Earth's Radiation Balance Research Articles

Two-year-long high-time-resolution apportionment of primary and secondary carbonaceous aerosols in the Los Angeles Basin using an advanced total carbon–black carbon (TC-BC(λ)) method

In recent years, carbonaceous aerosols (CA) have been recognized as a significant contributor to the concentration of particles smaller than 2.5 μm (i.e., PM2.5), with a negative impact on public health and Earth's radiative balance. In this study, we present a method for CA apportionment based on high-time-resolution measurements of total carbon (TC), black carbon (BC), and spectral dependence of absorption coefficient using a recently developed Carbonaceous Aerosol Speciation System (CASS). Two-year-long CA measurements at two different locations within California's Los Angeles Basin are presented. CA was apportioned based on its optical absorption properties, organic or elemental carbon composition, and primary or secondary origin. We found that the secondary organic aerosols (SOA), on average, represent >50 % of CA in the study area, presumably resulting from the oxidation of anthropogenic and biogenic volatile organic components. Remarkable peaks of SOA in summer afternoons were observed, with a fractional contribution of up to 90 %. On the other hand, the peak of primary emitted CA, consisting of BC and primary organic aerosol (POA), contributed >80 % to the CA during morning rush hours on winter working days. The light absorption of BC dominated over the brown carbon (BrC), which contributed to 20 % and 10 % of optical absorption at the lower wavelength of 370 nm during winter nights and summer afternoons, respectively. The highest contribution of BrC, up to 50 %, was observed during the wildfire periods. Although the uncertainty levels can be high for some CA components (such as split between primary emitted and secondary formed BrC during winter nights), further research focused on the optical properties of CA at different locations may help to better constrain the parameters used in CA apportionment studies. We believe that the CASS system combined with the apportionment method presented in this study can offer simplified and cost-effective insights into the composition of carbonaceous aerosols.

Effect of dust on rainfall over the Red Sea coast based on WRF-Chem model simulations

Abstract. Water is the single most important element of life. Rainfall plays an important role in the spatial and temporal distribution of this precious natural resource, and it has a direct impact on agricultural production, daily life activities, and human health. One of the important elements that govern rainfall formation and distribution is atmospheric aerosol, which also affects the Earth's radiation balance and climate. Therefore, understanding how dust compositions and distributions affect the regional rainfall pattern is crucial, particularly in regions with high atmospheric dust loads such as the Middle East. Although aerosol and rainfall research has garnered increasing attention as both an independent and interdisciplinary topic in the last few decades, the details of various direct and indirect pathways by which dust affects rainfall are not yet fully understood. Here, we explored the effects of dust on rainfall formation and distribution as well as the physical mechanisms that govern these phenomena, using high-resolution WRF-Chem simulations (∼ 1.5 km × 1.5 km) configured with an advanced double-moment cloud microphysics scheme coupled with a sectional eight-bin aerosol scheme. Our model-simulated results were realistic, as evaluated from multiple perspectives including vertical profiles of aerosol concentrations, aerosol size distributions, vertical profiles of air temperature, diurnal wind cycles, and spatio-temporal rainfall patterns. Rainfall over the Red Sea coast is mainly caused by warm rain processes, which are typically confined within a height of ∼ 6 km over the Sarawat mountains and exhibit a strong diurnal cycle that peaks in the evening at approximately 18:00 local time under the influence of sea breezes. Numerical experiments indicated that dust could both suppress or enhance rainfall. The effect of dust on rainfall was calculated as total, indirect, and direct effects, based on 10-year August-average daily-accumulated rainfall over the study domain covering the eastern Red Sea coast. For extreme rainfall events (domain-average daily-accumulated rainfall of ≥ 1.33 mm), the net effect of dust on rainfall was positive or enhancement (6.05 %), with the indirect effect (4.54 %) and direct effect (1.51 %) both causing rainfall increase. At a 5 % significance level, the total and indirect effects were statistically significant whereas the direct effect was not. For normal rainfall events (domain-average daily-accumulated rainfall < 1.33 mm), the indirect effect enhanced rainfall (4.76 %) whereas the direct effect suppressed rainfall (−5.78 %), resulting in a negative net suppressing effect (−1.02 %), all of which were statistically significant. We investigated the possible physical mechanisms of the effects and found that the rainfall suppression by dust direct effects was mainly caused by the scattering of solar radiation by dust. The surface cooling induced by dust weakens the sea breeze circulation, which decreases the associated landward moisture transport, ultimately suppressing rainfall. For extreme rainfall events, dust causes net rainfall enhancement through indirect effects as the high dust concentration facilitates raindrops to grow when the water vapor is sufficiently available. Our results have broader scientific and environmental implications. Specifically, although dust is considered a problem from an air quality perspective, our results highlight the important role of dust on sea breeze circulation and associated rainfall over the Red Sea coastal regions. Our results also have implications for cloud seeding and water resource management.

Absorption enhancement of black carbon particles in a Mediterranean city and countryside: effect of particulate matter chemistry, ageing and trend analysis

Abstract. Black carbon (BC) is recognized as the most important warming agent among atmospheric aerosol particles. The absorption efficiency of pure BC is rather well-known, nevertheless the mixing of BC with other aerosol particles can enhance the BC light absorption efficiency, thus directly affecting Earth's radiative balance. The effects on climate of the BC absorption enhancement due to the mixing with these aerosols are not yet well constrained because these effects depend on the availability of material for mixing with BC, thus creating regional variations. Here we present the mass absorption cross-section (MAC) and absorption enhancement of BC particles (Eabs), at different wavelengths (from 370 to 880 nm for online measurements and at 637 nm for offline measurements) measured at two sites in the western Mediterranean, namely Barcelona (BCN; urban background) and Montseny (MSY; regional background). The Eabs values ranged between 1.24 and 1.51 at the urban station, depending on the season and wavelength used as well as on the pure BC MAC used as a reference. The largest contribution to Eabs was due to the internal mixing of BC particles with other aerosol compounds, on average between a 91 % and a 100 % at 370 and 880 nm, respectively. Additionally, 14.5 % and 4.6 % of the total enhancement at the short ultraviolet (UV) wavelength (370 nm) was due to externally mixed brown carbon (BrC) particles during the cold and the warm period, respectively. On average, at the MSY station, a higher Eabs value was observed (1.83 at 637 nm) compared to BCN (1.37 at 637 nm), which was associated with the higher fraction of organic aerosols (OA) available for BC coating at the regional station, as denoted by the higher organic carbon to elemental carbon (OC:EC) ratio observed at MSY compared to BCN. At both BCN and MSY, Eabs showed an exponential increase with the amount of non-refractory (NR) material available for coating (RNR-PM). The Eabs at 637 nm at the MSY regional station reached values up to 3 during episodes with high RNR-PM, whereas in BCN, Eabs kept values lower than 2 due to the lower relative amount of coating materials measured at BCN compared to MSY. The main sources of OA influencing Eabs throughout the year were hydrocarbon OA (HOA) and cooking-related OA (COA), i.e. primary OA (POA) from traffic and cooking emissions, respectively, at both 370 and 880 nm. At the short UV wavelength (370 nm), a strong contribution to Eabs from biomass burning OA (BBOA) and less oxidized oxygenated OA (LO-OOA) sources was observed in the colder period. Moreover, we found an increase of Eabs with the ageing state of the particles, especially during the colder period. This increase of Eabs with particle ageing was associated with a larger relative amount of secondary OA (SOA) compared to POA. The availability of a long dataset at both stations from offline measurements enabled a decade-long trend analysis of Eabs at 637 nm, that showed statistically significant (s.s.) positive trends of Eabs during the warmer months at the MSY station. This s.s. positive trend in MSY mirrored the observed increase of the OC:EC ratio over time. Moreover, in BCN during the COVID-19 lockdown period in spring 2020 we observed a sharp increase of Eabs due to the observed sharp increase of the OC:EC ratio. Our results show similar values of Eabs to those found in the literature for similar background stations.

Lunar Observatory for monitoring the radiation balance of the Earth and climate

Continuous uniform high-precision data on the Earth's radiation balance over many decades and hundreds of years are essential for reliable research and prediction of climate change. This necessarily requires the use of long-term fixed, stable space platforms at a sufficient distance from Earth. However, measurements by all low-orbit and geostationary satellites cannot provide high-precision identifications of the radiation balance, since they observe its limited areas. Two identical especial optical robotic telescopes, of which the Lunar Observatory (LO) consists of, are a single system of sequentially functioning in the automatic mode of Earth observations. Telescopes will be installed along the equatorial zone at opposite edges of the Moon, 80.9±0.1° distant from its visible center. LO provides measurements of the variations in the magnitude of the specific power reflected by the planet into the space of the total solar irradiance in the spectral range Δλ=0.2–4 μm and outgoing intrinsic thermal radiation of the Earth in the ranges Δλ=4–50 μm and the atmospheric transparency window Δλ=8–13 μm. The system of two telescopes, replacing each other, will sequentially survey the entire Earth's surface continuously for more than 94% of the time of a lunar day only at lunar night time. LO will make it possible to determine the parameters of the average annual values of total energies departing from the Earth into space with an error of the order of 0.1%, which is almost an order of magnitude more accurate than the errors in the measurement of these parameters by instruments of any orbital spacecraft. Monitoring of energy variations in the range Δλ=8–13 μm will make it possible to control and determine the relative contribution of the variation in the concentration of greenhouse gases in the atmosphere to climate change.

Contrasting mineral dust abundances from X-ray diffraction and reflectance spectroscopy

Abstract. Mineral dust particles dominate aerosol mass in the atmosphere and directly modify Earth's radiative balance through absorption and scattering. This radiative forcing varies strongly with mineral composition, yet there is still limited knowledge on the mineralogy of atmospheric dust. In this study, we performed X-ray diffraction (XRD) and reflectance spectroscopy measurements on 37 different dust deposition samples collected as airfall in an urban setting to determine mineralogy and the relative proportions of minerals in the dust mixture. Most commonly, XRD has been used to characterize dust mineralogy; however, without prior special sample preparation, this technique is less effective for identifying poorly crystalline or amorphous phases. In addition to XRD measurements, we performed visible and short-wave infrared (VSWIR) reflectance spectroscopy for these natural dust samples as a complementary technique to determine mineralogy and mineral abundances. Reflectance spectra of dust particles are a function of a nonlinear combination of mineral abundances in the mixture. Therefore, we used a Hapke radiative transfer model along with a linear spectral mixing approach to derive relative mineral abundances from reflectance spectroscopy. We compared spectrally derived abundances with those determined semi-quantitatively from XRD. Our results demonstrate that total clay mineral abundances from XRD are correlated with those from reflectance spectroscopy and follow similar trends; however, XRD underpredicts the total amount of clay for many of the samples. On the other hand, calcite abundances are significantly underpredicted by SWIR compared to XRD. This is caused by the weakening of absorption features associated with the fine particle size of the samples, as well as the presence of dark non-mineral materials (e.g., asphalt) in these samples. Another possible explanation for abundance discrepancies between XRD and SWIR is related to the differing sensitivity of the two techniques (crystal structure vs. chemical bonds). Our results indicate that it is beneficial to use both XRD and reflectance spectroscopy to characterize airfall dust because the former technique is good at identifying and quantifying the SWIR-transparent minerals (e.g., quartz, albite, and microcline), while the latter technique is superior for determining abundances for clays and non-mineral components.

The Effect of Convective Injection of Ice on Stratospheric Water Vapor in a Changing Climate

AbstractStratospheric water vapor affects the Earth's radiative balance and stratospheric chemistry, yet its future changes are uncertain and not fully understood. The influence of deep convection on stratospheric water vapor remains subject to debate. This letter presents a detailed process‐based model study of the impact of convective ice sublimation on stratospheric water vapor in response to CO2 forced climate change. The influence of convective injection is found to be limited by the vertical profile of temperature and saturation vapor pressure in the tropical tropopause layer, not by the frequency of occurrence. Lagrangian trajectory analysis shows the relative contributions to stratospheric water vapor from sublimation and large‐scale transport are approximately unchanged when CO2 is increased. The results indicate the role of convective ice injection for stratospheric water vapor in a warmer climate remains constrained by large‐scale temperatures.

Assessing the potential for simplification in global climate model cloud microphysics

Abstract. Cloud properties and their evolution influence Earth's radiative balance. The cloud microphysical (CMP) processes that shape these properties are therefore important to represent in global climate models. Historically, parameterizations in these models have grown more detailed and complex. However, a simpler formulation of CMP processes may leave the model results mostly unchanged while enabling an easier interpretation of model results and helping to increase process understanding. This study employs sensitivity analysis of an emulated perturbed parameter ensemble of the global aerosol–climate model ECHAM-HAM to illuminate the impact of selected CMP cloud ice processes on model output. The response to the perturbation of a process serves as a proxy for the effect of a simplification. Autoconversion of ice crystals is found to be the dominant CMP process in influencing key variables such as the ice water path and cloud radiative effects, while riming of cloud droplets on snow has the most influence on the liquid phase. Accretion of ice and snow and self-collection of ice crystals have a negligible influence on model output and are therefore identified as suitable candidates for future simplifications. In turn, the dominating role of autoconversion suggests that this process has the greatest need to be represented correctly. A seasonal and spatially resolved analysis employing a spherical harmonics expansion of the data corroborates the results. This study introduces a new application for the combination of statistical emulation and sensitivity analysis to evaluate the sensitivity of a complex numerical model to a specific parameterized process. It paves the way for simplifications of CMP processes leading to more interpretable climate model results.

The effect of marine ice-nucleating particles on mixed-phase clouds

Abstract. Shallow marine mixed-phase clouds are important for the Earth's radiative balance, but modelling their formation and dynamics is challenging. These clouds depend on boundary layer turbulence and cloud top radiative cooling, which is related to the cloud phase. The fraction of frozen droplets depends on the availability of suitable ice-nucleating particles (INPs), which initiate droplet freezing. While mineral dust is the dominating INP type in most regions, high-latitude boundary layer clouds can be dependent on local marine INP emissions, which are often related to biogenic sources including phytoplankton. Here we use high resolution large eddy simulations to examine the potential effects of marine emissions on boundary layer INP concentrations and their effects on clouds. Surface emissions have a direct effect on INP concentration in a typical well-mixed boundary layer whereas a steep inversion can block the import of background INPs from the free troposphere. The importance of the marine source depends on the background INP concentration, so that marine INP emissions become more important with lower background INP concentrations. For the INP budget it is also important to account for INP recycling. Finally, with the high-resolution model we show how ice nucleation hotspots and high INP concentrations are focused on updraught regions. Our results show that marine INP emissions contribute directly to the boundary layer INP budget and therefore have an influence on mixed-phase clouds.

Detecting Supercooled Water Clouds Using Passive Radiometer Measurements

AbstractSupercooled water clouds (SWCs) have significant impacts on the Earth's radiation balance, aircraft ice accretion and precipitation augmentation. This study introduces an efficient algorithm to detect SWCs from passive radiometers, which combines information from the reflectance difference between 1.61 and 2.25 μm channels, the brightness temperature difference between the 8.5 and 11 μm channels, and the cloud top temperature. Validated by space radar and lidar measurements, our algorithm can correctly detect 91% of SWC pixels, better than current Visible Infrared Imager Radiometer Suite operational product. SWCs are found mostly over the mid‐ to high‐latitude oceans and have a global occurrence frequency of ∼8% in cloudy skies. Since the channels used for the detection are available in most current operational polar and geostationary satellite radiometers, this SWC detection algorithm can be easily implemented for operations such as cloud monitoring, aviation safety, and SWC‐related weather modification.

Use of large-eddy simulations to design an adaptive sampling strategy to assess cumulus cloud heterogeneities by remotely piloted aircraft

Abstract. Trade wind cumulus clouds have a significant impact on the Earth's radiative balance due to their ubiquitous presence and significant coverage in subtropical regions. Many numerical studies and field campaigns have focused on better understanding the thermodynamic, microphysical, and macroscopic properties of cumulus clouds with ground-based and satellite remote sensing as well as in situ observations. Aircraft flights have provided a significant contribution, but their resolution remains limited by rectilinear transects and fragmented temporal data for individual clouds. To provide a higher spatial and temporal resolution, remotely piloted aircraft (RPA) can now be employed for direct observations using numerous technological advances to map the microphysical cloud structure and to study entrainment mixing. In fact, the numerical representation of mixing processes between a cloud and the surrounding air has been a key issue in model parameterizations for decades. To better study these mixing processes as well as their impacts on cloud microphysical properties, the paper aims to improve exploration strategies that can be implemented by a fleet of RPA. Here, we use a large-eddy simulation (LES) of shallow maritime cumulus clouds to design adaptive sampling strategies. An implementation of the RPA flight simulator within high-frequency LES outputs (every 5 s) allows tracking individual clouds. A rosette sampling strategy is used to explore clouds of different sizes that are static in time and space. The adaptive sampling carried out by these explorations is optimized using one or two RPA and with or without Gaussian process regression (GPR) mapping by comparing the results obtained with those of a reference simulation, in particular the total liquid water content (LWC) and the LWC distribution in a horizontal cross section. Also, a sensitivity test of length scale for GPR mapping is performed. The results of exploring a static cloud are then extended to a dynamic case of a cloud evolving with time to assess the application of this exploration strategy to study the evolution of cloud heterogeneities. While a single RPA coupled to GPR mapping remains insufficient to accurately reconstruct individual clouds, two RPA with GPR mapping adequately characterize cloud heterogeneities on scales small enough to quantify the variability of important parameters such as total LWC.

Molecular size of surfactants affects their degree of enrichment in the sea spray aerosol formation

Sea spray aerosol (SSA), the largest source of natural primary aerosol, plays an important role in atmospheric chemical processes and the earth radiation balance. Its formation process is controlled by many factors. In this study, ethylene glycol (EG) and polyethylene glycol (PEG) with three different molecular weights (200, 400, 600) were used to investigate the influence of molecular size on the properties of submicron SSA produced by plunging jet from an adjustable home-built SSA generator. Different parameters were tested to obtain the optimum experimental conditions. The addition of EG and PEG inhibited the production of SSA and increased the geometric mean diameter (GMD) between 10 and 35 nm. However, PEG with a molecular weight of 600 could promote the production of SSA at higher concentrations, which means that the molecular weight and concentration of the polymer would affect the production efficiency of SSA. Combining with the measurement of surface tension, we found no clear relationship between surface tension and the yield of SSA, due to the properties of the substances themselves. Transmission electron microscopy images show that the addition of EG and PEG could significantly change the structure of salt nuclei in SSA. PEG was significantly enriched in SSA (with enrichment factors within the range 92.9–133.4), and the enrichment was independent of the sampling time, while increasing with the increase of molecular weight. Our results highlight the influence of polymer molecular weight on the properties of SSA, and their importance to improve the accuracy of aerosol emission model parameters.

Global evaluation of the precipitable-water-vapor product from MERSI-II (Medium Resolution Spectral Imager) on board the Fengyun-3D satellite

Abstract. Atmospheric water vapor plays a key role in Earth's radiation balance and hydrological cycle, and the precipitable-water-vapor (PWV) product under clear-sky conditions has been routinely provided by the advanced Medium Resolution Spectral Imager (MERSI-II) on board Fengyun-3D since 2018. The global evaluation of the PWV product derived from MERSI-II is performed herein by comparing it with PWV from the Integrated Global Radiosonde Archive (IGRA) based on a total of 462 sites (57 219 matchups) during 2018–2021. The monthly averaged PWV from MERSI-II presents a decreasing distribution of PWV from the tropics to the polar regions. In general, a sound consistency exists between PWV values of MERSI-II and IGRA; their correlation coefficient is 0.951, and their root mean squared error (RMSE) is 0.36 cm. The histogram of mean bias (MB) shows that the MB is concentrated around zero and mostly located within the range from −1.00 cm to 0.50 cm. For most sites, PWV is underestimated with the MB between −0.41 and 0.05 cm. However, there is also an overestimated PWV, which is mostly distributed in the area surrounding the Black Sea and the middle of South America. There is a slight underestimation of MERSI-II PWV for all seasons with the MB value below −0.18 cm, with the bias being the largest magnitude in summer. This is probably due to the presence of thin clouds, which weaken the radiation signal observed by the satellite. We also find that there is a larger bias in the Southern Hemisphere, with a large value and significant variation in PWV. The binned error analysis revealed that the MB and RMSE increased with the increasing value of PWV, but there is an overestimation for PWV smaller than 1.0 cm. In addition, there is a higher MB and RMSE with a larger spatial distance between the footprint of the satellite and the IGRA station, and the RMSE ranged from 0.33 to 0.47 cm. There is a notable dependency on solar zenith angle of the deviations between MERSI-II and IGRA PWV products.

Evolution of organic carbon during COVID-19 lockdown period: Possible contribution of nocturnal chemistry

Carbonaceous aerosol is one of the main components of atmospheric particulate matter, which is of great significance due to its role in climate change, earth's radiation balance, visibility, and human health. In this work, carbonaceous aerosols were measured in Shijiazhuang and Beijing using the OC/EC analyzer from December 1, 2019 to March 15, 2020, which covered the Coronavirus Disease 2019 (COVID-19) pandemic. The observed results show that the gas-phase pollutants, such as NO, NO2, and aerosol-phase pollutants (Primary Organic Compounds, POC) from anthropogenic emissions, were significantly reduced during the lockdown period due to limited human activities in North China Plain (NCP). However, the atmospheric oxidation capacity (Ox/CO) shows a significantly increase during the lockdown period. Meanwhile, additional sources of nighttime Secondary Organic Carbon (SOC), Secondary Organic Aerosol (SOA), and babs, BrC(370 nm) are observed and ascribed to the nocturnal chemistry related to NO3 radical. The Potential Source Contribution Function (PSCF) analysis indicates that the southeast areas of the NCP region contributed more to the SOC during the lockdown period than the normal period. Our results highlight the importance of regional nocturnal chemistry in SOA formation.

Heterogeneous Nucleation Drives Particle Size Segregation in Sequential Ozone and Nitrate Radical Oxidation of Catechol

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O3 and NO3 in the presence of (NH4)2SO4 seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO3 oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.

Rapid cloud removal of dimethyl sulfide oxidation products limits SO2 and cloud condensation nuclei production in the marine atmosphere

Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth's radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO2) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τHPMTF < 2 h) and terminates DMS oxidation to SO2 When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO2 production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO2 concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate.

Importance of Pressure Changes in High Cloud Area Feedback Due to Global Warming

AbstractHigh clouds have large impacts on the Earth's radiation balance, and a better understanding of their change in warmer climates is essential for climate projection science. Previous studies have focused on the role of large‐scale circulations in high cloud change, and little attention is paid to the local processes in high clouds. Here, we show that the evolution of ice clouds in the upper troposphere due to the cloud microphysical processes is dominated by the collection, sublimation, deposition, and sedimentation using a model with an explicit cloud microphysics scheme. Furthermore, we show that these processes can be enhanced and can lead to the reduction of high clouds in warmer climates through the pressure dependencies of fall speeds of ice particles and the molecular diffusivity of water vapor. Our results highlight the importance of change of the high cloud pressure level in the high cloud area feedback.

A synergic study on estimating surface downward shortwave radiation from satellite data

Surface downward shortwave radiation (DSR) is a fundamental variable in determining the Earth's radiation balance and is essential in many applications. Considerable efforts have been devoted to algorithm development, product generation, and validation. However, few studies have focused on comparing retrieval approaches, examining their strengths and weaknesses, and identifying the most suitable scenarios for each approach. In this study, we implemented and evaluated five representative DSR retrieval algorithms, including the forward parameterization approach, two physical inversion methods (look-up table (LUT) and optimization), and two statistical inversion methods (direct estimation and neural networks). We then proposed an algorithm-integration framework that combined the results of these DSR retrieval methods to further improve DSR estimation accuracy and consistency. To validate the DSR retrievals, we used in-situ data collected at 25 stations of the Baseline Surface Radiation Network (BSRN) over one year. Validation revealed that forward parameterization consistently performed best, with an overall root mean square error (RMSE) of 91.7 W/m2 or a relative RMSE of 16.9%, although it generated the fewest valid retrievals. For an identical data set, the LUT approach generated results comparable to those of parameterization. The neural network-based algorithm-integration approach reduced the RMSE by 11.0 W/m2 or the relative RMSE by 2.0%, compared to the best individual retrieval algorithm. Our analysis demonstrates that algorithm integration is a promising way to obtain DSR data that are superior to estimates from any individual retrieval algorithm.

Contribution of sea spray to aerosol size distributions measured in a South African coastal zone

The atmospheric aerosol has a large influence on global climate through its ability of scattering and absorbing electromagnetic radiation, which affect the Earth's radiative balance. Characteristics, such as size distribution, chemical composition and hygroscopic properties are important for improving global climate model projections of natural sea spray aerosols, which are numerically dominant in the global aerosol budget. Previous experimental studies in coastal locations revealed a complex mixing between natural and anthropogenic aerosols. This complexity makes it difficult to characterize the behavior of natural sea spray aerosols in a simplified form that is suitable for global models. The aim of the present paper is to characterize the size distributions of natural sea spray aerosols at a coastal site in False Bay, South Africa. The opening of False Bay towards the Southern Ocean with unlimited fetch from Antarctica provides a unique opportunity to measure and focus on pristine marine air masses solely. A large observational data set from the First European-South African Transmission Experiment (FESTER) was analyzed and two case studies are presented to illustrate the contribution of sea spray to aerosol size distributions under pure marine and mixed air mass conditions, respectively. A set of conditions for measuring pure marine aerosols in False Bay was established. The results of the relationship of aerosol concentrations with wind speed reveals the distinct behavior of natural sea spray aerosols and demonstrates the pure marine influence of the Southern Ocean in False Bay. Comparison of the aerosol size distributions to a coastal site in the Mediterranean Sea, attest to the contribution of sea spray to aerosol size distributions in False Bay.

The impact of solar activity on ambient ultrafine particle concentrations: An analysis based on 19-year measurements in Boston, USA

Solar radiation plays a major role in atmospheric photochemistry, contributing to the formation and growth of ultrafine particles (PN). PN affect global Earth's radiation balance, climate system, and human health. However, the impact of solar activity on ambient PN remains unclear. In this study, we investigated the associations between daily ambient PN concentrations [particle number (PN)/cm3] and solar radio flux [solar activity index (F10.7 in sfu)] as a solar activity parameter, shortwave solar radiation (SWR), daylight time (DL), cosmic ray-induced ionization (CRII), and air pollution [PM2.5, black carbon (BC) and SO2] over a 19-year period in Boston, MA. We used generalized additive models adjusted for local environmental conditions. We found that F10.7 was the strongest predictor for daily PN concentrations over all time lags (0–28 days of lags) and seasons. The effects were higher in winter and fall. In winter, an interquartile (IQR) of 60 sfu F10.7 corresponded to an increase of 5770 PN/cm3 in the day of PN collection. In fall, an IQR of 75.5 sfu F10.7 was associated with an increase of 5429 PN/cm3. The effects of F10.7 on PN concentrations were slightly greater when the models were adjusted for air pollution. In summer, ambient PN concentrations were statistically significantly associated with F10.7, SWR, and BC, with the strongest association found for PN and BC in the day of PN collection. Unlike the effects of F10.7, SWR and local pollutants on PN concentrations, DL and CRII were negatively associated with ambient PN in the analyses. These findings suggest that solar activity may have a significant impact on daily ambient PN concentrations that affect the Earth's climate system and human health.

Constraining the Impact of Dust‐Driven Droplet Freezing on Climate Using Cloud‐Top‐Phase Observations

AbstractDespite advances in our understanding of ice‐nucleating particles, the effect of cloud glaciation on the Earth's radiation balance has remained poorly constrained. Particularly, dust ice nuclei are believed to enhance cloud glaciation in the Northern Hemisphere. We used satellite observations of the hemispheric and seasonal contrast in cloud top phase to assess the dust‐driven droplet freezing in a climate model. The required freezing efficiency for dust ice nuclei suggests that climate models glaciate too few clouds through immersion droplet freezing. After tuning, the model leads to more realistic cloud‐top‐phase contrasts and a dust‐driven glaciation effect of 0.14 ± 0.13 W m−2 between 30°N and 60°N. Observations of cloud‐top‐phase contrasts provide a strong constraint for ice formation in mixed‐phase clouds and may provide a weak constraint for the associated impact on radiation and precipitation. Future studies should therefore consider both the mean‐state cloud‐phase partitioning and cloud‐phase contrasts to achieve a more accurate simulation of dust‐driven cloud glaciation.