 μm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm⁻³, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.     

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.     

Seasonal and global NOx production by lightning estimated from the Optical Transient Detector (OTD)

The Optical Transient Detector (OTD) lightning data for the 12‐month period of 1996 are used to estimate the seasonal and global distributions of lightning‐produced NO _x . The relatively small viewing footprint and the low detection efficiency of the OTD sensor and other difficulties require extrapolations of the OTD data to the actual global flash distributions. Furthermore, available measurements for the ratios of intracloud (IC) to cloud‐to‐ground (CG) flashes have been used to partition lightning counts for IC versus CG flashes from the OTD observations. The resulting lightning distributions are then used to calculate the global and seasonal production of NO _x , assuming a NO production rate of 6.2×10²⁵ molecules for each CG flash and 8.7×10²⁴ molecules for each IC flash. Consequently, we find that CG flashes produce more NO _x than IC flashes despite fewer CG flashes by a factor of 3 or more. NO _x production by lightning varies seasonally in accordance with the global lightning distribution, with the maximum production occurring in the Northern Hemisphere in the local summer. The latitudinal distribution of NO _x production exhibits a strong seasonal variation outside the tropics with the production occurring mainly in the summer hemisphere, whereas in the tropics the production is high throughout the year. The annual contribution to NO _x production by lightning is higher in the Northern Hemisphere than that in the Southern Hemisphere.     

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.     

Microphysical properties of stratocumulus clouds during ACE‐2

Microphysical measurements performed during 8 flights of the CLOUDYCOLUMN component of ACE‐2, with the Meteo‐France Merlin‐IV, are analyzed in terms of droplet number concentration and size. The droplet concentration is dependent upon the aerosol properties within the boundary layer. Its mean value over a flight varies from 55 cm⁻³, for the cleanest conditions, to 244 cm⁻³, for the most polluted one. For each flight, the variability of the concentration, in selected cloud regions that are not affected by mixing with dry air or drizzle scavenging, ranges from 0.5 to 1.5 of the mean value. The mean volume diameter increases with altitude above cloud base according to the adiabatic cloud model. The frequency distribution of mean droplet volume normalized by the adiabatic value, for the selected regions, shows the same dispersion as the distribution of normalized concentration. The values of droplet concentration versus mean volume diameter are then examined in sub‐adiabatic samples to characterize the effects of mixing and drizzle scavenging. Finally, the ratio of mean volume diameter to effective diameter is analyzed and a simple relationship between these 2 crucial parameters is proposed.     

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.     

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).     

Excess 210Po in the coastal atmosphere

Many researchers have reported the widespread occurrence of excess ²¹⁰ Po in the global atmosphere and suggested probable sources such as resuspension of top soils, stratospheric aerosols, sea spray of the surface micro‐layer, volcanic emission, and bio‐volatile ²¹⁰Po species from the productive ocean. We have observed excess ²¹⁰Po on aerosols in the coastal atmosphere of the Chesapeake and Delaware Bays. On‐board measurements in the Chesapeake Bay atmosphere show that the increase of this excess ²¹⁰Po is dependent upon wind speed. Simultaneously measured activity ratios of ⁷Be/²¹⁰Pb and ²¹⁰Pb/²²²Rn argue against either higher altitude air or continental soils as the source of this excess. We hypothesize that the excess ²¹⁰Po originates mainly from surface waters either by the sea‐spray of the surface microlayer, or more likely, by gas exchange. We conclude gas exchange as the mechanism since the polonium excess increases linearly with wind speed over a threshold of 3 m s⁻¹(mean) similar to other gases (i.e., CO₂, SF₆ , and DMS). In addition, higher ²¹⁰Po excess with lower ²²²Rn is observed in on‐shore marine air at Lewes, DE. This suggests sea‐air exchange of volatile Po along with other bio‐volatile species (i.e., DMS, DMSe, and MMHg) in the coastal productive ocean during high wind speeds.     

Vertical fluxes and micrometeorology during aerosol particle formation events

Fluxes of aerosol particles with sizes larger than 10 nm together with fluxes of momentum, sensible and latent heat and CO₂ were measured 10 m above a Scots pine forest with the eddy covariance method. During days when nucleation events were observed particle size distribution measurements showed particle growth from 3 nm sizes to the Aitken mode. Analysis of the experimental data showed systematic differences in fluxes during the days when new particle production was observed compared to other days. During the nucleation events the particle flux measurements showed downward aerosol particle transport, i.e., indicating an elevated source, with respect to the measurement level, of particles larger than 10 nm. Furthermore the turbulence intensity and the heat fluxes were observed to be significantly higher. Evidences of mesoscale circulation were observed in wind speed records as well as in turbulent fluxes on nucleation days. The measurement results show that micrometeorology, the synoptic scale conditions and the particle formation are closely related.     

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.     

Size‐segregated chemical, gravimetric and number distribution‐derived mass closure of the aerosol in Sagres, Portugal during ACE‐2

During the ACE‐2 field campaign in the summer of 1997 an intensive, ground‐based physical and chemical characterisation of the clean marine and continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3–10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RH) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity of 60%. A size segregated 3‐way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically‐determined mass with both number distribution‐derived (slope=1.23/1.09; R²>0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope=1.02; R²= 0.79) for particles smaller than 3 μm in diameter. Except for the smallest impactor size range relatively good correlations (slope=0.86–1.42) with small deviations (R²=0.76–0.98) for the different size fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size‐segregated mass fractions can be explained by the measurement uncertainties. However, the number distribution‐derived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.     

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.