The assumption of chemical equilibrium is widely used to derive information about poorly measured characteristics of the mesosphere–lower thermosphere from rocket and satellite data and to study the physicochemical processes at these altitudes. In this work, we analyze the fundamental aspects of chemical equilibria of two important trace gases and discuss their possible applications.

The Copernicus Atmosphere Monitoring Service (CAMS) provides global analyses and forecasts of aerosols and trace gases in the atmosphere. On 27 June 2023 a major upgrade, Cy48R1, became operational. Comparisons with in situ, surface remote sensing, aircraft, and balloon and satellite observations show that the new CAMS system is a significant improvement. The results quantify the skill of CAMS to forecast impactful events, such as wildfires, dust storms and air pollution peaks.

To serve the uncertainty quantification (UQ) needs of 4D-Var data assimilation (DA) practitioners, we describe and justify a UQ algorithm from carbon flux inversion and incorporate its sampling uncertainty into the final reported UQ. The algorithm is mathematically proved, and its performance is shown for a carbon flux observing system simulation experiment. These results legitimize and generalize this algorithm's current use and make available this effective algorithm to new DA domains.

The supercooled liquid fraction (SLF) in mixed-phase clouds is retrieved for the first time using passive geostationary satellite observations based on differences in liquid droplet and ice particle radiative properties. The retrieved results are comparable to global distributions observed by active instruments, and the feasibility of the retrieval method to analyze the observed trends of the SLF has been validated.

The determination of ions in the mass spectra of individual particles remains uncertain. We have developed a standard-free mass calibration algorithm applicable to more than 98 % of ambient particles. With our algorithm, ions with ~ 0.05 Th mass difference could be determined. Therefore, many more atmospheric species could be determined and involved in the source apportionment of aerosols, the study of chemical reaction mechanisms, and the analysis of single-particle mixing states.

pH is a key property of ambient aerosols, which affect many atmospheric processes. As aerosol pH is a non-conservative parameter, diverse averaging metrics and temporal resolutions may influence the pH values calculated by thermodynamic models. This technical note seeks to quantitatively evaluate the average pH using varied metrics and resolutions. The ultimate goal is to establish standardized reporting practices in future research endeavors.

A great majority of observational trend studies of free tropospheric ozone use sparsely sampled ozonesonde and aircraft measurements as reference data sets. A ubiquitous assumption is that trends are accurate and reliable so long as long-term records are available. We show that sampling bias due to sparse samples can persistently reduce the trend accuracy, and we highlight the importance of maintaining adequate frequency and continuity of observations.

Models are used to make forecasts of volcanic ash dispersion during eruptions. These models have unknown inputs relating to the eruption itself, physical processes, and meteorological conditions. We use statistical models to predict the output of the expensive physical model and show we can account for the effects of the different inputs. We compare the model to real-world observations and show that accounting for all sources of uncertainty may lead to different conclusions about the inputs.

Chemistry–climate models show biases compared to surface ozone observations and thus require bias correction for impact studies and the assessment of air quality changes. We compare the performance of commonly used correction techniques for model outputs available via CMIP6. While all methods can reduce model biases, better results are obtained from more complex approaches. Thus, our study suggests broader use of these techniques in studies seeking to inform air quality management and policy.

An aerosol optical tweezer (AOT) Raman spectroscopy system is developed to capture a single aerosol droplet for phase transition monitoring and morphology studies. Rapid droplet capture is achieved and accurate droplet size and refractive index are retrieved. Results indicate that mixed inorganic/organic droplets are more inclined to form core–shell morphology when RH decreases. The phase transitions of secondary mixed organic aerosol/inorganic droplets vary with their precursors.

We developed a method that uses aqueous S(IV) to quantitatively convert NO₂ to NO₂⁻, which allows both species to be quantified using the Griess method. As an example of the utility of the method, we quantified both photolysis channels of nitrate, with and without a scavenger for hydroxyl radical (^·OH). The results show that without a scavenger, ^·OH reacts with nitrite to form nitrogen dioxide, suppressing the apparent quantum yield of NO₂⁻ and enhancing that of NO₂.