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Abstract:
HOx (≡OH + HO2) radicals, act as the most important catalytic species in the air, controlling most of the daytime chemical transformations of VOCs and NOx. Ozone and secondary aerosols are generated as by-products during cyclic reactions. However, the closure of the radical budget and the oxidizing capacity of the air remain poorly understood. Using a top-down approach, we conducted measurements of OH and HO2 reactivity by using Laser Photolysis–Laser Induced Fluorescence, which determines the decay rates of corresponding radicals and makes it possible to capture the HOx performance. Field measurements and chamber experiments were conducted in various conditions to understand the OH-initiated oxidation process, quantify the missing HOx cycle mechanism and evaluate its further impact on ozone production. On the other hand, HO2 is a reservoir of OH and is less active than OH in gas-phase reactions; however, aerosol particles can absorb HO2, which acts as a potential sink for HOx radicals. These multi-phase processes alter the HOx budget and require detailed investigation to fully understand HOx chemistry. We measured the HO2 uptake using synthetic and ambient aerosols. This was done to examine how the physicochemical properties of the aerosols could vary the heterogeneous processes and to parameterize the HO2 uptake onto aerosols for a future simulation of the HOx budget.

In this talk, I will address the following topics: 1) The results of field campaigns and chamber experiments focusing on the breakdown of OH reactivity, and 2) The factors controlling the heterogeneous processes for HO2 uptake based on HO2 reactivity measurement. These topics provide a general understanding of the HOx chemistry under typical atmospheric conditions and enable the ozone production sensitivity to be assessed.