Tropospheric degradation by solar photolysis of biogenic unsaturated aldehydes.

THILO MINICH; ELIZABETH GAONA COLMAN; MARIANO TERUEL; BLANCO MARÍA BELÉN

Congreso; XIII ELAFOT; 2017

Volatile Organic Compounds (VOCs) emitted from vegetation often referred to as biogenic volatile organic compounds (BVOCs) are considered to be the main source of reactive species in the troposphere1. Aldehydes play an important role in atmospheric chemistry since they are recognized as important key precursors in the formation of tropospheric ozone and OH radicals2 and are also potential prolific sources of secondary organic aerosols through their reactions with atmospheric oxidants and photolysis.3Aldehydes are ubiquitous gaseous chemical constituents in the atmosphere and are emitted by a variety of biogenic sources.2In the troposphere aldehydes can be photolyzed2 and will also be subject to reaction with the atmospheric oxidants OH radicals, NO3 radicals and O3 molecules and also Cl atoms. The combination of the photodissociation of aldehydes and reaction with the atmospheric oxidants represents, in many cases, an important source of free radicals in the lower atmosphere which can significantly influence the atmospheric oxidation capacity.2We present here, a photochemical study in the actinic region of a series of unsaturated aldehydes as: (E)-2-heptenal (CH3(CH2)3CH=CHC(O)H), (E)-2-octenal (CH3(CH2)4CH=CHC(O)H) and (E)-2-nonenal (CH3(CH2)5CH=CHC(O)H) of atmospheric interest, since as mentioned above they are emitted by different types of vegetation.1Photolysis of the aldehydes follows first order kinetics and can be represented by:Aldehyde + h  products, jwhere j is the photolysis rate coefficient. We report the photolysis rate coefficients of these VOCs by direct photolysis of the sun using a 80 L teflon bag coupled to Gas Cromatography with Flame ptIonization (GC-FID) as detection system. Values of j((E)-2-heptenal)= (3.4  0.4)  10-5 s-1, j((E)-2-octenal)= (1.5  0.5)  10-5 s-1 and j((E)-2-nonenal)= (4.3  0.5)  10-5 s-1 were determinedThe photolysis processes of these aldehydes are very important with tropospheric lifetimes of 6 to 18 hours which would indicate that the photolysis reactions of these long chain aldehydes could compete with the OH-radical-initiated reactions. In general mechanistic studies of short chain aldehydes demonstrate that the photolysis channel has a yield of 50% and the remaining 50% is the reaction pathway with OH radical during the day.4Therefore, we can consider that the atmospheric sinks of (E)-2-heptenal, (E)-2-octenal and (E)-2-nonenal in the atmosphere during the day is the reaction with OH and solar photolysis. Thephotolysis of these unsaturated aldehydes in the atmosphere is expected to be generate several species including 2-hexenal, 2-heptenal, 2-octenal, CO and HOx radicals.