Yolanda Díaz-De-Mera

SO₂-Criegee intermediate reactions enhance the formation and growth of new particles in the atmosphere.

In this work we report the study of the ozonolysis of 2,5-dihydrofuran and 2,3-dihydrofuran and the reaction conditions leading to the formation of secondary organic aerosols. The reactions have been carried out in a Teflon chamber filled with synthetic air mixtures at atmospheric pressure and room temperature. The ozonolysis only produced particles in the presence of SO₂. Rising relative humidity from 0 to 40 % had no effect on the production of secondary organic aerosol in the case of 2,5-dihydrofuran, while it reduced the particle number and particle mass concentrations from the 2,3-dihydrofuran ozonolysis. The water-to-SO₂ rate constant ratio for the 2,3-dihydrofuran Criegee intermediate was derived from the secondary organic aerosol (SOA) yields in experiments with different relative humidity values, kH₂O/kSO₂ = (9.8 ± 3.7) × 10⁻⁵. The experimental results show that SO₃ may not be the only intermediate involved in the formation or growth of new particles in contrast to the data reported for other Criegee intermediate–SO₂ reactions. For the studied reactions, SO₂ concentrations remained constant during the experiments, behaving as a catalyst in the production of condensable products. Computational calculations also show that the stabilised Criegee intermediates from the ozonolysis reaction of both 2,5-dihydrofuran and 2,3-dihydrofuran may react with SO₂, resulting in the regeneration of SO₂ and the formation of low-volatility organic acids.

Smog chamber/GC techniques were used to investigate the atmospheric degradation of two hydrofluoroesters (allyl trifluoroacetate (CF₃C(O)OCH₂CHCH₂) and vinyl trifluoroacetate (CF₃C(O)OCHCH₂)) by oxidation with OH radicals and Cl atoms at 298 K and an atmospheric pressure of N₂ or air. The measured rate coefficients were (in units of cm³ per molecule per s): k_{allyl trifluoroacetate+OH} = (9.27 ± 3.81) × 10⁻¹²; k_{vinyl trifluoroacetate+OH} = (8.07 ± 1.92) × 10⁻¹²; k_{allyl trifluoroacetate+Cl} = (1.75 ± 0.21) × 10⁻¹⁰ and k_{vinyl trifluoroacetate+Cl} = (2.08 ± 0.16) × 10⁻¹⁰. In the OH-initiated oxidation of allyl trifluoroacetate, the identified product can arise from OH addition to both carbons in the double bond and the later decomposition of the alkoxy radical formed. However, in the reactions of both fluoroesters with Cl atoms, the main product detected arises from Cl addition to the terminal carbon atom and the subsequent reaction of the chloroalkoxy radical formed with O₂. Infrared spectra of the studied esters were collected and their contribution to global warming was assessed by calculating their radiative efficiencies. Combining these results with the kinetic data we found that their global warming potentials are negligible. Finally, the photochemical ozone creation potentials were calculated, obtaining values lower than those of non-fluorinated unsaturated hydrocarbons.

Ultrafine particles, PM_2.5 and PM₁₀ mass concentration, NO_x, Ozone, SO₂, back-trajectories of air masses and meteorological parameters were studied in a small city over the period February, 2013 to June, 2014. The profiles of PM_2.5 and PM₁₀ particles are provided, showing averaged values of 16.6 and 21.6 μg m⁻³, respectively. The average number concentration of particles in the range of diameters 5.6–560 nm was 1.2 × 10⁴#/ cm³ with contributions of 42, 51 and 7 % from the nucleation, Aitken, and accumulation modes, respectively. The average number concentration of ultrafine particles was 1.1 × 10⁴#/ cm³. The results obtained are evidence for some differences in the pollution of ambient air by particles in the studied town in comparison to bigger cities. Nucleation events due to emissions from the city were not observed, and traffic emissions amount to a small contribution to PM_2.5 and PM₁₀ particles which are mainly due to crustal origin from the arid surroundings and long-range transport from the Sahara Desert.