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Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations

Seasonal variations in cloud droplet number concentration (NCD) in low-level stratiform clouds over the boreal forest are estimated from MODIS observations of cloud optical and microphysical properties, using a sub-adiabatic cloud model to interpret vertical profiles of cloud properties. An uncertainty analysis of the cloud model is included to reveal the main sensitivities of the cloud model. We compared the seasonal cycle in NCD, obtained using 9 yr of satellite data, to surface concentrations of potential cloud activating aerosols, measured at the SMEAR II station at Hyytiälä in Finland. The results show that NCD and cloud condensation nuclei (CCN) concentrations have no clear correlation at seasonal time scale. The fraction of aerosols that actually activate as cloud droplet decreases sharply with increasing aerosol concentrations. Furthermore, information on the stability of the atmosphere shows that low NCD is linked to stable atmospheric conditions. Combining these findings leads to the conclusion that cloud droplet activation for the studied clouds over the boreal forest is limited by convection. Our results suggest that it is important to take the strength of convection into account when studying the influence of aerosols from the boreal forest on cloud formation, although they do not rule out the possibility that aerosols from the boreal forest affect other types of clouds with a closer coupling to the surface.
Variation of upper tropospheric clouds and water vapor over the Indian ocean

The upper tropospheric (UT) ice water content (IWC) and water vapor (H2O) observed by the Microwave Limb Sounder (MLS) show dominant dipole mode variability over the Indian Ocean. This is characterized by the oscillating differences between the Western and Eastern Indian Ocean (WIO and EIO) with greater amplitude in JJA and SON than in other seasons. We denote δ X = X_WIO–X_EIO, with X being H2O and IWC at three UT levels (215 hPa, 147 hPa and 100 hPa) as well as sea surface temperature (SST), following the definition for previously identified Indian Ocean Dipole (IOD) variability. We found a strong positive correlation of δIWC at three UT levels with δSST, and a relatively weak positive correlation of δIWC with Nino 3.4 SST, suggesting that the UT clouds over the Indian Ocean are largely controlled by local thermally-driven circulation while teleconnection to ENSO plays a secondary role. The change per degree of δSST for δIWC in SON is 5.5 mg m−3 C−1 at 215 hPa, 1.6 mg m−3 C−1 at 147 hPa and 0.13 mg m−3 C−1 at 100 hPa (the 7-yr mean δIWC is −4.7 mg m−3, −1.6 mg m−3 and −0.13 mg m−3 at 215 hPa, 147 hPa and 100 hPa respectively). For δH2O, the per degree δSST change of 41.2 ppmv C−1 corresponds to a strong increase at 215 hPa and a decrease of −0.23 ppmv C−1 (−0.18 ppmv C−1) at 100 hPa (147 hPa), respectively. The Nino 3.4 SST has a relatively weak positive (negative) correlation with δ H2O at 215 hPa (100 hPa). The increase of δH2O at 215 hPa with increasing δSST is associated with the sharper contrast in convective intensity while the decrease of δH2O at 100 hPa with increasing δSST is a signature of the "convective cold top" and temperature control of 100 hPa H2O. For H2O, the 147 hPa marks a transition from the convection-controlled 215 hPa to the temperature-controlled 100 hPa.
Surface modification of mineral dust particles by sulphuric acid processing: implications for ice nucleation abilities

The ability of coated mineral dust particles to act as ice nuclei (IN) was investigated at LACIS (Leipzig Aerosol Cloud Interaction Simulator) during the FROST1- and FROST2-campaigns (Freezing of dust). Sulphuric acid was condensed on the particles which afterwards were optionally humidified, treated with ammonia vapour and/or heat. By means of aerosol mass spectrometry we found evidence that processing of mineral dust particles with sulphuric acid leads to surface modifications of the particles. These surface modifications are most likely responsible for the observed reduction of the IN activation of the particles. The observed particle mass spectra suggest that different treatments lead to different chemical reactions on the particle surface. Possible chemical reaction pathways and products are suggested and the implications on the IN efficiency of the treated dust particles are discussed.
Relativistic electron beams above thunderclouds

Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency &sim;40–400 kHz which they radiate. The electron beams occur &sim;2–9 ms after positive cloud-to-ground lightning discharges at heights between &sim;22–72 km above thunderclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the electron beam. One electron beam was detected without any luminous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning electric field above thunderclouds and thereby gain a mean energy of &sim;7 MeV to transport a total charge of &sim;−10 mC upwards. The impulsive current &sim;3 × 10−3 Am−2 associated with relativistic electron beams above thunderclouds is directed downwards and needs to be considered as a novel element of the global atmospheric electric circuit.
Spatial features of rain frequency change and pollution and associated aerosols

A spatial-temporal analysis has been conducted using satellite observed distributions of rain frequency, NO2 concentration and aerosol, with focus on the spring season in East Asia. As NO2 is a key precursor of secondary aerosols, especially in urban areas, an increase of NO2 emission is generally accompanied by an increase of fine aerosol particles. Comparison between trends in rain frequency and in precipitation amount shows that the changes in precipitation are more due to changes in precipitation occurrence than in precipitation amount. The overall feature emerged from the region-by-region analyses is that there is an inverse relationship between the rain frequency and the pollution and associated aerosols at continental scale in spring. The change in rain frequency is associated with changes in pollution-produced aerosols and long-range transport mineral dust. The inverse relationship at large temporal and spatial scales illustrates potential climatological consequence of changed pollution and aerosols on precipitation. Due to relatively short duration of observation and the potential uncertainty and bias associated with satellite measurements, more robust longer-term statistical study at various temporal and spatial scales and detailed modeling investigation are warranted to understand the physical causality of observed relationship between the rain frequency and the pollution and associated aerosols.
The sensitivity of Secondary Organic Aerosol component partitioning to the predictions of component properties – Part 2: Determination of particle hygroscopicity and its dependence on "apparent" volatility

A large number of calculations of absorptive partitioning of organic compounds have been conducted, making use of several methods to estimate pure component vapour pressures and activity coefficients (p0 and γi). The sensitivities of the predicted particle properties (density, hygroscopicity, CCN activation potential) to the choice of p0 and γi models and to the number of components used to represent the organic mixture have been systematically compared. The variability in theoretical hygroscopic growth factor attributable to the choice of estimation technique increases with decreasing mixture complexity. Generally there is low sensitivity to the choice of vapour pressure predictive technique. The inclusion of non-ideality is responsible for a larger difference in predicted growth factor, though still relatively minor. Assuming instantaneous equilibration of all semi-volatile on drying the aerosol to 0 % RH massively increases the sensitivity. Without such re-equilibration, the calculated growth factors are comparable to the low hygroscopicity of organic material widely measured in the laboratory and atmosphere. Allowing re-equilibration on drying produces a calculated hygroscopicity greater than measured for ambient organic material, and frequently close to those of common inorganic salts. Such a result has substantial implications on aerosol behaviour in instruments designed to measure hygroscopicity and on the degree of equilibration of semi-volatile components in the ambient atmosphere. The impacts of this variability on behaviour of particles as cloud condensation nuclei, on predicted cloud droplet number and uncertainty in radiative forcing are explored. When it is assumed only water evaporates on drying, the sensitivity in radiative forcing, "ΔF" to choice of p0 and γi estimation technique is low when the particle organic volume fraction is less than 55 %. Sensitivities increase with decreasing component complexity. If all components re-equilibrate on drying, the sensitivity of ΔF increases substantially for organic volume fractions as low as between 16 and 22 % depending on the complexity of the organic composition and assumed aerosol size distribution. The current study ignores the impact of predicted changes in particle size which will increase uncertainty in droplet number and forcing.
Radon activity in the lower troposphere and its impact on ionization rate: a global estimate using different radon emissions

The radioactive decay of radon and its progeny can lead to ionization of air molecules and consequently influence aerosol size distribution. In order to provide a global estimate of the radon-related ionization rate, we use the global atmospheric model ECHAM5 to simulate transport and decay processes of the radioactive tracers. A global radon emission map is put together using regional fluxes reported recently in the literature. Near-surface radon concentrations simulated with this new map compare well with measurements. Radon-related ionization rate is calculated and compared to that caused by cosmic rays. The contribution of radon and its progeny clearly exceeds that of the cosmic rays in the mid- and low-latitude land areas in the surface layer. During cold seasons, at locations where high concentration of sulfuric acid gas and low temperature provide potentially favorable conditions for nucleation, the coexistence of high ionization rate may help enhance the particle formation processes. This suggests that it is probably worth investigating the impact of radon-induced ionization on aerosol-climate interaction in global models.
Measurements of gaseous H2SO4 by AP-ID-CIMS during CAREBeijing 2008 Campaign

As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBeijing 2008), measurements of gaseous sulfuric acid (H2SO4) have been conducted at an urban site in Beijing, China from 7 July to 25 September 2008 using atmospheric pressure ion drift – chemical ionization mass spectrometry (AP-ID-CIMS). This represents the first gaseous H2SO4 measurements in China. Diurnal profile of sulfuric acid is strongly dependent on the actinic flux, reaching a daily maximum around noontime and with an hourly average concentration of 5 × 106 molecules cm−3. Simulation of sulfuric acid on the basis of the measured sulfur dioxide concentration, photolysis rates of ozone and nitrogen dioxide, and aerosol surface areas captures the trend of the measured H2SO4 diurnal variation within the uncertainties, indicating that photochemical production and condensation onto preexisting particle surface dominate the observed diurnal H2SO4 profile. The frequency of the peak H2SO4 concentration exceeding 5 × 106 molecules cm−3 increases by 16 % during the period of the summer Olympic Games (8–24 August 2008), because of the implementation of air quality control regulations. Using a multivariate statistical method, the critical nucleus during nucleation events is inferred, containing two H2SO4 molecules (R2 = 0.85). The calculated condensation rate of H2SO4 can only account for 10–25 % of PM1 sulfate formation, indicating that either much stronger sulfate production exists at the SO2 source region or other sulfate production mechanisms are responsible for the sulfate production.
Global dust model intercomparison in AeroCom phase I

This study presents the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations related to desert dust aerosols, their direct radiative effect, and their impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional comparisons to Angström exponent (AE), coarse mode AOD and dust surface concentrations are included to extend the assessment of model performance and to identify common biases present in models. These data comprise a benchmark dataset that is proposed for model inspection and future dust model development. There are large differences among the global models that simulate the dust cycle and its impact on climate. In general, models simulate the climatology of vertically integrated parameters (AOD and AE) within a factor of two whereas the total deposition and surface concentration are reproduced within a factor of 10. In addition, smaller mean normalized bias and root mean square errors are obtained for the climatology of AOD and AE than for total deposition and surface concentration. Characteristics of the datasets used and their uncertainties may influence these differences. Large uncertainties still exist with respect to the deposition fluxes in the southern oceans. Further measurements and model studies are necessary to assess the general model performance to reproduce dust deposition in ocean regions sensible to iron contributions. Models overestimate the wet deposition in regions dominated by dry deposition. They generally simulate more realistic surface concentration at stations downwind of the main sources than at remote ones. Most models simulate the gradient in AOD and AE between the different dusty regions. However the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models simulate the offshore transport of West Africa throughout the year but they overestimate the AOD and they transport too fine particles. The models also reproduce the dust transport across the Atlantic in the summer in terms of both AOD and AE but not so well in winter-spring nor the southward displacement of the dust cloud that is responsible of the dust transport into South America. Based on the dependency of AOD on aerosol burden and size distribution we use model bias with respect to AOD and AE to infer the bias of the dust emissions in Africa and the Middle East. According to this analysis we suggest that a range of possible emissions for North Africa is 400 to 2200 Tg yr−1 and in the Middle East 26 to 526 Tg yr−1.
Simulating the oxygen content of ambient organic aerosol with the 2D volatility basis set

A module predicting the oxidation state of organic aerosol (OA) has been developed using the two-dimensional volatility basis set (2D-VBS) framework. This model is an extension of the 1D-VBS framework and tracks saturation concentration and oxygen content of organic species during their atmospheric lifetime. The host model, a one-dimensional Lagrangian transport model, is used to simulate air parcels arriving at Finokalia, Greece during the Finokalia Aerosol Measurement Experiment in May 2008 (FAME-08). Extensive observations were collected during this campaign using an aerosol mass spectrometer (AMS) and a thermodenuder to determine the chemical composition and volatility, respectively, of the ambient OA. Although there are several uncertain model parameters, the consistently high oxygen content of OA measured during FAME-08 (O:C = 0.8) can help constrain these parameters and elucidate OA formation and aging processes that are necessary for achieving the high degree of oxygenation observed. The base-case model reproduces observed OA mass concentrations (measured mean = 3.1 μg m−3, predicted mean = 3.3 μg m−3) and O:C (predicted O:C = 0.78) accurately. A suite of sensitivity studies explore uncertainties due to (1) the anthropogenic secondary OA (SOA) aging rate constant, (2) assumed enthalpies of vaporization, (3) the volatility change and number of oxygen atoms added for each generation of aging, (4) heterogeneous chemistry, (5) the oxidation state of the first generation of compounds formed from SOA precursor oxidation, and (6) biogenic SOA aging. Perturbations in most of these parameters do impact the ability of the model to predict O:C well throughout the simulation period. By comparing measurements of the O:C from FAME-08, several sensitivity cases including a high oxygenation case, a low oxygenation case, and biogenic SOA aging case are found to unreasonably depict OA aging, keeping in mind that this study does not consider possibly important processes like fragmentation that may offset mass gains and affect the prediction bias. On the other hand, many of the cases chosen for this study predict average O:C estimates that are consistent with the observations, illustrating the need for more thorough experimental characterizations of OA parameters including the enthalpy of vaporization and oxidation state of the first generation of SOA products. The ability of the model to predict OA concentrations is less sensitive to perturbations in the model parameters than its ability to predict O:C. In this sense, quantifying O:C with a predictive model and constraining it with AMS measurements can reduce uncertainty in our understanding of OA formation and aging.