When exposed to glyoxal in chamber experiments, dry ammonium or methylammonium sulfate particles turn brown immediately and reversibly without increasing in size. Much less browning was observed on wet aerosol particles, and no browning was observed with sodium sulfate aerosol. While estimated dry aerosol light absorption caused by background glyoxal (70 ppt) is insignificant compared to that of secondary brown carbon overall, in polluted regions this process could be a source of brown carbon.

The spread in effective radiative forcing for both CO₂ and aerosols is narrower in the latest CMIP6 (Coupled Model Intercomparison Project) generation than in CMIP5. For the case of CO₂ it is likely that model radiation parameterisations have improved. Tropospheric and stratospheric radiative adjustments to the forcing behave differently for different forcing agents, and there is still significant diversity in how clouds respond to forcings, particularly for total anthropogenic forcing.

We have calculated an 11.5-year water vapour climatology using the Raman Lidar for Meteorological Observations (RALMO), located in Payerne, Switzerland. The climatology shows that the highest water vapour concentrations are in the summer months and the lowest in the winter months. We present for the first time height-resolved water vapour trends, which show that water vapour increases specific humidity by between 5 % and 15 % per decade depending on the altitude.

Through the cruise campaign conducted over marginal seas in China, we found that the concentrations of condensation nuclei (N_cn) and cloud condensation nuclei (N_ccn) were 1 order of magnitude larger than those in remote clear marine atmospheres, indicating overwhelming contributions from marine traffic emissions and long-range continental transport. Moreover, we derived regression equations used to estimate N_cn and N_ccn from SO₂ when the direct observations of N_cn and N_ccn are not available.

It is widely known that the impact of aerosol–radiation and aerosol–cloud interactions on the radiative forcing is subject to large uncertainties. This is mainly due to the lack of understanding of aerosol optical properties and vertical distribution, whose uncertainties come from different processes. This work attempts to quantify the sensitivity of aerosol optical properties and their vertical distribution to key physico-chemical processes.

The understanding of the absorption Ångstrom exponent (AAE) of aged black carbon (BC) particles affected by their microphysics is numerically evaluated. With key sensitive microphysical parameters, a simple parameterization of the AAE of coated BC with a size distribution is proposed. It is found that BC coated by thin brown carbon with more large particles shows an AAE smaller than pure BC particles. Our findings improve the understanding and application of the AAE of BC with brown coatings.

We found that Br atoms in the marine boundary layer are the most probable oxidizer that transform gaseous elemental mercury into gaseous oxidized mercury, according to the mercury isotopes in the total gaseous mercury. On the other hand, Br or Cl atoms are not the primary oxidizers that produced oxidized mercury on particles. This study showed that mercury isotopes can provide new evidence that help us to fully understand the transformations of atmospheric mercury.

This work expands upon the recently developed multivariate relative rate technique, presented in Shaw et al. (2019), for the measurement of rates of reaction between aromatic and aliphatic volatile organic compounds (VOCs) and OH. Knowledge of the rates of such reactions are important for understanding air quality in urban environments. This work also provides a key validation of structure–activity relationship models, which provide a theoretical method for estimating OH + VOC kinetics.

Recent lower stratospheric ozone decreases remain unexplained. We show that chemistry–climate models are not generally able to reproduce mid-latitude ozone and water vapour changes. Our analysis of observations provides evidence that climate change may be responsible for the ozone trends. While model projections suggest that extratropical ozone should recover by 2100, our study raises questions about their efficacy in simulating lower stratospheric changes in this region.

We demonstrate that AirCore technology, which is based on small low-cost balloons, can provide access to trace gas measurements such as CFCs at ultra-low abundances. This is a new way to quantify ozone-depleting, and related, substances in the stratosphere, which is largely inaccessible to aircraft. We show two potential uses: (a) tracking the stratospheric circulation, which is predicted to change, and (b) assessing three common meteorological reanalyses driving a global stratospheric model.

Aromatic volatile organic compounds (VOCs) emitted from anthropogenic activity are important for tropospheric ozone and secondary organic aerosol (SOA) formation. Here we present a detailed chemical characterisation of SOA from four C9-aromatic isomers and a polycyclic aromatic hydrocarbon (PAH). We identify and compare their oxidation products in the gas and particle phases, showing the different relative importance of oxidation pathways and proportions of highly oxygenated organic molecules.

Nitrogen isotopic compositions of atmospheric reactive nitrogen are widely used to infer their sources. However, the reactions between NO and NO₂ strongly impact their isotopes, which was not well understood. We conducted a series of experiments in an atmospheric simulation chamber to determine the isotopic effects of (1) direct isotopic exchange between NO and NO₂ and (2) the isotopic fractionations during NO_x photochemistry, then developed an equation to quantify the overall isotopic effect.

Biogenic iodine emission from macroalgae and microalgae could initiate atmospheric new particle formation (NPF). But it is unknown if other species are needed to drive the growth of new iodine particles in the marine boundary layer. Unlike the deeper understanding of organic compounds driving continental NPF, little is known about the organics involved in coastal or open-ocean NPF. This article reveals a new group of important organic compounds involved in this process.

For the first time, we provide a joint nonlinear optimal estimate of NO_x and NMVOC emissions during the KORUS-AQ campaign by simultaneously incorporating SAO's new product of HCHO columns from OMPS and OMI tropospheric NO₂ columns into a regional model. Results demonstrate a promising improvement in the performance of the model in terms of HCHO and NO₂ concentrations, which in turn enables us to quantify the impact of the emission changes on different pathways of ozone formation and loss.

We present an approach to track separate contributions to PM_2.5 by gasoline and diesel vehicles through a positive matrix factorization analysis of online monitoring data measurable by relatively inexpensive analytical instruments. They are PM_2.5 organic and elemental carbon, C₂–C₉ volatile organic compounds, and nitrogen oxide concentrations. The method was demonstrated to be effective by applying monitoring data spanning 6 years (2011–2017) from a roadside environment in Hong Kong.

This paper explores the month-to-month variability of aerosol concentrations (ACs) over China. The AC variability is dominated by the monopole mode and the meridional dipole mode. The associated dynamic and thermal impacts of the climate systems are examined to explain their contributions to the formation of the two modes. The result suggests the variations are originating from the tropical Pacific, and extratropical atmospheric systems contribute to the dominant variabilities of ACs over China.

We provide a statistical framework for detecting trends of multiple autocorrelated time series from sparsely sampled profile data. The result is a better and more consistent quantification of trend estimates of vertical profile data. The focus was placed on the long-term ozone time series from commercial aircraft and balloon-borne ozonesonde measurements. This framework can be applied to other trace gases in the atmosphere.

Cirrus clouds appearing in the upper troposphere and lower stratosphere have important impacts on the radiation budget and climate change. We revisited global stratospheric cirrus clouds with CALIPSO and for the first time with MIPAS satellite observations. Stratospheric cirrus clouds related to deep convection are frequently detected in the tropics. At middle latitudes, MIPAS detects more than twice as many stratospheric cirrus clouds due to higher detection sensitivity.

We present an updated projection of Antarctic ozone hole recovery using an ensemble of chemistry–climate models. To do so, we employ a method, more advanced and skilful than the current multi-model mean standard, which is applicable to other ensemble analyses. It calculates the performance and similarity of the models, which we then use to weight the model. Calculating model similarity allows us to account for models which are constructed from similar components.

China is one of the largest agricultural countries in the world. Some of the major PM_2.5 particles that cause the atmospheric haze and impact the climate change were converted from agricultural NH₃ emission. This paper applied the numerical modeling system, coupled with a high-resolution agricultural NH₃ emissions inventory, to investigate the contribution of agricultural NH₃ to PM_2.5 mass burden in China and obtained some interesting results.

Polycyclic aromatic hydrocarbons (PAHs) are not declining in Arctic air despite reductions in global emissions. We studied PAHs and oxy- and nitro-PAHs in gas and particulate phases of Arctic aerosol, collected in autumn 2018 in Longyearbyen, Svalbard. PAHs were found at comparable levels as at other background Scandinavian and European air sampling stations. Statistical analysis confirmed that a coal-fired power plant and vehicle and marine traffic are the main local contributors of PAHs.

Water vapour has a complex spectrum and absorbs from the microwave to the near-UV where it dissociates. There is limited knowledge of the absorption features in the near-UV, and there is a large disagreement for the available models and experiments. We created a new ab initio model that is in good agreement with observation at 363 nm. At lower wavelengths, our calculations suggest that the latest experiments overestimate absorption. This has implications for trace gas retrievals in the near-UV.

New particle formation is a key process influencing both local air quality and climatically active cloud condensation nuclei concentrations. This study has carried out fundamental measurements of nucleation processes in Barcelona, Spain, and concludes that a mechanism involving stabilisation of sulfuric acid clusters by low molecular weight amines is primarily responsible for new particle formation events.

Comparisons of sea salt and size-resolved dust mass concentration measurements over southeast Florida to those from the MERRA-2/GEOS-5 FP aerosol reanalysis show the reanalysis depicts excessive sea salt and puts too much dust in larger intermediate sizes than do the measurements. The vertical distribution of the dust mass is approximately correct. The incorrect reanalysis aerosol speciation and dust sizes have implications for the modeling of their transport, deposition, and radiative impact.

The amount of energy reflected back into space because of man-made particles is highly uncertain. Processes related to naturally occurring particles cause most of the uncertainty, but these processes are poorly constrained by present-day measurements. We show that measurements over the Southern Ocean, far from pollution sources, efficiently reduce climate model uncertainties. Our results pave the way to designing experiments and measurement campaigns that reduce this uncertainty even further.

Uncertainties in marine boundary layer clouds arise in the presence of biomass burning aerosol, as is the case over the southeast Atlantic Ocean. Heating due to this aerosol has the potential to alter the thermodynamic profile as the aerosol is transported across the Atlantic Ocean. Radiation transfer experiments indicate local shortwave aerosol heating is ~2–8 K d⁻¹; however uncertainties in this quantity exist due to the single-scattering albedo and back trajectories of the aerosol plume.

Gravity waves are an important coupling mechanism in the atmosphere. Measurements by two research aircraft during a mountain wave event over Scandinavia in 2016 revealed changes of the horizontal scales in the vertical velocity field and of momentum fluxes in the vicinity of the tropopause inversion. Idealized simulations revealed the presence of interfacial waves. They are found downstream of the mountain peaks, meaning that they horizontally transport momentum/energy away from their source.