Aerosol-Cloud Interaction Determined by Both in Situ and Satellite Data Over a Northern High-Latitude Site

The first aerosol indirect effect over a clean, northern high-latitude site was investigated by determining the aerosol cloud interaction (ACI) using three different approaches; ground-based in situ measurements, combined ground-based in situ measurements 5 and satellite retrievals and using only satellite retrievals. The obtained values of ACI were highest for in situ ground-based data, clearly lower for combined ground-based and satellite data, and lowest for data relying solely on satellite retrievals. One of the key findings of this study was the high sensitivity of ACI to the definition of the aerosol burden. We showed that at least a part of the variability in ACI can be explained by 10 how different investigators have related dierent cloud properties to "aerosol burden"

Growth of sulphuric acid nanoparticles under wet and dry conditions

New particle formation, which greatly influences the number concentrations and size distributions of an atmospheric aerosol, is often followed by a rapid growth of freshly formed particles. The initial growth of newly formed aerosol is the crucial process determining the fraction of nucleated particles growing to cloud condensation nuclei sizes, which have a significant influence on climate. In this study, we report the laboratory observations of the growth of nanoparticles produced by nucleation of H₂SO₄ and water in a laminar flow tube at temperatures of 283, 293 and 303 K, under dry (a relative humidity of 1%) and wet conditions (relative humidity of 30%) and residence times of 30, 45, 60 and 90 s. The initial H₂SO₄ concentration spans the range from 2 × 10⁸ to 1.4 × 10¹⁰ molecule cm⁻³ and the calculated wall losses of H₂SO₄ were assumed to be diffusion limited. The detected particle number concentrations, measured by the Ultrafine Condensation Particle Counter (UCPC) and Differential Mobility Particle Sizer (DMPS), were found to depend strongly on the residence time. Hygroscopic particle growth, presented by growth factors, was found to be in good agreement with the previously reported studies. The experimental growth rates ranged from 20 nm h⁻¹ to 890 nm h⁻¹ at relative humidity (RH) 1% and from 7 nm h⁻¹ to 980 nm h⁻¹ at RH 30% and were found to increase significantly with the increasing concentration of H₂SO₄. Increases in the nucleation temperature had a slight enhancing effect on the growth rates under dry conditions. The influence of relative humidity on growth was not consistent – at lower H₂SO₄ concentrations, the growth rates were higher under dry conditions while at H₂SO₄ concentrations greater than 1 × 10¹⁰ molecule cm⁻³, the growth rates were higher under wet conditions. The growth rates show only a weak dependence on the residence time. The experimental observations were compared with predictions made using a numerical model, which investigates the growth of particles with three different extents of neutralization by ammonia, NH₃: (1) pure H₂SO₄ – H₂O particles; (2) particles formed by ammonium bisulphate, (NH₄)HSO₄; (3) particles formed by ammonium sulphate, (NH₄)₂SO₄. The highest growth rates were found for ammonium sulphate particles. Since the model accounting for the initial H₂SO₄ concentration predicted the experimental growth rates correctly, our results suggest that the commonly presumed diffusional wall losses of H₂SO₄ in case of long-lasting experiments are not so significant. We therefore assume that there are not only losses of H₂SO₄ on the wall, but also a flux of H₂SO₄ molecules from the wall into the flow tube, the effect being more profound under dry conditions and at higher temperatures of the tube wall. Based on a comparison with the atmospheric observations, our results indicate that sulphuric acid alone cannot explain the growth rates of particles formed in the atmosphere

Boundary layer concentrations and landscape scale emissions of volatile organic compounds in early spring

International audienceBoundary layer concentrations of several volatile organic compounds (VOC) were measured during two campaigns in springs of 2003 and 2006. The measurements were conducted over boreal landscapes near SMEAR II measurement station in Hyytiälä, Southern Finland. In 2003 the measuremens were performed using a light aircraft and in 2006 using a hot air balloon. Isoprene concentrations were low, usually below detection limit. This can be explained by low biogenic production due to cold weather, phenological stage of the isoprene emitting plants, and snow cover. Monoterpenes were observed frequently. The average total monoterpene concentration in the boundary layer was 33 pptv. Many anthropogenic compounds such as benzene, xylene and toluene, were observed in high amounts. Ecosystem scale surface emissions were estimated using a simple mixed box budget methodology. Total monoterpene emissions varied up to 80 ?g m?2 h?1, ?-pinene contributing typically more than two thirds of that. These emissions were somewhat higher that those calculated using emission algorithm. The highest emissions of anthropogenic compounds were those of p/m xylene

New particle formation in air mass transported between two measurement sites in Northern Finland

This study covers four years of aerosol number size distribution data from Pallas and Värriö sites 250 km apart from each other in Northern Finland and compares new particle formation events between these sites. In air masses of eastern origin almost all events were observed to start earlier at the eastern station Värriö, whereas in air masses of western origin most of the events were observed to start earlier at the western station Pallas. This demonstrates that particle formation in a certain air mass type depends not only on the diurnal variation of the parameters causing the phenomenon (such as photochemistry) but also on some properties carried by the air mass itself. The correlation in growth rates between the two sites was relatively good, which suggests that the amount of condensable vapour causing the growth must have been at about the same level in both sites. The condensation sink was frequently much higher at the downwind station. It seems that secondary particle formation related to biogenic sources dominate in many cases over the particle sinks during the air mass transport between the sites. Two cases of transport from Pallas to Värriö were further analysed with an aerosol dynamics model. The model was able to reproduce the observed nucleation events 250 km down-wind at Värriö but revealed some differences between the two cases. The simulated nucleation rates were in both cases similar but the organic concentration profiles that best reproduced the observations were different in the two cases indicating that divergent formation reactions may dominate under different conditions. The simulations also suggested that organic compounds were the main contributor to new particle growth, which offers a tentative hypothesis to the distinct features of new particles at the two sites: Air masses arriving from the Atlantic Ocean typically spent approximately only ten hours over land before arriving at Pallas, and thus the time for the organic vapours to accumulate in the air and to interact with the particles is relatively short. This can lead to low nucleation mode growth rates and even to suppression of detectable particle formation event due to efficient scavenging of newly formed clusters, as was observed in the case studies

Measurements of optical properties of atmospheric aerosols in Northern Finland

International audienceThree years of continuous measurements of aerosol optical properties and simultaneous aerosol number size distribution measurements at Pallas GAW station, a remote subarctic site in the northern border of the boreal forest zone, have been analysed. The scattering coefficient at 550 nm varied from 0.2 to 94.4 Mm?1 with an average of 7.1±8.6 Mm?1. Both the scattering and backscattering coefficients had a clear seasonal cycle with an autumn minimum and a 4?5 times higher summer maximum. The scattering was dominated by submicron aerosols and especially so during late summer and autumn. The Ångström exponent had a clear seasonal pattern with maximum values in late summer and minimum values during wintertime. The highest hemispheric backscattering fraction values were observed in autumn, indicating clean air with few scattering particles and a particle size distribution strongly dominated by ultrafine particles. To analyse the influence of air mass origin on the aerosol optical properties a trajectory climatology was applied to the Pallas aerosol data. The most polluted trajectory patterns represented air masses from the Kola Peninsula, Scandinavia and Russia as well as long-range transport from Britain and Eastern Europe. These air masses had the largest average scattering and backscattering coefficients for all seasons. Higher than average values of the Ångström exponent were also observed in connection with transport from these areas

Airborne measurements of nucleation mode particles I: coastal nucleation and growth rates

International audienceA light aircraft was equipped with a bank of Condensation Particle Counters (CPCs) (50% cut from 3?5.4?9.6 nm) and a nano-Scanning Mobility Particle Sizer (nSMPS) and deployed along the west coast of Ireland, in the vicinity of Mace Head. The objective of the exercise was to provide high resolution micro-physical measurements of the coastal nucleation mode in order to map the spatial extent of new particle production regions and to evaluate the evolution, and associated growth rates of the coastal nucleation-mode aerosol plume. Results indicate that coastal new particle production is occurring over most areas along the land-sea interface with peak concentrations at the coastal plume-head in excess of 106 cm?3. Pseudo-Lagrangian studies of the coastal plume evolution illustrated significant growth of new particles to sizes in excess of 8 nm approximately 10 km downwind of the source region. Close to the plume head (?1, decreasing gradually to 53?72 nm h?1 at 3 km. Further along the plume, at distances up to 10 km, the growth rates are calculated to be 17?32 nm h?1. Growth rates of this magnitude suggest that after a couple of hours, coastal nucleation mode particles can reach significant sizes where they can contribution to the regional aerosol loading

Homogeneous nucleation of sulfuric acid and water mixture: experimental setup and first results

In this study we introduce a new flow tube suitable for binary and ternary homogeneous nucleation studies. The production of sulfuric acid and water vapor mixture, the experimental setup and the method of sulfuric acid concentration determination are discussed in detail. Wall losses were estimated from the measured sulfuric acid concentration profiles along the flow tube and compared to a theoretical prediction. In this investigation the experimental evidence of new particle formation was observed at a concentration of 10<sup>9</sup> molecules cm<sup>−3</sup> of sulfuric acid and the nucleation rates measured at three relative humidities (RH) 10, 30 and 50%, cover six orders of magnitude, from 10<sup>−3</sup> to 10<sup>3</sup> particles cm<sup>−3</sup>. Particle free air was used as a carrier gas. Our initial results are compared to the theoretical prediction of binary homogeneous nucleation, to results obtained by other investigators, and to atmospheric nucleation

Observed metre scale horizontal variability of elemental carbon in surface snow

Peer reviewe

Evaluating the Assumptions of Surface Reflectance and Aerosol Type Selection Within the MODIS Aerosol Retrieval Over Land: The Problem of Dust Type Selection

Aerosol Optical Depth (AOD) and Angstrom exponent (AE) values derived with the MODIS retrieval algorithm over land (Collection 5) are compared with ground based sun photometer measurements at eleven sites spanning the globe. Although, in general, total AOD compares well at these sites (R2 values generally over 0.8), there are cases (from 2 to 67% of the measurements depending on the site) where MODIS clearly retrieves the wrong spectral dependence, and hence, an unrealistic AE value. Some of these poor AE retrievals are due to the aerosol signal being too small (total AOD<0.3) but in other cases the AOD should have been high enough to derive accurate AE. However, in these cases, MODIS indicates AE values close to 0.6 and zero fine model weighting (FMW), i.e. dust model provides the best fitting to the MODIS observed reflectance. Yet, according to evidence from the collocated sun photometer measurements and back-trajectory analyses, there should be no dust present. This indicates that the assumptions about aerosol model and surface properties made by the MODIS algorithm may have been incorrect. Here we focus on problems related to parameterization of the land-surface optical properties in the algorithm, in particular the relationship between the surface reflectance at 660 and 2130 nm

Driving factors of aerosol properties over the foothills of central Himalayas based on 8.5 years continuous measurements

This study presents analysis of in situ measurements conducted over the period 2005–2014 in the Indian Himalayas to give a thorough overview of the factors and causes that drive aerosol properties. Aerosol extensive properties (namely, particle number concentration, scattering coefficient, equivalent black carbon, PM2.5, and PM10) have 1.5–2 times higher values in the early to late afternoon than during the night, and a strong seasonality. The interannual variability is ±20% for both PM2.5 and total particle number concentration. Analysis of the data shows statistically significant decreasing trends of −2.3 μg m−3 year−1 and −2.7 μg m−3 year−1 for PM2.5 and PM10, respectively, over the study period. The mountainous terrain site (Mukteshwar, MUK) is primarily under the influence of air from the plains. This is due to convective transport processes that are enhanced by local and mesoscale topography, leading to pronounced valley/mountain winds and consequently to atmospheric boundary layer air lifting from the plains below. The transport from plains is evident in seasonal‐diurnal patterns observed at MUK. The timing of the patterns corresponds with changes in turbulence and water vapor (q). According to our analysis, using these as proxies is a viable method for examining boundary layer influence in the absence of direct atmospheric boundary layer height measurements. Comparing the measurements with climate models shows that even regional climate models have problems capturing the orographic influence accurately at MUK, highlighting the importance of long‐term direct measurements at multiple points to understand aerosol behavior in mountainous areas