Estimates of the organic aerosol volatility in a boreal forest using two independent methods

Abstract

Abstract. The volatility distribution of secondary organic aerosols that formed and had undergone aging – i.e., the particle mass fractions of semi-volatile, low-volatility and extremely low volatility organic compounds in the particle phase – was characterized in a boreal forest environment of Hyytiälä, southern Finland. This was done by interpreting field measurements using a volatility tandem differential mobility analyzer (VTDMA) with a kinetic evaporation model. The field measurements were performed during April and May 2014. On average, 40 % of the organics in particles were semi-volatile, 34 % were low-volatility organics and 26 % were extremely low volatility organics. The model was, however, very sensitive to the vaporization enthalpies assumed for the organics (ΔHVAP). The best agreement between the observed and modeled temperature dependence of the evaporation was obtained when effective vaporization enthalpy values of 80 kJ mol−1 were assumed. There are several potential reasons for the low effective enthalpy value, including molecular decomposition or dissociation that might occur in the particle phase upon heating, mixture effects and compound-dependent uncertainties in the mass accommodation coefficient. In addition to the VTDMA-based analysis, semi-volatile and low-volatility organic mass fractions were independently determined by applying positive matrix factorization (PMF) to high-resolution aerosol mass spectrometer (HR-AMS) data. The factor separation was based on the oxygenation levels of organics, specifically the relative abundance of mass ions at m∕z 43 (f43) and m∕z 44 (f44). The mass fractions of these two organic groups were compared against the VTDMA-based results. In general, the best agreement between the VTDMA results and the PMF-derived mass fractions of organics was obtained when ΔHVAP = 80 kJ mol−1 was set for all organic groups in the model, with a linear correlation coefficient of around 0.4. However, this still indicates that only about 16 % (R2) of the variation can be explained by the linear regression between the results from these two methods. The prospect of determining of extremely low volatility organic aerosols (ELVOAs) from AMS data using the PMF analysis should be assessed in future studies.

Supplementary material to "Secondary organic aerosol formation from α-pinene, alkanes and oil sands related precursors in a new oxidation flow reactor"

“…As a result, a OFR fate correction for S/IVOCs is not feasible. Previous field measurements and laboratory studies demonstrated that SOA from various sources were mainly ~40%-80% L/ELVOCs and ~20%-60% S/IVOCs (Hong et al, 2017;Saha et al, 2017;D'Ambro et al, 2018;Sato et al, 2018;Saha et al, 2018). In our experiments, the low-NOx yields are significantly higher than the high-NOx yields (paper in preparation), indicating a lower volatility for SOA formed under low-NOx conditions.…”

Long-term sub-micrometer aerosol chemical composition in the boreal forest: inter- and intra-annual variability

“…The chemical composition of aerosol particles at SMEAR II has been studied previously in multiple short (< 1-10 month) measurement campaigns with both offline (Saarikoski et al, 2005;Kourtchev et al, 2005;Cavalli et al, 2006;Finessi et al, 2012;Corrigan et al, 2013;Kourtchev et al, 2013;Kortelainen et al, 2017) and online methods (Allan et al, 2006;Finessi et al, 2012;Häkkinen et al, 2012;Corrigan et al, 2013;Crippa et al, 2014;Hong et al, , 2017Makkonen et al, 2014;Äijälä et al, 2017Kortelainen et al, 2017;Riva et al, 2019). The previous studies include several important discoveries regarding SMEAR II aerosol composition.…”

Reply to comments #1

“…A scanning mobility particle sizer (SMPS, TSI, Inc.) was used to measure the particle number size distribution (mobility diameter: 15-600 nm) with a time resolution of 5 min. By assuming the densities of the components obtained in the literature (Kuwata et al, 2012;Poulain et al, 2014;Hu et al, 2017), the corresponding mass concentration can be calculated from the particle number size distribution. The AE-31 and SMPS were used only for ambient sampling.…”