17O18O and 18O18O in ice core O2 from Greenland: implications to reconstruct past atmospheric photochemistry

<p>Abundances of <sup>17</sup>O<sup>18</sup>O and <sup>18</sup>O<sup>18</sup>O (also called clumped isotopes and denoted by Δ<sub>35</sub> and Δ<sub>36</sub>) of O<sub>2</sub>  in firn and ice core air are novel tracers that can be useful to study past changes in atmospheric photochemistry and temperature. We present Δ<sub>35</sub> and Δ<sub>36</sub> values measured in firn and ice core air O<sub>2</sub> from North Greenland (NEEM; 77.45°N 51.06°W). The aim is to reconstruct the preindustrial-industrial, Holocene and glacial-interglacial variation in the tropospheric ozone photochemistry and temperature. Measurements of Δ<sub>35</sub> and Δ<sub>36</sub> are carried out using a high-resolution stable isotope ratio mass spectrometer Thermo Fisher 253 ULTRA[1]. Our measurements of Δ<sub>35</sub> and Δ<sub>36</sub>  across past air, from archive samples, to the modern-day show significant changes in the atmospheric photochemistry via ozone burdening and stratospheric- tropospheric transport processes. We will present the measurement results along with a detailed discussion on the dominant process using explicit dynamic simulations of ∆<sub>36 </sub>in the AC-GCM EMAC model [2,3,4].</p><p> </p>

Photoinduced Production of Chlorine Molecules from Titanium Dioxide Surfaces Containing Chloride

<div>Titanium dioxide (TiO<sub>2</sub>) is extensively used with the process of urbanization and potentially inflfluences atmospheric chemistry, which is yet unclear. In this work, we demonstrated strong production of Cl<sub>2</sub> from illuminated KCl-coated TiO<sub>2</sub> membranes and suggest an important daytime source of chlorine radicals. We found that water and oxygen were required for the reactions to proceed, and Cl<sub>2</sub> production increased linearly with the amount of coated KCl, humidity of the carrier gas, and light intensity. These results suggested that water promotes the reactivity of coated KCl via interaction with the crystal lattice to release free chloride ions (Cl<sup>−</sup>). The free Cl<sup>−</sup> transfer charges to O<sub>2</sub> via photoactivated TiO<sub>2</sub> to form Cl<sub>2</sub> and probably the O<sub>2</sub><sup>−</sup> radical. In addition to Cl<sub>2</sub>, ClO and HOCl were also observed via the complex reactions between Cl/Cl<sub>2</sub> and HOx. An intensive campaign was conducted in Shanghai, during which evident daytime peaks of Cl<sub>2</sub> were observed. Estimated Cl<sub>2</sub> production from TiO<sub>2</sub> photocatalysis can be up to 0.2 ppb/h when the TiO<sub>2</sub>-containing surface reaches 20% of the urban surface, and this is highly correlated to the observed Cl<sub>2</sub>. Our results suggest a non-negligible role of TiO<sub>2</sub> in atmospheric photochemistry via altering the radical budget.</div>

Irradiation intensity dependent heterogeneous formation of sulfate and dissolution of ZnO nanoparticles

Atmospheric photochemistry is largely influenced by the irradiation intensity.

NH₃-promoted hydrolysis of NO₂ induces explosive growth in HONO

The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous NO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether SO2 directly takes part in the HONO production process and what roles NH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with 4< pH <6) and haze episodes under high relative humidity (pH ≈4) – that NH3 was the key factor that promoted the hydrolysis of NO2, leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions. The results also suggest that SO2 plays a minor or insignificant role in HONO formation during fog and haze events, but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms. Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation, further promoting HONO production as a positive feedback. For future photochemical and aerosol pollution abatement, it is crucial to introduce effective NH3 emission control measures, as NH3-promoted NO2 hydrolysis is a large daytime HONO source, releasing large amounts of OH radicals upon photolysis, which will contribute largely to both atmospheric photochemistry and secondary aerosol formation.