Suppressed atmospheric chemical aging of cooking organic aerosol particles in wintertime conditions

Cooking organic aerosol (COA) is one of the major constituents of particulate matter in urban areas. COA is oxidized by atmospheric oxidants such as ozone, changing its physical, chemical and toxicological properties. However, atmospheric chemical lifetimes of COA and its tracers such as oleic acid are typically longer than those that have been estimated by laboratory studies. We tackled the issue by considering temperature. Namely, we hypothesize that increased viscosity of COA at ambient temperature accounts for its prolonged atmospheric chemical lifetimes in wintertime. Laboratory-generated COA particles from cooking oil were exposed to ozone in an aerosol flow tube reactor for the temperature range of −20 to 35 °C. The pseudo-second-order chemical reaction rate constants (k2) were estimated from the experimental data by assuming a constant ozone concentration in the flow tube. The estimated values of k2 decreased by orders of magnitude for lower temperatures. The temperature dependence in k2 was fit well by considering the diffusion-limited chemical reaction mechanism. The result suggested that increased viscosity was likely the key factor to account for the decrease in chemical reactivity at the reduced temperature range, though the idea will still need to be verified by temperature-dependent viscosity data in the future. In combination with the observed global surface temperature, the atmospheric chemical lifetimes of COA were estimated to be much longer in wintertime (&gt; 1 h) than in summertime (a few minutes) for temperate and boreal regions. Our present study demonstrates that the oxidation lifetimes of COA particles will need to be parameterized as a function of temperature in the future for estimating environmental impacts and fates of this category of particulate matter.</p

Particle Classification by the Tandem Differential Mobility Analyzer–Particle Mass Analyzer System

<div><p>Particle mass analyzers, such as the aerosol particle mass analyzer (APM) and the Couette centrifugal particle mass analyzer (CPMA), are frequently combined with a differential mobility analyzer (DMA) to measure particle mass <i>m</i>_<i>p</i> and effective density ρ_eff distributions of particles with a specific electrical mobility diameter <i>d</i>_<i>m</i>. Combinations of these instruments, which are referred to as the DMA–APM or DMA–CPMA system, are also used to quantify the mass-mobility exponent <i>D</i>_<i>m</i> of non-spherical particles as well as to eliminate multiple charged particles. This study investigates the transfer functions of these setups, focusing especially on the DMA–APM system. The transfer function of the DMA–APM system was derived by multiplying the transfer functions of DMA and APM. The APM transfer function can be calculated using either the uniform or parabolic flow models. The uniform flow model provides an analytical function, while the parabolic flow model is more accurate. The resulting DMA–APM transfer functions were plotted on log(<i>m</i>_<i>p</i>)-log(<i>d</i>_<i>p</i>) space. A theoretical analysis of the DMA–APM transfer function demonstrated that the resolution of the setup is maintained when the rotation speed ω of APM is scanned to measure distribution. In addition, an equation was derived to numerically calculate the minimum values of the APM resolution parameter λ_<i>c</i> for eliminating multiple charged particles.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

1-octanol-water partitioning as a classifier of water soluble organic matters: Implication for solubility distribution

<p>Water-soluble organic matters (WSOMs) play an important role in determining magnitudes of climatic and environmental impacts of organic aerosol particles because of their contributions to hygroscopic growth and cloud formation. These processes are dependent on water solubility as well as distribution of this property in a particle, yet no method has been available to quantify such characteristics. In this study, we developed a theoretical framework to classify WSOM by 1-octanol-water partitioning that has a strong correlation with water solubility. 1-octanol-water partitioning coefficient also has a strong correlation with a traditional solid phase extraction method, facilitating interpretation of data from the technique. The theoretical analysis demonstrated that the distributions of WSOM classified by 1-octanol-water partitioning depend on (1) the volume ratio of 1-octanol and aqueous phases, and (2) extraction steps. The method was tested by using organic aerosol particles generated by smoldering of a mosquito coil, which serves as a surrogate for biomass burning particles. The WSOM extracted from the mosquito coil burning particles was classified by 1-octanol-water partitioning at different volume ratios. These solutions, including both the 1-octanol and aqueous phases, were nebulized to generate particles for measurements using an online aerosol mass spectrometer. The mass spectra indicated that highly oxygenated species tend to be highly soluble, while high molecular weight compounds are less soluble. Linear combinations of these mass spectra allowed the estimation of the mass fractions of WSOM partitioned to 1-octanol and aqueous phases, thereby facilitating the evaluation of the mass fractions of cloud condensation nuclei (CCN) active materials.</p> <p>© 2017 American Association for Aerosol Research</p

Roles of relative humidity and particle size on chemical aging of tropical peatland burning particles: potential influence of phase state and implications for hygroscopic property

Peatland fires in Southeast Asia are an important source of primary organic aerosol (POA). Chemical aging of POA in the atmosphere produces oxygenated POA (OPOA). The OPOA production influences optical and hygroscopic properties, modulating the regional climate. However, the roles of environmental parameters such as relative humidity (RH) on chemical aging of peatland burning particles have rarely been investigated. Utilizing the potential aerosol mass (PAM) reactor, we conducted laboratory experiments for POA aging of particles generated from smoldering combustions of surface peat, fern, and Acacia leaves. The corresponding experiments for secondary organic aerosol formation were also separately conducted. Properties and chemical compositions of the resulting particle were quantified by both the hygroscopic tandem differential mobility analyzer and time-of-flight aerosol chemical speciation monitor (ToF-ACSM). Conversion of peat combustion from POA to OPOA was pronounced when RH in the PAM reactor was higher. Considering that previous electromicroscopic observations demonstrated that peat combustion POA is likely (semi)solid, we postulate that oxidation of fresh peatland burning particles is faster at an elevated RH due to reduced viscosity following hygroscopic growth. Hygroscopicity parameter (κ) of aged POA particles linearly correlated with the mass fraction of OPOA that was quantified by the ToF-ACSM. The above results highlight the importance of simultaneously measuring the chemical aging and particle phase state of peatland burning POA for quantifying their climatic impacts.Nanyang Technological UniversityPublished versionThis work was supported by the National Natural Science Founda-tion of China (Grant Nos. 42105075 and no. 42175121), the Singapore National Research Foundation (NRF) under its Singapore National Research Fellow-ship scheme (National Research Fellow Award, NRF2012NRF-NRFF001-031), the NRF Campus for Research Excellence and Technological Enterprise (CREATE) program (NRF2016-ITCOO1-021), and Nanyang Technological University. M.I. was funded by the Ministry of Education, Culture, Sports, Science, and Technology for Science Research of Japan (18H02238 and 19H05666) and Research Institute for Humanity and Nature (RIHN; Project No. 14200117)

An Analytic Equation for the Volume Fraction of Condensationally Grown Mixed Particles and Applications to Secondary Organic Material Produced in Continuously Mixed Flow Reactors

<div><p>Secondary condensation of organic material onto primary seed particles is one pathway of particle growth in the atmosphere, and many properties of the resulting mixed particles depend on organic volume fraction. Environmental chambers can be used to simulate the production of these types of particles, and the optical, hygroscopic, and other properties of the mixed particles can be studied. In the interpretation of the measured properties, the probability density function <i>p</i>(<i>ϵ</i>;<i>d</i>) of volume fraction <i>ϵ</i> of the condensing material for particle diameter <i>d</i> in the outflow of the chamber is typically needed. In this article, analytic equations are derived <i>p</i>(<i>ϵ</i>;<i>d</i>) for condensational growth in a continuously mixed flow reactor. The equation predictions are compared to measurements for the condensation of secondary organic material on quasi-monodisperse sulfate seed particles. Equations are presented herein for discrete, Gaussian, and triangular distribution functions for the seed particle number–diameter distributions, including generalization to any linearly segmented distributions. The analytic equations are useful both for the interpretation of laboratory data from environmental chambers, such as the construction of probability density functions for use in interpretation of hygroscopic growth data, cloud–condensation–nuclei data, or other laboratory data sets dependent on organic volume fraction, as well as for understanding atmospheric processes at times that condensational growth processes prevail.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

Secondary aerosol formation promotes water uptake by organic-rich wildfire haze particles in equatorial Asia

The diameter growth factor (GF) of 100nm haze particles at 85% relative humidity (RH) and their chemical characteristics were simultaneously monitored at Singapore in October 2015 during a pervasive wildfire haze episode that was caused by peatland burning in Indonesia. Non-refractory submicron particles (NR-PM1) were dominated by organics (OA; approximating 77.1% in total mass), whereas sulfate was the most abundant inorganic constituent (11.7% on average). A statistical analysis of the organic mass spectra showed that most organics (36.0% of NR-PM1 mass) were highly oxygenated. Diurnal variations of GF, number fractions of more hygroscopic mode particles, mass fractions of sulfate, and mass fractions of oxygenated organics (OOA) synchronized well, peaking during the day. The mean hygroscopicity parameter (κ) of the haze particles was 0.189±0.087, and the mean κ values of organics were 0.157±0.108 (κorg, bulk organics) and 0.266±0.184 (κOOA, OOA), demonstrating the important roles of both sulfate and highly oxygenated organics in the hygroscopic growth of organics-dominated wildfire haze particles. κorg correlated with the water-soluble organic fraction insignificantly, but it positively correlated with f44 (fraction of the ion fragment at m∕z44 in total organics) (R=0.070), implying the oxygenation degree of organics could be more critical for the water uptake of organic compounds. These results further suggest the importance of sulfate and secondary organic aerosol formation in promoting the hygroscopic growth of wildfire haze particles. Further detailed size-resolved as well as molecular-level chemical information about organics is necessary for the profound exploration of water uptake by wildfire haze particles in equatorial Asia.NRF (Natl Research Foundation, S’pore)Published versio

Using Elemental Ratios to Predict the Density of Organic Material Composed of Carbon, Hydrogen, and Oxygen

A governing equation was developed to predict the density ρ_org of organic material composed of carbon, oxygen, and hydrogen using the elemental ratios O:C and H:C as input parameters: ρ_org = 1000 [(12 + 1­(H:C) + 16­(O:C)]/[7.0 + 5.0­(H:C) + 4.15­(O:C)] valid for 750 < ρ_org < 1900 kg m^–3. Comparison of the actual to predicted ρ_org values shows that the developed equation has an accuracy of 12% for more than 90% of the 31 atmospherically relevant compounds used in the training set. The equation was further validated for secondary organic material (SOM) produced by isoprene photo-oxidation and by α-pinene ozonolysis. Depending on the conditions of SOM production, ρ_org/SOM ranged from 1230 to 1460 kg m^–3, O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric chemistry models that simulate particle production and growth can employ the developed equation to simulate particle physical properties. The equation can also extend atmospheric measurements presented as van Krevelen diagrams to include estimates of the material density of particles and their components. Use of the equation, however, is restricted to particle components having negligible quantities of additional elements, most notably nitrogen

The role of sulfur emission from the petroleum industry on ultrafine particle number concentration in Singapore

Ultrafine particles, defined as particles with a diameter (dp) smaller than 100 nm, serve as an important component of cloud condensation nuclei, in addition to impacting human health. The dominant sources of ultrafine particles include traffic emissions and nucleation. Singapore is a tropical city that hosts petrochemical industries. To identify the sources of ultrafine particles, a year-long observation of the number size distribution was conducted in Singapore in 2018 and 2019. The concentrations of CO, CO2, CH4, and SO2 were also monitored. The particle number concentration during the southwest monsoon season was high, while that during the northeast monsoon period was relatively low. The CO concentration increased during the morning traffic rush hours, which was associated with relatively minor enhancements in ultrafine particle number concentration. The events for a high number concentration of the Aitken mode particles (dp 50 nm) correlated with the enhancements in CO concentration (ΔCO) for CH4-dominant air masses, suggesting that incomplete combustion processes, such as traffic emission, are important for the size range. Conversely, the number concentration of the Aitken mode particles (dp < 50 nm) increased for SO2-dominant air masses, suggesting the importance of industrial plume.National Research Foundation (NRF)Published versionThis work was supported by the Singapore National Research Foundation (NRF) under its Singapore National Research Fellowship scheme (National Research Fellow Award, NRF2012NRFNRFF001-031) and the National Natural Science Foundation of China (42175121 and 4215061048)

Using Elemental Ratios to Predict the Density of Organic Material Composed of Carbon, Hydrogen, and Oxygen

A governing equation was developed to predict the density ρ_org of organic material composed of carbon, oxygen, and hydrogen using the elemental ratios O:C and H:C as input parameters: ρ_org = 1000 [(12 + 1­(H:C) + 16­(O:C)]/[7.0 + 5.0­(H:C) + 4.15­(O:C)] valid for 750 < ρ_org < 1900 kg m^–3. Comparison of the actual to predicted ρ_org values shows that the developed equation has an accuracy of 12% for more than 90% of the 31 atmospherically relevant compounds used in the training set. The equation was further validated for secondary organic material (SOM) produced by isoprene photo-oxidation and by α-pinene ozonolysis. Depending on the conditions of SOM production, ρ_org/SOM ranged from 1230 to 1460 kg m^–3, O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric chemistry models that simulate particle production and growth can employ the developed equation to simulate particle physical properties. The equation can also extend atmospheric measurements presented as van Krevelen diagrams to include estimates of the material density of particles and their components. Use of the equation, however, is restricted to particle components having negligible quantities of additional elements, most notably nitrogen