Relating hygroscopicity and optical properties to chemical composition and structure of secondary organic aerosol particles generated from the ozonolysis of α-pinene

Abstract

Abstract. Secondary organic aerosol (SOA) were generated from the ozonolysis of α-pinene in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) simulation chamber. The SOA formation and aging were studied by following their optical, hygroscopic and chemical properties. The optical properties were investigated by determining the particle complex refractive index (CRI). The hygroscopicity was quantified by measuring the effect of relative humidity (RH) on the particle size (size growth factor, GF) and on the scattering coefficient (scattering growth factor, f (RH)). The oxygen to carbon atomic ratios (O : C) of the particle surface and bulk were used as a sensitive parameter to correlate the changes in hygroscopic and optical properties of the SOA composition during their formation and aging in CESAM.The real CRI at 525 nm wavelength decreased from 1.43-1.60 (±0.02) to 1.32-1.38 (±0.02) during the SOA formation. The decrease in the real CRI correlated to the O : C decrease from 0.68 (±0.20) to 0.55 (±0.16). In contrast, the GF remained roughly constant over the reaction time, with values of 1.02-1.07 (±0.02) at 90 % (±4.2 %) RH. Simultaneous measurements of O : C of the particle surface revealed that the SOA was not composed of a homogeneous mixture, but contained less oxidised species at the surface which may limit water absorption. In addition, an apparent change in both mobility diameter and scattering coefficient with increasing RH from 0 to 30 % was observed for SOA after 14 h of reaction. We postulate that this change could be due to a change in the viscosity of the SOA from a predominantly glassy state to a predominantly liquid state.

Determining the link between hygroscopicity and composition for semi-volatile aerosol species

“…6). This low value mirrors other studies of semi-volatile organics (Meyer et al, 2009;Raatikainen et al, 2010) and is within the range of α-Pinene HGFs observed by Prenni et al (2007) and Denjean et al (2015).…”

Determining the link between hygroscopicity and composition for semi-volatile aerosol species

“…6). This low value mirrors other studies of semi-volatile organics (Meyer et al, 2009;Raatikainen et al, 2010), and is within the range of α-pinene HGFs observed by Prenni et al (2007) and Denjean et al (2015).…”

Complex refractive index of secondary organic aerosol generated from isoprene/NOx photooxidation in the presence and absence of SO2

“…Recently, positive correlations between the n value and O/C ratio were also reported for the SOA generated during the photooxidation of toluene in the presence of NO x [ Nakayama et al ., ], ozonolysis and OH oxidation of the α ‐pinene/limonene/ p ‐xylene mixture [ Flores et al ., ], and ozonolysis of α ‐pinene [ Denjean et al ., ]. In contrast, Lambe et al [] reported a negative relation between n and O/C for SOA generated in the OH oxidation of naphthalene, guaiacol, tricyclo [5.2.1.0 2,6 ] decane, and α ‐pinene, in the absence of NO x .…”