Aerosol CCN Research Articles

A multiple-charging correction algorithm for a broad-supersaturation scanning cloud condensation nuclei (BS2-CCN) system

Abstract. High time resolution (∼1 s) of aerosol hygroscopicity and CCN activity can be obtained with a broad-supersaturation scanning cloud condensation nuclei (BS2-CCN) system. Based on a commercial CCNC (CCN counter), the newly designed diffusive inlet in the BS2-CCN realizes a broad-supersaturation distribution in a chamber with a stable, low sheath-to-aerosol-flow ratio (SAR). In this way, a monotonic relation between the activation fraction of aerosols (Fact) and critical activation supersaturation (Saerosol) can be obtained. The accuracy of the size-resolved aerosol hygroscopicity, κ, measured by the BS2-CCN system can be, however, hampered by multiply charged particles, i.e., resulting in the overestimation of κ values. As the BS2-CCN system uses multiple and continuous supersaturations in the chamber and the size-resolved Fact value is directly used to derive κ values, the multiple-charging correction algorithm of the traditional CCNC where single supersaturation is applied does not work for the BS2-CCN observation. Here, we propose a new multiple-charging correction algorithm to retrieve the true Fact value. Starting from the largest size bin, a new Fact value at a specific particle diameter (Dp) is updated from a measured activation spectrum after removing both aerosol and the CCN number concentration of multiply charged particles using a kernel function with a given particle number size distribution. We compare the corrected activation spectrum with laboratory aerosols for a calibration experiment and ambient aerosols during the 2021 Yellow-Sea Air Quality Studies (YES-AQ) campaign. It is noted that this algorithm is only applied to the monomodal particle distribution. The difference between corrected and measured κ values can be as large as 0.08 within the measured κ values that range between 0.11 and 0.37 among the selected samples, highlighting that the multiple-charge effect should be considered for the ambient aerosol measurement. Furthermore, we examine how particle number size distribution is linked to the deviation of activation spectra and κ values.

African smoke particles act as cloud condensation nuclei in the wintertime tropical North Atlantic boundary layer over Barbados

Abstract. The number concentration and properties of aerosol particles serving as cloud condensation nuclei (CCN) are important for understanding cloud properties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure the low-albedo ocean surface. Studies linking aerosol source, composition, and water uptake properties in this region have been conducted primarily during the summertime dust transport season, despite the region receiving a variety of aerosol particle types throughout the year. In this study, we compare size-resolved aerosol chemical composition data to the hygroscopicity parameter κ derived from size-resolved CCN measurements made during the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. We observed unexpected periods of wintertime long-range transport of African smoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN number concentrations as well as the proportions of dust and smoke particles increased, whereas the average κ slightly decreased (κ=0.46±0.10) from marine background conditions (κ=0.52±0.09) when the submicron particles were mostly composed of marine organics and sulfate. Size-resolved chemical analysis shows that smoke particles were the major contributor to the accumulation mode during long-range transport events, indicating that smoke is mainly responsible for the observed increase in CCN number concentrations. Earlier studies conducted at Barbados have mostly focused on the role of dust on CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Our findings highlight the importance of African smoke for atmospheric processes and cloud formation over the Caribbean.

Seasonal significance of new particle formation impacts on cloud condensation nuclei at a mountaintop location

Abstract. New particle formation (NPF) events are defined as a sudden burst of aerosols followed by growth and can impact climate by growing to larger sizes and under proper conditions, potentially forming cloud condensation nuclei (CCN). Field measurements relating NPF and CCN are crucial in expanding regional understanding of how aerosols impact climate. To quantify the possible impact of NPF on CCN formation, it is important to not only maintain consistency when classifying NPF events but also consider the proper timeframe for particle growth to CCN-relevant sizes. Here, we analyze 15 years of direct measurements of both aerosol size distributions and CCN concentrations and combine them with novel methods to quantify the impact of NPF on CCN formation at Storm Peak Laboratory (SPL), a remote, mountaintop observatory in Colorado. Using the new automatic method to classify NPF, we find that NPF occurs on 50 % of all days considered in the study from 2006 to 2021, demonstrating consistency with previous work at SPL. NPF significantly enhances CCN during the winter by a factor of 1.36 and during the spring by a factor of 1.54, which, when combined with previous work at SPL, suggests the enhancement of CCN by NPF occurs on a regional scale. We confirm that events with persistent growth are common in the spring and winter, while burst events are more common in the summer and fall. A visual validation of the automatic method was performed in the study. For the first time, results clearly demonstrate the significant impact of NPF on CCN in montane North American regions and the potential for widespread impact of NPF on CCN.

On the submicron aerosol distributions and CCN activity in and around the Korean Peninsula measured onboard the NASA DC-8 research aircraft during the KORUS-AQ field campaign

Aerosol and CCN number concentrations, and aerosol size distributions in and around the Korean Peninsula were measured onboard the NASA DC-8 research aircraft during the KORUS-AQ campaign in May and June of 2016. The average and standard deviation values of total aerosol number concentrations (diameter > 10 nm, NCN10) in the boundary layer (below 1 km altitude) over Seoul, the Korean Peninsula, the Yellow Sea, the East Sea, and the South Sea were 12,318±7808, 10,721±8636, 7599±8583, 5759±6051, and 4186±4777 cm−3, respectively. Corresponding values for CCN number concentration at 0.6% supersaturation (NCCN06) were 3358±1916, 3613±2242, 3422±1707, 2313±1039, and 1819±1328 cm−3, respectively. NCN10 was highly variable according to region and period. In contrast, NCCN06 was relatively invariant regardless of region and period. With the PMF analysis on the aircraft measured aerosol size distribution and NCCN06 data, it was found that aerosol size distributions were composed of several distinct modes and they were different depending on regions. The aerosol hygroscopicity parameter κ was also estimated based on the PMF analysis results. The estimated κ values over Seoul, the Korean Peninsula, and the Yellow Sea were 0.11, 0.11, and 0.36, respectively. Lastly, the CCN closure experiments indicated that CCN concentration could be successfully estimated with the aerosol size distribution data and the estimated κ information from the aircraft measurements. Overall, it was demonstrated that PMF analysis technique could be applied to extract valuable information from limited aircraft measurement dataset.

Characterization of aerosol hygroscopicity, mixing state, and CCN activity at a suburban site in the central North China Plain

Abstract. This study investigates aerosol hygroscopicity, mixing state, and cloud condensation nucleation as part of the Atmosphere–Aerosol–Boundary Layer–Cloud Interaction Joint Experiment performed in the summer of 2016 at Xingtai (XT), a suburban site located in the center of the North China Plain (NCP). In general, the probability density function (PDF) of the hygroscopicity parameter (κ) for 40–200 nm particles had a unimodal distribution, and mean κ-PDF patterns for different sizes were similar, suggesting that the particles were highly aged and internally mixed because of strong photochemical reactions. The κ calculated from the hygroscopic growth factor in the daytime and at night suggests that photochemical reactions largely enhanced the aerosol hygroscopicity. This effect became weaker as the particle size increased. In addition, the aerosol hygroscopicity was much larger at XT than at other sites in the NCP. This is because new particle formation takes place much more frequently in the central NCP, which is heavily polluted from industrial activities, than elsewhere in the region. The evolution of the planetary boundary layer played a dominant role in dictating aerosol mass concentration. Particle size was the most important factor influencing the ability of aerosols to activate, whereas the effect of chemical composition was secondary, especially when supersaturation was high. Using a fixed value of κ=0.31 to calculate the cloud condensation nuclei number concentration in this region suffices.

Growth rates of fine aerosol particles at a site near Beijing in June 2013

Growth of fine aerosol particles is investigated during the Aerosol–CCN–Cloud Closure Experiment campaign in June 2013 at an urban site near Beijing. Analyses show a high frequency (∼ 50%) of fine aerosol particle growth events, and show that the growth rates range from 2.1 to 6.5 nm h−1 with a mean value of ∼ 5.1 nm h−1. A review of previous studies indicates that at least four mechanisms can affect the growth of fine aerosol particles: vapor condensation, intramodal coagulation, extramodal coagulation, and multi-phase chemical reaction. At the initial stage of fine aerosol particle growth, condensational growth usually plays a major role and coagulation efficiency generally increases with particle sizes. An overview of previous studies shows higher growth rates over megacity, urban and boreal forest regions than over rural and oceanic regions. This is most likely due to the higher condensational vapor, which can cause strong condensational growth of fine aerosol particles. Associated with these multiple factors of influence, there are large uncertainties for the aerosol particle growth rates, even at the same location.

Aerosol processing and CCN formation of an intense Saharan dust plume during the EUCAARI 2008 campaign

Abstract. Atmospheric processing and CCN formation of Saharan dust is illustrated through the analysis of a case of dust transport over northern Europe. This spread of dust is investigated by combining satellite, airborne and ground-based observations and the non-hydrostatic meso-scale model Meso-NH. The altitude of the dust plume during its transport to northwestern Europe was assessed using the CALIPSO observations and our model results. The major dust plume was transported toward Mediterranean and European regions between 2 and 5 km above sea level (a.s.l.). This is confirmed by an average particle depolarization ratio equal to 30%. Due to transport, this layer split into two layers over northern Europe, and we analyzed in this paper possible mixing of the European pollution aerosol with dust particles in the lower layer. The simulations have shown that the lower dust layer has interacted with the anthropogenic aerosol mainly over Belgium and the Netherlands. The analyses of numerical simulation results show that mineral dust particles accumulated soluble material through internal mixing over the Netherlands. The value of the CCN0.2 / CN ratio obtained over the Netherlands (~ 70%) is much greater than those observed over the Saharan region. In addition over the Netherlands, the CCN measurement reached 14 000 particles cm−3 at 0.63% supersaturation level on 30 May. Our model results reveal that more than 70% of the CCN concentration observed on 30 May can be explained by the presence of Saharan aged dust. The study reveals that heterogeneous reactions with inorganic salts converted this Saharan mineral dust into compounds that were sufficiently soluble to impact hygroscopic growth and cloud droplet activation over the Netherlands.

On the submicron aerosol distributions and CCN number concentrations in and around the Korean Peninsula

Abstract. Total number concentrations of particles having a diameter larger than 10 nm (NCN), cloud condensation nuclei at several supersaturation (S) values (NCCN) and number size distributions of particles with 10–414 nm diameter were measured in Seoul between 2004 and 2010. Overall average values of NCN and geometric mean diameter were 17 811 ± 5581 cm−3 and 48 ± 6 nm. Average NCCN at 0.4, 0.6 and 0.8% S were 4145 ± 2016, 5323 ± 2453 and 6067 ± 2780 cm−3 and corresponding NCCN / NCN were 0.26 ± 0.11, 0.33 ± 0.11 and 0.37 ± 0.12. There is a clear seasonal variation in aerosol concentration, which seems to be due to the monsoon. NCN and NCCN are also found to depend on the volume of traffic and the height of the planetary boundary layer, respectively. During aircraft campaigns in 2009 and 2011, NCN and NCCN at 0.6% S (N0.6%) were measured in and around the Korean Peninsula. During the 2011 campaign, the aerosol scattering coefficient was also measured. NCN and N0.6% in the lower altitudes were generally higher than at higher altitudes, except for cases when particle formation and growth events were thought to occur at higher altitudes. NCN and N0.6% generally show a positive correlation with aerosol scattering coefficients but this correspondence tends to vary with altitude. Occasional instances of low (< 0.3) N0.6% / NCN in the boundary layer are demonstrated to be associated with particle formation and growth events. With the support of ground measurements, it is confirmed that a particle formation and growth event did indeed occur over the Yellow Sea on a flight day, and the areal extent of this event is estimated to be greater than 100 km × 450 km. With the combination of the current and several relevant previous studies, a composite map of NCN and NCCN in and around the Korean Peninsula is produced. Overall, the exhibited concentrations are typical of values measured over polluted regions elsewhere on the globe. Moreover, there is a generally decreasing trend from west to east over the region, implying that the region is constantly under the dominant influence of continental outflow.

Aerosol mixing state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006

Abstract. Observations of aerosol hygroscopic growth and CCN activation spectra for submicron particles are reported for the T1 ground site outside of Mexico City during the MIRAGE 2006 campaign. κ-Köhler theory is used to evaluate the characteristic hygroscopicity parameter, κ*, for the CCN active aerosol population using both size-resolved HTMDA and size-resolved CCNc measurements. Organic mass fractions (forg) are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which predictions of the hygroscopicity parameter are compared against κ*. Strong diurnal changes in aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased κ* and CCN-active fraction during the daytime, with greater impact on smaller particles. During NPF events, the number concentration of 40 nm particles acting as CCN at 0.51% ± 0.06% supersaturation can surpass by more than a factor of two the corresponding concentrations of 100 nm particles. We also find that at 06:00–08:00 LT throughout the campaign, fresh traffic emissions result in substantial changes to the chemical distribution of the aerosol, with on average 65% externally mixed fraction for 40 nm particles and 30% externally mixed fraction for 100 nm particles, whereas at midday nearly all particles of both sizes can be described as "internally mixed". Average activation spectra and growth factor distributions are analyzed for different time periods characterizing the daytime (with and without NPF events), the early morning "rush hour" and the entire campaign. We show that κ* derived from CCNc measurements decreases as a function of size during all time periods, while the CCN-active fraction increases as a function of size. Size-resolved AMS measurements do not predict the observed trend for κ* versus particle size, which can be attributed to unresolved mixing state and the presence of refractory material not measured by the AMS. Measured κ* typically ranges from 0.2 to 0.35, and organics typically make up 60–85 % of the aerosol mass in the size range studied. We show that κAMS is able to describe CCN concentrations reasonably well, provided mixing-state information is available, especially at the highest CCN concentrations. This is consistent with other CCN studies carried out in urban environments, and is partly due to the fact that the highest CCN concentrations occur during the daytime when the aerosol is internally mixed. During the early morning rush hour, however, failing to account for the aerosol mixing state results in systematic overestimation of CCN concentrations by as much as 50–100% on average.

Ternary homogeneous nucleation of H<sub>2</sub>SO<sub>4</sub>, NH<sub>3</sub>, and H<sub>2</sub>O under conditions relevant to the lower troposphere

Abstract. Ternary homogeneous nucleation (THN) of H2SO4, NH3 and H2O has been used to explain new particle formation in various atmospheric regions, yet laboratory measurements of THN have failed to reproduce atmospheric observations. Here, we report first laboratory observations of THN made under conditions relevant to the lower troposphere ([H2SO4] of 106–107 cm−3, [NH3] of 0.08–20 ppbv, and a temperature of 288 K). Our observations show that NH3 can enhance atmospheric H2SO4 aerosol nucleation and the enhancement factor (EF) in nucleation rate (J) due to NH3 (the ratio of J measured with vs. without NH3) increases linearly with increasing [NH3] and increases with decreasing [H2SO4] and RH. Two chemical ionization mass spectrometers (CIMS) are used to measure [H2SO4] and [NH3], as well as possible impurities of amines in the nucleation system. Aerosol number concentrations are measured with a water condensation counter (CPC, TSI 3786). The slopes of Log J vs. Log [H2SO4], Log J vs. Log RH, and Log J vs. Log [NH3] are 3–5, 1–4, and 1, respectively. These slopes and the threshold of [H2SO4] required for the unity nucleation vary only fractionally in the presence and absence of NH3. These observations can be used to improve aerosol nucleation models to assess how man-made SO2 and NH3 affect aerosol formation and CCN production at the global scale.

Adaptive method of lines for multi-component aerosol condensational growth and CCN activation

Abstract. The process of formation of cloud droplets on an ensemble of aerosol particles is modelled by numerous investigators using the method of lines (MOL). The method involves discretisation of the aerosol size spectrum into bins whose positions evolve with time. One of the drawbacks of the method is its poor representation of the aerosol spectrum shape in the region between the unactivated aerosol mode and the activated droplet mode. An adaptive grid refinement procedure is introduced. The procedure splits any overly wide bins into several narrower ones during integration. The number of new bins added is a function of particle concentration in the bin being split. Application of the grid refinement procedure results in suppression of the sensitivity of the computed cloud droplet spectrum characteristics such as droplet number concentration or effective radius to the initial grid choice. A model of droplet formation on multi-component aerosol is formulated for the purpose of the study. Model formulation includes explicit treatment of the droplet temperature evolution. Several examples of the model set-up are used to demonstrate model capabilities. Model results are compared to those without adaptivity. A C++ implementation of the model is available as an electronic supplement of the paper.

The importance of aerosol mixing state and size-resolved composition on CCN concentration and the variation of the importance with atmospheric aging of aerosols

Abstract. Aerosol microphysics, chemical composition, and CCN concentrations were measured at the T0 urban supersite in Mexico City during Megacity Initiative: Local and Global Research Observations (MILAGRO) in March 2006. The aerosol size distribution and composition often showed strong diurnal variation associated with traffic emissions and aging of aerosols through coagulation and local photochemical production of secondary aerosol species. CCN concentrations (NCCN) are derived using Köhler theory from the measured aerosol size distribution and various simplified aerosol mixing state and chemical composition, and are compared to concurrent measurements at five supersaturations ranging from 0.11% to 0.35%. The influence of assumed mixing state on calculated NCCN is examined using both aerosols observed during MILAGRO and representative aerosol types. The results indicate that while ambient aerosols often consist of particles with a wide range of compositions at a given size, NCCN may be derived within ~20% assuming an internal mixture (i.e., particles at a given size are mixtures of all participating species, and have the identical composition) if great majority of particles has an overall κ (hygroscopicity parameter) value greater than 0.1. For a non-hygroscopic particle with a diameter of 100 nm, a 3 nm coating of sulfate or nitrate is sufficient to increase its κ from 0 to 0.1. The measurements during MILAGRO suggest that the mixing of non-hygroscopic primary organic aerosol (POA) and black carbon (BC) particles with photochemically produced hygroscopic species and thereby the increase of their κ to 0.1 take place in a few hours during daytime. This rapid process suggests that during daytime, a few tens of kilometers away for POA and BC sources, NCCN may be derived with sufficient accuracy by assuming an internal mixture, and using bulk chemical composition. The rapid mixing also indicates that, at least for very active photochemical environments such as Mexico City, the timescale during daytime for the conversion of hydrophobic POA and BC to hydrophilic particles is substantially shorter than the 1–2 days used in some global models. The conversion time scale is substantially longer during night. Most POA and BC particles emitted during evening hours likely remain non-hygroscopic until efficiently internally mixed with secondary species in the next morning. The results also suggest that the assumed mixing state strongly impacts calculated NCCN only when POA and BC represent a large fraction of the total aerosol volume. One of the implications is that while physically unrealistic, external mixtures, which are used in many global models, may also sufficiently predict NCCN for aged aerosol, as the contribution of non-hygroscopic POA and BC to overall aerosol volume is often substantially reduced due to the condensation of secondary species.

Consistency between parameterisations of aerosol hygroscopicity and CCN activity during the RHaMBLe discovery cruise

Abstract. Results from a measurement study performed in the Tropical Atlantic on board the RHaMBLe Discovery Cruise D319 are presented. Measurements of aerosol composition, hygroscopicity and CCN activity were used to test the ability of a single parameter model to describe water uptake in sub- and supersaturated conditions. It was found that the magnitude and temporal variability of the sub-saturated water uptake could be well represented using the non-refractory composition to derive the model input for 2 periods when the large majority of the aerosol mass was non-refractory. As may be expected, when a significant fraction of the aerosol volume is refractory the sub-saturated water uptake is not well predicted by the non-refractory composition. When predicting the cloud activation potential from the composition and the hygroscopicity there is a consistent under-prediction of the CCN activity. The prediction of CCN activity from the sub-saturated water uptake gives a better prediction of the CCN activity than the composition when the non-refractory components are not fully representative of the aerosol composition. Based on these observations it appears that a single parameter cannot always capture the behavior fully across the sub- and supersaturated regimes. Measurements made at relative humidities (RHs) up to 94% showed that the water activity appears satisfactorily represented by a single parameter derived at 90% RH. It therefore appears that the change in the observed hygroscopicity take place between 94% RH and the point of activation. This change may be due in part to a change solution non-ideality, surface tension effects or the presence of sparingly soluble compounds for example, but cannot be reconciled without measurements at higher RHs.

On the additivity of climate response to anthropogenic aerosols and CO2, and the enhancement of future global warming by carbonaceous aerosols

Climate responses to aerosol forcing at present-day and doubled CO 2 -levels are studied based on equilibrium simulations with the CCM-Oslo atmospheric GCM coupled to a slab ocean. Aerosols interact on-line with meteorology through life-cycling of sulphate and black carbon (BC), and tables for aerosol optics and CCN activation. Anthropogenic aerosols counteract the warming by CO 2 through a negative radiative forcing dominated by the indirect effect. Anthropogenic aerosols reduce precipitation by 4%, while CO 2 doubling gives a 5% increase, mainly through enhanced convective activity, including a narrower ITCZ. Globally, the aerosol cooling is insensitive to CO 2 , and the effects of CO 2 doubling are insensitive to aerosols. Hence, global climate responses to these sources of forcing are almost additive, although sulphate and BC burdens are slightly increased due to reduced stratiform precipitation over major anthropogenic source regions and a modified ITCZ. Regionally, positive cloud feedbacks give up to 5 K stronger aerosol cooling at present-day CO 2 than after CO 2 doubling. Aerosol emissions projected for year-2100 (SRES A2) strongly increase BC and change the sign of the direct effect. This results in a 0.3 K warming and 0.1% increase in precipitation compared to the year 2000, thus enhancing the global warming by greenhouse gases.

On the additivity of climate response to anthropogenic aerosols and CO2, and the enhancement of future global warming by carbonaceous aerosols

Climate responses to aerosol forcing at present-day and doubledCO2-levels are studied based on equilibrium simulations with the CCM-Oslo atmospheric GCM coupled to a slab ocean. Aerosols interact on-line with meteorology through life-cycling of sulphate and black carbon (BC), and tables for aerosol optics and CCN activation. Anthropogenic aerosols counteract the warming by CO2 through a negative radiative forcing dominated by the indirect effect. Anthropogenic aerosols reduce precipitation by 4%, while CO2 doubling gives a 5% increase, mainly through enhanced convective activity, including a narrower ITCZ. Globally, the aerosol cooling is insensitive to CO2, and the effects of CO2 doubling are insensitive to aerosols. Hence, global climate responses to these sources of forcing are almost additive, although sulphate and BC burdens are slightly increased due to reduced stratiform precipitation over major anthropogenic source regions and a modified ITCZ. Regionally, positive cloud feedbacks give up to 5 K stronger aerosol cooling at present-day CO2 than after CO2 doubling. Aerosol emissions projected for year-2100 (SRES A2) strongly increase BC and change the sign of the direct effect. This results in a 0.3 K warming and 0.1% increase in precipitation compared to the year 2000, thus enhancing the global warming by greenhouse gases.

Submicron aerosol size distributions and cloud condensation nuclei concentrations measured at Gosan, Korea, during the Atmospheric Brown Clouds–East Asian Regional Experiment 2005

Submicron aerosol size distributions, CN and CCN concentrations at a constant supersaturation of 0.6% were measured at a relatively remote coastal site at Gosan in Jeju Island, Korea, during the ABC‐EAREX from 11 March to 8 April 2005. The average CN concentrations were 6088 ± 3988, 5231 ± 2454 and 3513 ± 1790 cm−3, respectively, for the three major air mass types classified by their origins. The corresponding CCN concentrations were 2393 ± 1156, 2897 ± 1226 and 1843 ± 585 cm−3. The type III air mass was the closest to maritime origins, but these lowest concentrations at Gosan were an order of magnitude higher than those of clean marine boundary layer, indicating that regardless of air mass designation springtime submicron aerosols at Gosan were under steady continental influences. Distinct new particle formation and growth events occurred on 6 d, when clear sky weather conditions prevailed that brought air from northern China, Mongolia or Russia by anticyclonic circulations. Simultaneous occurrence of these events at a western coastal site in the Korean Peninsula 350 km north of Gosan suggests that these events were not local but at least regional‐scale events. CCN concentrations were predicted with the aerosol size distributions and the assumption of particles being composed of (NH4)2SO4. The predicted to measured CCN concentration ratio was 1.27 ± 0.29 and the r2 was 0.77 for the whole measurement period. The type I air mass that has the most continental influences showed a slight tendency to overpredict CCN concentrations but the good agreement overall suggests that springtime Gosan aerosols act almost like ammonium sulfate as far as CCN activity is concerned, almost regardless of air mass origin.

Response of the climate system to aerosol direct and indirect forcing: Role of cloud feedbacks

In this study, the response of the climate system to aerosol direct and indirect radiative forcing is investigated. Several multidecadal equilibrium simulations are carried out, using the NCAR CCM3 framework coupled to a separately developed aerosol treatment. The aerosol treatment includes, e.g., a life cycle scheme for particulate sulfate and black carbon (natural and anthropogenic), calculations of aerosol size distributions and CCN activation, as well as computations of direct and indirect forcing (1st and 2nd indirect effect). In all the simulations the full aerosol treatment is run online, hence responding interactively to changes in climate. By far the largest response is caused by the indirect forcing, with a globally averaged cooling of −1.25 K due to anthropogenic aerosols. The largest temperature reduction is found in the Northern Hemisphere because of a larger aerosol burden there. As a result of this cooling pattern, the Intertropical Convergence Zone is displaced southward by a few hundred kilometers. Interestingly, a similar, though less significant displacement is also found in the experiments with the direct effect alone, even though the globally averaged aerosol induced cooling is much weaker in that case, i.e., −0.08 K. The direct radiative forcing is much stronger at the surface than at the top of the atmosphere, and this leads to a slight weakening of the hydrological cycle. Comparing simulations with direct and indirect forcing combined to those with indirect and direct forcing separately, a residual, caused by nonlinear model feedbacks, is manifested through a reduction in precipitation. This reduction amounts to −0.5% in a global average and exceeds −2.5% in the Arctic, highlighting the role of high‐latitude climate feedbacks. Globally, cloud feedback is negative in the sense that in the colder climate resulting from anthropogenic aerosol forcing, net cloud forcing is reduced by 15% compared to the original climate state. This is caused by a general cloud thinning, especially at high latitudes, while in the most polluted regions, clouds are thicker through the 2nd indirect effect. The 1st indirect effect, on the other hand, remains intact in the presence of climate feedbacks, yielding a similar signature of cloud droplet reduction as in the pure forcing simulations.

On the retrieval of columnar aerosol mass and CCN concentration by MODIS

A parameterization is introduced for the derivation of columnar aerosol mass concentration (AMC) and cloud condensation nucleus concentration (CCNC) from the primary aerosol products of Moderate‐Resolution Imaging Spectroradiometer (MODIS). The method relies on the scaling between AMC and optical depth with a proportionality constant dependent on MODIS‐derived reff, η (contribution of the accumulation mode radiance to the total radiance), ambient RH, and an assumed constant aerosol composition. The CCNC is derived from a parameterization of concentration as a function of the retrieved aerosol volume. By comparing with in situ data, it is shown that retrievals in dry ambient conditions (dust) are improved when using a proportionality constant dependent on reff and η obtained in the same pixel. In high‐humidity environments, the improvement in the new method is inconclusive because of the difficulty in accounting for the uneven vertical distribution of relative humidity. Additionally, two detailed comparisons of AMC and CCNC retrieved by the MODIS Airborne Simulator (MAS) algorithm and the new method are shown. The new method and MAS retrievals of AMC are within the same order of magnitude with respect to the in situ measurements of aerosol mass. However, the proposed method is closer to the in situ measurements than the MODIS retrievals. The retrievals of CCNC are also within the same order of magnitude for both methods. Finally, the new method is applied to an actual MODIS retrieval. Although no in situ data is available for comparison, it is shown that the proposed method yields more credible values than the MODIS retrievals.

A 2-D spectral model simulation of the scavenging of gaseous and particulate sulfate by a warm marine cloud

Our 2-D dynamic model including spectral microphysics and a detailed treatment of the scavenging and processing of (NH 4) 2SO 4 and NaCl aerosol particles as well as SO 2, H 2O 2, and O 3 has been evaluated for a warm precipitating convective cloud at Day 261 (September 18, 1974) of the GATE campaign. The model determined the rate at which S(IV) was converted to S(VI) inside the cloud and rain water and the rate at which its pH changed. The rate of drop sulfate formation due to scavenging of particles was compared with the rate of sulfate formation due to the uptake and oxidation of SO 2. It was found that of the sulfate processed by the cloud 87% was scavenged by nucleation of sulfate containing aerosol particles, 6% of sulfate entered the drop through impaction scavenging below cloud base, almost 7% was produced by oxidation by H 2O 2, but only 0.2% by oxidation of O 3. Averaging over space and time we derived from the model that about 2 mg/l in the rain was due to particulate sulfate and 1 mg/l was due to sulfate from oxidized SO 2 in the drops. Oxidation of SO 2 was accompanied by an average pH of 4.4 in the rain water. For sulfur a washout ratio of 600 and a scavenging coefficient of about 10 −4 s −1 was calculated. In addition, a comparison of the aerosol particle and CCN spectrum at one point in the atmosphere before and after the cloud event showed that the size of the aerosol particles had increased and their number decreased such that the new CCN spectrum activates the particles at lower supersaturations.