Volatility of mixed atmospheric humic-like substances and ammonium sulfate particles
Wei Nie, Juan Hong, Silja A. K. Häme, Aijun Ding, Yugen Li, Chao Yan, Liqing Hao, Jyri Mikkilä, Longfei Zheng, Yuning Xie, Caijun Zhu, Zheng Xu
(+11 others)
2017
Atmospheric Chemistry and Physics
<p><strong>Abstract.</strong> The volatility of organic aerosols remains poorly understood due to the complexity of speciation and multiphase processes. In this study, we extracted humic-like substances (HULIS) from four atmospheric aerosol samples collected at the SORPES station in Nanjing, eastern China, and investigated the volatility behavior of particles at different sizes using a Volatility Tandem Differential Mobility Analyzer (VTDMA). In spite of the large differences in particle mass
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... ncentrations, the extracted HULIS from the four samples all revealed very high-oxidation states (O<span class="thinspace"></span>:<span class="thinspace"></span>C<span class="thinspace"></span>&gt;<span class="thinspace"></span>0.95), indicating secondary formation as the major source of HULIS in Yangtze River Delta (YRD). An overall low volatility was identified for the extracted HULIS, with the volume fraction remaining (VFR) higher than 55<span class="thinspace"></span>% for all the regenerated HULIS particles at the temperature of 280<span class="thinspace"></span>°C. A kinetic mass transfer model was applied to the thermodenuder (TD) data to interpret the observed evaporation pattern of HULIS, and to derive the mass fractions of semi-volatile (SVOC), low-volatility (LVOC) and extremely low-volatility components (ELVOC). The results showed that LVOC and ELVOC dominated (more than 80<span class="thinspace"></span>%) the total volume of HULIS. Atomizing processes led to a size-dependent evaporation of regenerated HULIS particles, and resulted in more ELVOC in smaller particles. In order to understand the role of interaction between inorganic salts and atmospheric organic mixtures in the volatility of an organic aerosol, the evaporation of mixed samples of ammonium sulfate (AS) and HULIS was measured. The results showed a significant but nonlinear influence of ammonium sulfate on the volatility of HULIS. The estimated fraction of ELVOC in the organic part of the largest particles (145<span class="thinspace"></span>nm) increased from 26<span class="thinspace"></span>%, in pure HULIS samples, to 93<span class="thinspace"></span>% in 1<span class="thinspace"></span>:<span class="thinspace"></span>3 (mass ratio of HULIS<span class="thinspace"></span>:<span class="thinspace"></span>AS) mixed samples, to 45<span class="thinspace"></span>% in 2<span class="thinspace"></span>:<span class="thinspace"></span>2 mixed samples, and to 70<span class="thinspace"></span>% in 3<span class="thinspace"></span>:<span class="thinspace"></span>1 mixed samples, suggesting that the interaction with ammonium sulfate tends to decrease the volatility of atmospheric organic compounds. Our results demonstrate that HULIS are important low-volatility, or even extremely low-volatility, compounds in the organic-aerosol phase. As important formation pathways of atmospheric HULIS, multiphase processes, including oxidation, oligomerization, polymerization and interaction with inorganic salts, are indicated to be important sources of low-volatility and extremely low-volatility species of organic aerosols.</p>
doi:10.5194/acp-17-3659-2017
fatcat:yspq6cejwfeefepvqkc4bht5he
