Photochemistry of 4-chlorophenol in liquid and frozen aqueous media studied by chemical, compound-specific isotope, and DFT analyses.

The photochemistry of 4-chlorophenol in liquid and frozen aqueous solutions and on the surface of ice grains yields substantially different photoproducts. Several complementary experimental and theoretical methods, such as trace analyses of the photoproducts, trapping experiments, compound-specific isotope analyses, and quantum chemical calculations, were used to study the reaction mechanism differences. A similar carbon kinetic isotope effect determined for the photolysis of 4-chlorophenol samples in the temperature range of 20 to -40 °C and the results of trapping experiments suggest that heterogeneous cleavage of the C-Cl bond in the excited state is probably a common key step that leads to the formation of carbene and hydroxyphenyl cation intermediates. We conclude that the subsequent specific reactions of these species under various conditions are responsible for the formation of different final photoproducts.

Different approaches to explore the impact of COVID-19 lockdowns on carbonaceous aerosols at a European rural background site.

To prevent the fast spread of COVID-19, worldwide restrictions have been put in place, leading to a reduction in emissions from most anthropogenic sources. In this study, the impact of COVID-19 lockdowns on elemental (EC) and organic (OC) carbon was explored at a European rural background site combining different approaches: - "Horizontal approach (HA)" consists of comparing concentrations of pollutants measured at 4 m a.g.l. during pre-COVID period (2017-2019) to those measured during COVID period (2020-2021); - "Vertical approach (VA)" consists of inspecting the relationship between OC and EC measured at 4 m and those on top (230 m) of a 250 m-tall tower in Czech Republic. The HA showed that the lockdowns did not systematically result in lower concentrations of both carbonaceous fractions unlike NO2 (25 to 36 % lower) and SO2 (10 to 45 % lower). EC was generally lower during the lockdowns (up to 35 %), likely attributed to the traffic restrictions whereas increased OC (up to 50 %) could be attributed to enhanced emissions from the domestic heating and biomass burning during this stay-home period, but also to the enhanced concentration of SOC (up to 98 %). EC and OC were generally higher at 4 m suggesting a greater influence of local sources near the surface. Interestingly, the VA revealed a significantly enhanced correlation between EC and OC measured at 4 m and those at 230 m (R values up to 0.88 and 0.70 during lockdown 1 and 2, respectively), suggesting a stronger influence of aged and long distance transported aerosols during the lockdowns. This study reveals that lockdowns did not necessarily affect aerosol absolute concentrations but it certainly influenced their vertical distribution. Therefore, analyzing the vertical distribution can allow a better characterization of aerosol properties and sources at rural background sites, especially during a period of significantly reduced human activities.