The contribution of terpene oxidation by O3, OH and NO3 to aerosol formation and the influence of water vapour: particle size distribution studies.

Abstract

Secondary org. aerosol is formed during the reaction of O3 with biogenic alkenes; however, particle formation initiated by the reaction of terpenes with OH- or NO3- is still uncertain, but might also contribute significantly to secondary org. aerosol (SOA) formation. This lab. study on the SOA size distribution compared the ability to form new aerosol particles (no. and vol. concn.) via 3 different terpene oxidn. processes (O3, OH-, NO3-) using a- and b-pinene as surrogates for endo- and exocyclic alkenes. Results indicated the reaction with O3 is by far the most efficient reaction to initiate nucleation, thus producing new aerosol particles. Observations showed that oxidn. by OH- and NO3- cause much less intensive nucleation than by O3. Moreover, particle formation can be rather suppressed by lowering the initial monoterpene concn. to 50 ppbv, e.g., threshold for a-pinene >80 ppbv, at which nucleation during ozonolysis still occurs. Aerosol vol. produced in OH- or NO3- reactions contributes in a similar amt. to the atm. aerosol vol. obsd. in ozonolysis. This indicates that products of higher volatility are formed from OH- and NO3- reactions. Although expts. were performed at relatively high concns. vs. those occurring in the actual atm., it was concluded the most important process leading to SOA formation is ozonolysis. Also, expts. performed with different water vapor concns. showed only an influence of water concn. in ozonolysis; oxidn. studies with OH- and NO3- remain unaffected. This might be a good indicator for field studies to distinguish between different oxidn. processes leading to nucleation. Sesquiterpene-O3 reactions at initial concns. of 25 pptv and 50 ppbv showed nucleation depends on the water vapor concn., which might be of atm. relevance. [on SciFinder (R)]