Abstract

Radicals initiate the breakdown of the vast quantity and variety of volatile organic compounds (VOC) emitted to the atmosphere. Understanding radical budgets is crucial therefore to enable calculation of VOC lifetimes, assess impacts on air quality, chemistry and climate, and confidently predict future trends. Recent results from lab-, field-, model- and theoretical-work demonstrate that clean-air radical chemistry is poorly understood, even for important species such as OH. Radicals are consumed by VOC (R1), but may be recycled in subsequent oxidation steps such as (R2b & R3b).

OH + RH    ->    R     (R1)

R  O2 ( M)    ->    RO2 (+ M)    (R2a)

->    OH + (other products)    (R2b)

HO2  RO2     ->    ROOH  O2     (R3a)

->    OH  RO  O2    (R3b)

Whilst traditional chemical schemes would predict little radical recycling in clean-air, with formation of phytotoxic peroxides or peracids (R3a), large OH yields (R3b) have been identified for carbonyl-containing RO2 . In recent work conducted at the Max Planck Institute for Chemistry, Mainz, and at the School of Chemistry, University of Leeds, we have focussed on the clean-air oxidation of the important biogenic VOC isoprene (C5H8). Experimental techniques used have included Pulsed Laser Photolysis coupled to Laser Induced Fluorescence detection of OH (Mainz) and a suitably equipped environmental chamber (HIRAC, Leeds). A number of isoprene-related radical recycling yields will be presented, together with some mechanistic information for the most extensively studied system (R = CH3CO ). Atmospheric implications of these and other results will be discussed.