Abstract
Abstract. The chemical and physical properties of secondary organic aerosol (SOA) formed by the photochemical degradation of biogenic and anthropogenic volatile organic compounds (VOC) are as yet still poorly constrained. The evolution of the complex refractive index (RI) of SOA, formed from purely biogenic VOC and mixtures of biogenic and anthropogenic VOC, was studied over a diurnal cycle in the SAPHIR photochemical outdoor chamber in Jülich, Germany. The correlation of RI with SOA chemical and physical properties such as oxidation level and volatility was examined. The RI was retrieved by a newly developed broadband cavity-enhanced spectrometer for aerosol optical extinction measurements in the UV spectral region (360 to 420 nm). Chemical composition and volatility of the particles were monitored by a high-resolution time-of-flight aerosol mass spectrometer, and a volatility tandem differential mobility analyzer. SOA was formed by ozonolysis of either (i) a mixture of biogenic VOC (α-pinene and limonene), (ii) biogenic VOC mixture with subsequent addition of an anthropogenic VOC (p-xylene-d10), or (iii) a mixture of biogenic and anthropogenic VOC. The SOA aged by ozone/OH reactions up to 29.5 h was found to be non-absorbing in all cases. The SOA with p-xylene-d10 showed an increase of the scattering component of the RI correlated with an increase of the O / C ratio and with an increase in the SOA density. There was a greater increase in the scattering component of the RI when the SOA was produced from the mixture of biogenic VOCs and anthropogenic VOC than from the sequential addition of the VOCs after approximately the same ageing time. The increase of the scattering component was inversely correlated with the SOA volatility. Two RI retrievals determined for the pure biogenic SOA showed a constant RI for up to 5 h of ageing. Mass spectral characterization shows the three types of the SOA formed in this study have a significant amount of semivolatile components. The influence of anthropogenic VOCs on the oxygenated organic aerosol as well as the atmospheric implications are discussed.
Highlights
The interaction between aerosols and incoming solar radiation plays an important role in the radiative balance of Earth’s atmosphere
We have measured the evolution of the complex refractive index in the UV spectral region, between 360 and 420 nm, of biogenic SOA (BSOA) and ABSOA formed from three different mixtures of biogenic and anthropogenic (p-xylene-d10) volatile organic compounds (VOC) at low NOx levels
One experiment consisted of pure BSOA produced from a 1 : 1 mixture of α-pinene and limonene; the other two experiments consisted of ABSOA, one with the ABSOA produced from the sequential addition of a 1 : 1 mixture of α-pinene and limonene followed by pxylene-d10, and the other with the ABSOA produced from a mixture of α-pinene, limonene and p-xylene-d10
Summary
The interaction between aerosols and incoming solar radiation plays an important role in the radiative balance of Earth’s atmosphere. Black carbon, which is the dominant absorber of solar radiation in the atmosphere, has fairly well defined optical properties with an estimate of the industrial-era mean direct radiative forcing of approximately +1.1 W m−2 (Bond et al, 2013). The optical properties of light-absorbing organic particles, or “brown” carbon (Andreae and Gelencsér, 2006), which may account for 10–40 % of the total light absorption in the atmosphere, and on snow and ice (Park et al, 2010; Bahadur et al, 2012; Cappa et al, 2012; Chung et al, 2012; Kirchstetter and Thatcher, 2012; Feng et al, 2013; Bond et al, 2013), are still poorly constrained.
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