Trifluoroacetic acid (TFA) is a known and persistent pollutant in the environment. Although several direct anthropogenic sources exist, production from the atmospheric degradation of fluorocarbons such as some hydrofluorocarbons (HFCs) has been a known source for some time. The current transition from HFCs to HFOs (hydrofluoroolefins) is beneficial from a global warming viewpoint because HFOs are much shorter-lived and pose a much smaller threat in terms of warming, but the fraction of HFOs converted into TFA is higher than seen for the corresponding HFCs and the region in which TFA is produced is close to the source. Therefore, it is timely to review the role of TFA in the Earth’s environment. This review considers its toxicity, sources and removal processes, measurement in a variety of environments, and future prospects. New global model integrations are used to quantify the impacts of uncertainties on TFA levels using the Henry’s Law constant for TFA and the range of gas-phase kinetic parameters chosen for the reaction of OH radicals with a representative HFO (HFO-1234yf). Model runs suggest that TFA surface concentrations vary by up to 10% based on Henry’s Law data but could be up to 25% smaller than previously modelled values suggest depending on the kinetic analysis adopted. Therefore, future estimates of TFA surface concentrations based on HFO removal require updating and the kinetic analysis of TFA production warrants further investigation. The toxicity of TFA appears to be low, but further studies of a much wider range of animal and plant types are required.

Isoprene emissions can affect the oxidizing capacity of the atmosphere and are likely to increase with an increase in the world’s biomass. The emission of isoprene is strongest in tropical forested regions, suggesting a major portion of tropospheric chemistry occurs in the tropics. As well as deforestation and reforestation having a direct impact on the world’s climate through land cover, there is also an indirect environmental impact (e.g., global warming, air pollution) through the resulting change in isoprene emissions. Previously, incomplete understanding of isoprene oxidation chemistry caused a model-measurement breakdown for concentrations of HO_x radicals observed over certain low-NO_x regions, such as the pristine Amazon rainforest. Over the last decade, however, understanding of isoprene oxidation chemistry has been vastly improved. Numerous research studies have provided evidence for the involvement of 1,6-H and 1,5-H shift reactions in the isoprene oxidation mechanism, which increases the level of HO_x recycling that occurs. As well as helping to reduce the model-measurement breakdown observed, the updated isoprene oxidation mechanism affects the tropospheric burdens of other species, including carbon monoxide (CO), methane (CH₄), ozone (O₃), organic peroxides (ROOH), secondary organic aerosol (SOA), and organic nitrates (RONO₂). There are still gaps in the understanding of the impacts and oxidation chemistry of isoprene emissions, which this literature review identifies and discusses. In the future, there is still much scope for further research, including modeling future reforestation scenarios with isoprene emissions and their impacts on both global and regional scales.

The unprecedented reductions in anthropogenic emissions over the COVID-19 lockdowns were utilised to investigate the response of ozone (O₃) concentrations to changes in its precursors across various UK sites. Ozone, volatile organic compounds (VOCs) and NO_x (NO+NO₂) data were obtained for a 3-year period encompassing the pandemic period (January 2019–December 2021), as well as a pre-pandemic year (2017), to better understand the contribution of precursor emissions to O₃ fluctuations. Compared with pre-lockdown levels, NO and NO₂ declined by up to 63% and 42%, respectively, over the lockdown periods, with the most significant changes in pollutant concentrations recorded across the urban traffic sites. O₃ levels correspondingly increased by up to 30%, consistent with decreases in the [NO]/[NO₂] ratio for O₃ concentration response. Analysis of the response of O₃ concentrations to the NO_x reductions suggested that urban traffic, suburban background and suburban industrial sites operate under VOC-limited regimes, while urban background, urban industrial and rural background sites are NO_x-limited. This was in agreement with the [VOC]/[NO_x] ratios determined for the London Marylebone Road (LMR; urban traffic) site and the Chilbolton Observatory (CO; rural background) site, which produced values below and above 8, respectively. Conversely, [VOC]/[NO_x] ratios for the London Eltham (LE; suburban background) site indicated NO_x-sensitivity, which may suggest the [VOC]/[NO_x] ratio for O₃ concentration response may have had a slight NO_x-sensitive bias. Furthermore, O₃ concentration response with [NO]/[NO₂] and [VOC]/[NO_x] were also investigated to determine their relevance and accuracy in identifying O₃-NO_x-VOC relationships across UK sites. While the results obtained via utilisation of these metrics would suggest a shift in photochemical regime, it is likely that variation in O₃ during this period was primarily driven by shifts in oxidant (OX; NO₂ + O₃) equilibrium as a result of decreasing NO₂, with increased O₃ transported from Europe likely having some influence.

UK air pollutant data collected over a 10-year period (2010–2019) from 46 sites with Urban Traffic, Urban Background, Suburban Background, Rural Background, and Urban Industrial environmental types were analysed to study the relationships between [NO] vs. [PM_2.5] and [O₃] vs. [PM_2.5] during the summer for each site type. These results were used to describe the consequence of recent PM_2.5 reductions on NO and O₃ concentrations at different site types across the UK. The strongest positive [NO] vs. [PM_2.5] correlation was observed for the Urban Traffic site type overall, but it displayed the weakest positive [O₃] vs. [PM_2.5] correlation. Analysis of individual Urban Traffic sites revealed an overall negative [O₃] vs. [PM_2.5] gradient at the London Marylebone Road (LMR) site. A sharp 35% PM_2.5 decrease occurred at LMR between 2011 and 2015 before annual mean concentrations plateaued. Further examination of annual correlations revealed negative [O₃] vs. [PM_2.5] gradients in each year directly proceeding the sharp 35% PM_2.5 decrease at LMR. NO_x fluctuations were minimal and accompanied by comparable volatile organic compound (VOC) decreases; thus, VOC-limited chemistry at LMR was deemed to not be the primary cause of O₃ increases. Instead, PM_2.5 reductions are suggested to be a more significant factor in causing O₃ increases, as suppression of O₃ production by PM_2.5 chemistry decreases with declining [PM_2.5]. The remaining two Urban Traffic sites in Birmingham did not display a negative [O₃] vs. [PM_2.5] correlation in the years studied. This was partly ascribed to the Birmingham measurement sites not being under the influence of the street canyon effect like LMR. Principal attribution was to the lower-average absolute initial PM_2.5 concentrations and absence of a significant (>26%) continuous mean PM_2.5 decline of greater than 2 years. This study therefore proposed a threshold initial PM_2.5 concentration (t) above which O₃ suppression by PM_2.5 chemistry is sufficient to induce O₃ increases when average PM_2.5 concentrations significantly decline (by >26% across >2 years), where 17 μg m⁻³ < t < 26 μg m⁻³. Extending this analysis to additional cities across the UK as sufficient data become available would allow refinement of the proposed threshold and improved understanding of the influence from the street canyon effect. These results inform future air pollution policies, in the UK and across the globe, in which further joint reductions of PM_2.5 and O₃ are crucial to achieve maximum benefits to human health.

Quintero is a Chilean coastal city located 40 km north of Valparaiso. In the surroundings of Quintero, there are a number of industries that generate high levels of atmospheric pollutants such as sulfur dioxide (SO₂), nitrogen oxides (NO_x), carbon monoxide (CO) and non-methane hydrocarbons (NMHCs). These compounds may also be generated during oil handling in storage facilities in this area. Quintero Bay has a port that is being used mainly for oil and copper transportation. Since 2010, there have been reports of events producing nausea, vomiting and abdominal pain in the residents of Quintero, and some previous studies have correlated these medical events with high concentrations of SO₂ and NMHCs. One of the main sources of SO₂ in the area was identified to be the Ventanas copper foundry. Following public pressure, the government stopped the operation of the foundry by mid-2023, which led to a significant decrease in SO₂ levels. However, reports of health problems persisted to some extent. In this work, delayed cross-correlation, trajectory and dispersion analyses indicate that an upwind source of air pollution impacting Quintero originates near the oil refinery in Concón, located 20 km to the south. This source of air pollution could provide a background of NMHCs, over which local emissions add up to attain very high concentrations in Quintero. Our analysis shows that there is evidence of the transport of NMHCs from the Concón refinery to the Quintero area. In 2022, of the 20 days with NMHC concentrations greater than 200 ppvb, 50% of them were associated with prevalent southwest winds. Using trajectory and dispersion analyses for eight episodes in Quintero, it has been found that an approximate fraction of pollution generated in Concón that could arrive in Quintero is between 2 and 24%.