Towards closing the gap between hygroscopic growth and CCN activation for secondary organic aerosols – Part 3: Influence of the chemical composition on the hygroscopic properties and volatile fractions of aerosols

摘要

The influence of varying levels of water mixing ratio, r, during theformation of secondary organic aerosol (SOA) from the ozonolysis of α-pineneon the SOA hygroscopicity and volatility was investigated. Thereaction proceeded and aerosols were generated in a mixing chamber and thehygroscopic characteristics of the SOA were determined with the LeipzigAerosol Cloud Interaction Simulator (LACIS) and a Cloud Condensation Nucleicounter (CCNc). In parallel, a High-Resolution Time-of-Flight AerodyneAerosol Mass Spectrometer (HR-ToF-AMS) located downstream of a thermodenuder(TD) sampling from the mixing chamber, to collect mass spectra of particlesfrom the volatile and less-volatile fractions of the SOA. Results showedthat both hygroscopic growth and the volatile fraction of the SOA increasedwith increases in r inside the mixing chamber during SOA generation. Aneffective density of 1.40 g cm−3 was observed for the generated SOAwhen the reaction proceeded with r2+ and the sum ofC_xH_yO_z+ (short name CHO) and C_xH_y+ (shortname CH) fragments as measured by the HR-ToF-AMS were used to estimatechanges in the oxidation level of the SOA with reaction conditions, usingthe ratios CO₂+ to CH and CHO to CH. Under humid conditions, bothratios increased, corresponding to the presence of more oxygenatedfunctional groups (i.e., multifunctional carboxylic acids). This result isconsistent with the α-pinene ozonolysis mechanisms which suggestthat water interacts with the stabilized Criegee intermediate. Thevolatility and the hygroscopicity results show that SOA generation viaozonolysis of α-pinene in the presence of water vapour (r<16.9 g kg−1)leads to the formation of more highly oxygenated compounds thatare more hygroscopic and more volatile than compounds formed under dryconditions.