AIDA SOA08 campaign “MUCHACHAS”

Motivation and objectives

Motivation

Secondary organic aerosols (SOA) from the oxidation of biogenic volatile organic compounds (BVOC) are a large fraction of the tropospheric aerosol especially over tropical continental regions. Dominant SOA forming processes are the reactions of monoterpenes with OH radicals, NO₃ radicals, and ozone forming SOA mass in highly variable yields. Model calculations show that a substantial amount of SOA mass from oxidation of BVOCs may be formed in the free troposphere and hence at lower temperatures. During the estimated atmospheric lifetime of SOA of about at week multiphase reactions that transform the compounds and thus alter the volatility distribution of that material can have a profound effect on the aerosol mass and also on the characteristics of the organic aerosol (including cloud forming potential and health effects). The temperature dependence of SOA formation, yields, and ageing processes is one of the major uncertainties for understanding atmospheric SOA.

Therefore we investigated the yield and ageing of SOA material from the ozonolysis of α-pinene limonene, and isoprene under simulated tropospheric conditions in the large aerosol chamber AIDA on time scales of up to 50 hours and at temperatures between 243 and 313 K. The experimentally determined time evolutions of hydrocarbon, ozone, aerosol mass and number concentrations, as well as the particle size distributions were analysed using the aerosol behaviour code COSIMA, supplemented by a recently developed SOA module. For α-pinene and limonene the measured data could be consistently reproduced by assuming the SOA particles to consist of only two products with different vapour pressures. Detailed model analysis resulted in the parameters necessary for describing SOA yields e.g. as function of temperature and organic aerosol mass.

Objectives

Investigate later-generation oxidation of SOA formed from ozone + terpene reactions.

Ø Explore the effect of OH reactions with SOA (from α-pinene and limonene + ozone) and follow the O:C ratio of the products in the particulate phase using high resolution aerosol mass spectrometry at low temperatures in AIDA to constrain the evolving volatility of the higher-volatility products. Ozone + light alkenes (Tetramethylethylene) will be used as a dark OH source.

Some of the following issues will also be addressed :

Ø SOA yields for concentrations of atmospheric relevance,

Ø Formation and partitioning of semivolatile VOC's as indicators for temperature and humidity dependent reaction channels for SOA formation.