Radar and radiometer observations were used to study cloud liquid and snowfall in three types of snow clouds. While near-surface and shallow clouds have an area fraction of 90 %, deep clouds contribute half of the total snowfall volume. Deeper clouds have heavier snowfall, although cloud liquid is equally abundant in all three cloud types. The skills of a GMI Bayesian algorithm are examined. Snowfall in deep clouds may be reasonably retrieved, but it is challenging for near-surface clouds.

To study the role of organic matter in ice crystal formation, we investigated the ice nucleation ability of a subcomponent of organic aerosols, the biopolymer lignin, using a droplet-freezing technique. We found that lignin is an ice-active macromolecule with changing abilities based on dilutions. The effects of atmospheric processing and of physicochemical treatments on the ability of lignin solutions to freeze were negligible. Thus, lignin is a recalcitrant ice-nucleating macromolecule.

Airborne observations of ozone and related pollutants show smog was pervasive in spring 2016 over Hebei Province, China. We find high amounts of ozone precursors throughout and even above the PBL, continuing to generate ozone at high rates to be potentially transported downwind. Concentrations even in the rural areas of this highly industrialized province promote widespread ozone production, and we show that to improve air quality over Hebei both NO_x and VOCs should be targeted.

A first assessment is made of the historical and future changes in air pollutants from models participating in the 6th Coupled Model Intercomparison Project (CMIP6). Substantial benefits to future air quality can be achieved in future scenarios that implement measures to mitigate climate and involve reductions in air pollutant emissions, particularly methane. However, important differences are shown between models in the future regional projection of air pollutants under the same scenario.

We carried out a 1-year PM concurrent observation at 12 sites across six regions of China, and size-segregated PAHs were measured. We found both PAHs and BaP_eq were concentrated in PM_1.1, and northern China had higher PAHs' pollution and inhalation cancer risk than southern China. Nationwide increases in both PAH levels and inhalation cancer risk occurred in winter. We suggest reducing coal and biofuel consumption in the residential sector is an important option to mitigate PAHs' health risks.

Wildfires have become increasingly prevalent. Intense smoke consisting of particulate matter (PM) leads to an increased risk of morbidity and mortality. The record-breaking Camp Fire ravaged Northern California for two weeks in 2018. Here, we employ a comprehensive chemical transport model along with ground-based and satellite observations to characterize the PM concentrations across Northern California and to investigate the pollution sensitivity predictions to key parameters of the model.

This study investigates carbon monoxide pollution in East Asia during spring using a numerical model, satellite remote sensing, and aircraft measurements. We found an underestimation of emission sources. Correcting the emission bias can improve air quality forecasting of carbon monoxide and other species including ozone. Results also suggest that controlling VOC and CO emissions, in addition to widespread NO_x controls, can improve ozone pollution over East Asia.

This present article demonstrates how numerical simulations with very high horizontal resolution (150 m) can contribute to better understanding the key physical processes (turbulence and microphysics) that lead to Mediterranean heavy precipitation.

In winter 2015/16, the descent of the westerly quasi-biennial oscillation (QBO) jet was interrupted by easterly winds. We find that Rossby–gravity and inertia–gravity waves weaken the jet core in early stages, and small-scale convective gravity waves, as well as horizontal and vertical components of Rossby waves, reverse the wind sign in later stages. The strong negative wave forcing in the tropics results from the enhanced convection, an anomalous wind profile, and barotropic instability.

We present high-resolution measurements of pollutant trace gases (PAN, C₂H₂, and HCOOH) in the Asian monsoon UTLS from the airborne limb imager GLORIA during StratoClim 2017. Enhancements are observed up to 16 km altitude, and PAN and C₂H₂ even up to 18 km. Two atmospheric models, CAMS and EMAC, reproduce the pollutant's large-scale structures but not finer structures. Convection is investigated using backward trajectories of the models ATLAS and TRACZILLA with advanced detection of convection.

Methane accounts for ∼ 25 % of current climate warming. The current lack of methane measurements is a barrier for tracking major sources, which are key for near-term climate mitigation. We use mobile measurements to identify and quantify methane emission sources in Utrecht (NL) and Hamburg (DE) with a focus on natural gas pipeline leaks. The measurements resulted in fixing the major leaks by the local utility, but coordinated efforts are needed at national levels for further emission reductions.

A recent aircraft field campaign near the Azores in the summer of 2017 provides ample observations of aerosols and clouds with detailed vertical information. This study utilizes those observational data in combination with the aerosol-aware large-eddy simulations and aerosol reanalysis data to examine the significance of the long-range-transported aerosol effect on marine-boundary-layer clouds. It is the first time that the ACE-ENA aircraft campaign data are used for this topic.

Reanalysis products are an invaluable tool for representing variability and long-term trends in regions with limited in situ data. However, validation of these products is difficult because of that lack of station data. Here we present a novel assessment of eight reanalyses over the polar Southern Ocean, leveraging the close relationship between trends in sea ice cover and surface air temperature, that provides clear guidance on the most reliable product for Antarctic research.

Based on measurements from an online mass spectrometer, we quantify volatile organic compound (VOC) concentrations from numerous ions of the mass spectrometer, using information from laboratory-obtained calibration results. We find that most VOC concentrations are from oxygenated VOCs (OVOCs). We further show that these OVOCs also contribute significantly to OH reactivity. Our results suggest the important role of OVOCs in VOC emissions and chemistry in urban air.

The Chinese government has made major strides in curbing anthropogenic emissions. In this study, we constrain a state-of-the-art CTM by a reliable data assimilation method with extensive chemical and meteorological observations. This comprehensive technical design provides a crucial advance in isolating the influences of emission changes and meteorological perturbations over the Yangtze River Delta (YRD) from 2016 to 2019, thus establishing the first map of the PM_2.5 mitigation across the YRD.

We assess the effects of triboelectric charging on wind-blown sand via observations and a numerical model. The 3D electric field within a few centimetres of the ground is characterized for the first time. By using the discrete element method together with the particle-charging model, we explicitly account for the particle–particle charging during collisions. We find that triboelectric charging could enhance the total mass flux and saltation height by up to 20 % and 15 %, respectively.

Biomass burning (BB) plumes arriving at the Maïdo observatory located in the south-west Indian Ocean during August 2018 and August 2019 are studied using trace gas measurements, Lagrangian transport models and the CAMS near-real-time atmospheric composition service. We investigate (i) secondary production of volatile organic compounds during transport, (ii) efficacy of the CAMS model to reproduce the chemical makeup of BB plumes and (iii) the impact of BB on the remote marine boundary layer.

The paper details atmospheric chemistry in a megacity (Beijing), focussing on radicals which mediate the formation of secondary pollutants such as ozone and particles. Highly polluted conditions were experienced, including the highest ever levels of nitric oxide (NO), with simultaneous radical measurements. Radical concentrations were large during "haze" events, demonstrating active photochemistry. Modelling showed that our understanding of the chemistry at high NO_x levels is incomplete.

The Tibetan Plateau has been experiencing a rapid warming during the last 40 years, particularly in winter. The warming leads to an increase in the planetary boundary layer height and a decrease in the relative humidity in the Sichuan Basin, causing a reduction of PM_2.5 concentration by 17.5 % (~25.1 μg m⁻³), of which the reduction in secondary aerosols is 19.7 μg m⁻³. These findings indicate that the warming plateau plays an important role in mitigating air quality in downstream.

The properties of ice crystals smaller than 150 µm in diameter were investigated in a cold-room laboratory using digital holography and microscopy. Automated image processing has been used to determine the track of falling ice crystals, and collected crystals were melted and scanned under a microscope to infer particle mass. A parameterization relating particle size and mass was determined which describes ice crystals in this size range more accurately than existing relationships.

We use a set of model simulations of the 20th century to demonstrate that the uncertainty in the cooling effect of man-made aerosol emissions has a wide range of impacts on global monsoons. For the weakest cooling, the impact of aerosol is overpowered by greenhouse gas (GHG) warming and monsoon rainfall increases in the late 20th century. For the strongest cooling, aerosol impact dominates over GHG warming, leading to reduced monsoon rainfall, particularly from 1950 to 1980.

Significant stratification of aerosols was observed in North China. Polluted dust dominated above the PBL, and anthropogenic aerosols prevailed within the PBL, which is mainly driven by meteorological conditions. The key role of the elevated dust is to alter atmospheric thermodynamics and stability, causing the suppression of turbulence exchange and a decrease in PBL height, especially during the dissipation stage, thereby inhibiting dissipation of persistent heavy surface haze pollution.

Samples were collected from six ground-based sites located in Chengdu, a typical rapidly developing metropolitan area in Southwest China, and were analysed for seven OPEs in atmospheric PM_2.5 (Σ₇ OPEs). The concentrations of Σ₇ OPEs were higher in autumn and winter than in summer. In contrast to coastal cities, sustained and stable high local emissions in the inland city studied were identified, which is particularly noteworthy.

This study evaluated the impact of Asian haze from the three biggest industrial regions on Taiwan and analyzed the process during transport. The production and removal process revealed the mechanisms of long-range transport. This is the first time that the brute force method and process analysis technique has been applied in a Community Multiscale Air Quality Modeling System. Also, this study simulated the interesting transboundary transport of pollutants from southern mainland China to Taiwan.

Recent studies have reported up to a 10 % average decrease of lower stratospheric ozone at 20 km altitude following solar proton events (SPEs). Our study uses 49 events that occurred after the launch of Aura MLS (July 2004–now) and 177 events that occurred in the WACCM-D simulation period (Jan 1989–Dec 2012) to evaluate ozone changes following SPEs. The statistical and case-by-case studies show no solid evidence of SPE's direct impact on the lower stratospheric ozone.

We present observations of precipitating and non-precipitating Arctic liquid and mixed-phase clouds during a research cruise along the Russian shelf in summer and autumn of 2014. Active remote-sensing observations, radiosondes, and auxiliary measurements are combined in the synergistic Cloudnet retrieval. Cloud properties are analysed with respect to cloud-top temperature and boundary layer structure. About 8 % of all liquid clouds show a liquid water path below the infrared black body limit.

In order to perform proper model simulation of ozone near the ground in the coastal area of northeastern Asia, it has been found that it is very important to select appropriate dry deposition velocities of ozone on the oceanic water of specific area of the northwestern Pacific. Empirical measurement of the mixing ratios and dry deposition flux of ozone over the ocean in this area is highly recommended.

Volcanic aerosols endanger aircraft and thus disrupt air travel globally. For aviation safety, it is vital to know the location and lifetime of such aerosols in the atmosphere. Here we show that the interaction of volcanic particles with each other eventually reduces their atmospheric lifetime. Moreover, we demonstrate that sunlight heats these particles, which lifts them several kilometers in the atmosphere. These findings support a more reliable forecast of volcanic aerosol dispersion.

Tropical upper troposphere over the Atlantic and Pacific oceans is a major source region of new particles. These particles are associated with the outflow from deep convection. We investigate the processes that govern the formation of these particles and their initial growth and show that none of the formation schemes commonly used in global models are consistent with observations. Using newer schemes indicates that organic compounds are likely important as nucleating and initial growth agents.

Cities are characterized by their impact on various meteorological variables. Our study aims to generalize these modifications into a single phenomenon – the urban meteorology island (UMI). A wide ensemble of Weather Research and Forecasting (WRF) and Regional Climate Model (RegCM) simulations investigated urban-induced modifications as individual UMI components. Significant changes are found in most of the discussed meteorological variables with a strong impact of specific model simulations.

Anthropogenic pollution particles – aerosols – serve as cloud condensation nuclei and thus increase cloud droplet concentration and the clouds' reflection of sunlight (a cooling effect on climate). This Twomey effect is poorly constrained by models and requires satellite data for better quantification. The review summarizes the challenges in properly doing so and outlines avenues for progress towards a better use of aerosol retrievals and better retrievals of droplet concentrations.

Air quality in Beijing is of concern to both policy makers and the general public. In order to address concerns about air quality it is vital that the sources of atmospheric pollutants are understood. This work presents the first top-down measurement of volatile organic compound (VOC) emissions in Beijing. These measurements are used to evaluate the emissions inventory and assess the impact of VOC emission from the city centre on atmospheric chemistry.

Mineral dust aerosols blown mostly from desert areas are a key player in the climate system. We use a new desert dust aerosol low-altitude concentration data set as well as additional information on the surface state and low-altitude winds to infer desert dust emission and source maps over North Africa. With 9 years of data, we observe a full seasonal cycle of dust emissions, differentiating morning and afternoon/evening emissions and providing a first glance at long-term changes.

51 dust events over the Mediterranean from EARLINET were studied regarding the aerosol geometrical, optical and microphysical properties and radiative forcing. We found δ_p532 values of 0.24–0.28, LR₅₃₂ values of 49–52 sr and AOT₅₃₂ of 0.11–0.40. The aerosol mixing state was also examined. Depending on the dust properties, intensity and solar zenith angle, the estimated solar radiative forcing ranged from −59 to −22 W m⁻² at the surface and from −24 to −1 W m⁻² at the TOA (cooling effect).

Ship-based measurements of maritime ice nuclei concentrations encompassing all oceans are compiled. From this overview it is found that maritime ice nuclei concentrations are typically 10–100 times lower than over continents, while concentrations are surprisingly similar in different oceanic regions. The analysis of the influence of ship emissions shows no effect on the data, making ship-based measurements an efficient strategy for the large-scale exploration of ice nuclei concentrations.

Data assimilation provides a powerful tool to estimate emission inventories by feeding observations. This emission inversion relies on the correct assumption about the emission uncertainty, which describes the potential spatiotemporal spreads of sources. However, an unrepresentative uncertainty is unavoidable. Especially in the complex dust emission, the uncertainties can hardly all be taken into account. This study reports how adjoint can be used to detect errors in the emission uncertainty.

Modeling the stratosphere requires models with good representations of chemical transport. To do this, nearly all models divide the atmosphere into boxes. This creates some unwanted problems. However, the only other option is to divide the atmosphere into balloons, and this method is very complicated. Here, we use a model which uses this balloon-like method to estimate the impacts of this method on chemical transport. We find significant differences in sensitive regions of the stratosphere.

A novel lidar method to study cloud microphysical properties (of liquid water clouds) and to study aerosol–cloud interaction (ACI) is developed and presented in this paper. In Part 1, the theoretical framework including an error analysis is given together with an overview of the aerosol information that the same lidar system can obtain. The ACI concept based on aerosol and cloud information is also explained. Applications of the proposed approach to lidar measurements are presented in Part 2.

Part 2 presents the application of the dual-FOV polarization lidar technique introduced in Part 1. A lidar system was upgraded with a second polarization telescope, and it was deployed at the southernmost tip of South America. A comparison with alternative remote sensing techniques and the evaluation of the aerosol–cloud–wind relation in a convective boundary layer in pristine marine conditions are presented in two case studies, demonstrating the potential of the approach for ACI studies.

Particulate air pollution cools the climate and partially masks the greenhouse warming by reflecting sunlight and enhancing the reflection by clouds. The intensity of this cooling depends on interactions between pollution and desert dust within the atmosphere. Our simulations with a global atmospheric chemistry-climate model indicate that these interactions significantly weaken the cooling.

We used in situ aerosol measurements over the Atlantic, Caribbean, and Pacific to initialize a cloud model and study the impact of aerosol concentration and sizes on warm clouds. We show that high aerosol concentration increases cloud mass and reduces surface rain when giant particles (diameter > 9 µm) are present. The large aerosols changed the timing and magnitude of internal cloud processes and resulted in an enhanced evaporation below cloud base and dramatically reduced surface rain.

In this paper we present the analysis of the aerosol vertical features observed by CATS collected from 2015 to 2017 over three selected regions (North China, the Tibetan Plateau, and the Tarim Basin) over different timescales. This comprehensive information provides insights into the seasonal variations and diurnal cycles of the aerosol vertical features across East Asia.

Measurements of the multi-wavelength Raman polarization lidar Polly^XT have been combined with measurements of pollen type and concentration using a traditional pollen sampler at a rural forest site in Kuopio, Finland. The depolarization ratio was enhanced when there were pollen grains in the atmosphere, illustrating the potential of lidar to track pollen grains in the atmosphere. The depolarization ratio of pure pollen particles was assessed for birch and pine pollen using a novel algorithm.

Sea spray is the largest aerosol source on Earth. These aerosol particles can impact climate by inducing ice formation in clouds. The role that ocean biology plays in determining the composition and ice nucleation abilities of sea spray aerosol is unclarified. In this study, we demonstrate that atomized seawater from highly productive ocean regions is more effective at nucleating ice than seawater from lower-productivity regions.

Using dust AOD (DAOD) data from three aerosol reanalyses, we explored the correlative relationships between DAOD and multiple indices representing seasonal Atlantic TC activities. A robust negative correlation with Caribbean DAOD and Atlantic TC activity was found. We documented for the first time the regional differences of this relationship for over the Caribbean and the tropical North Atlantic. We also evaluated the impacts of potential confounding climate factors in this relationship.

The lowermost-stratosphere moist bias in ECMWF analyses and 12 h forecasts is diagnosed for the Arctic winter-spring 2016 period by using two-dimensional GLORIA water vapor observations. The bias is already present in the initial conditions (i.e., the analyses), and sensitivity forecasts on time scales of < 12 h show hardly any sensitivity to modified spatial resolution and output frequency.

This study uses 17 years (2001–2017) of observed rain rate, aerosol optical depth (AOD), meteorological reanalysis fields and outgoing long-wave radiation to investigate high precipitation events at the foothills of the Himalayas. Composite analysis of all data sets for high precipitation events (daily rainfall > 95th percentile) indicates clear and robust associations between high precipitation events, high aerosol loading and high moist static energy values.

We analyze global measurements of aerosol height from fires. A plume rise model reproduces measurements with a low bias in five regions, while a statistical model based on satellite measurements of trace gasses co-emitted from the fires reproduces measurements without bias in eight regions. We propose that the magnitude of the pollutants emitted may impact their height and subsequent downwind transport. Using satellite data allows better modeling of the global aerosol distribution.

Recently, China has promulgated a series of regulations to reduce air pollutants. The decreased black carbon (BC) and co-emitted pollutants could affect the interactions between BC and other aerosols, which in turn results in changes in BC. Herein, we re-assessed the characteristics of BC of a representative pollution site in northern China in the final year of the Chinese Action Plan for the Prevention and Control of Air Pollution.

Africa is estimated to account for approximately 52 % of global biomass burning (BB) carbon emissions. Despite this, there has been little previous in situ study of African BB emissions. This work presents BB emission factors for various atmospheric trace gases sampled from an aircraft in two distinct areas of Africa (Senegal and Uganda). Intracontinental variability in biomass burning methane emission is identified, which is attributed to difference in the specific fuel mixtures burnt.

Solar geoengineering has been introduced to mitigate human-caused global warming by reflecting sunlight back into space. This research investigates the impact of solar geoengineering on the tropical Pacific climate. We find that solar geoengineering can compensate some of the greenhouse-induced changes in the tropical Pacific but not all. In particular, solar geoengineering will result in significant changes in rainfall, sea surface temperatures, and increased frequency of extreme ENSO events.

With the sharp rise in coal seam gas (CSG) production in Queensland’s Surat Basin, there is much interest in quantifying methane emissions from this area and from unconventional gas production in general. We develop and apply a regional Bayesian inverse model that uses hourly methane concentration data from two sites and modelled backward dispersion to quantify emissions. The model requires a narrow prior and suggests that the emissions from the CSG areas are 33% larger than bottom-up estimates.