Search

Atmospheric particulate matter (PM) remains poorly understood due to the lack of comprehensive measurements at high time resolution for tracking its dynamic features and the lack of long-term observation for tracking its seasonal variability. Here, we present highly time-resolved and seasonal compositions and characteristics of non-refractory components in PM with a diameter less than 1 μm (NR-PM1) at a suburban site in Hong Kong. The measurements were made with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at the Hong Kong University of Science and Technology (HKUST) Air Quality Research Supersite for 4 months, with one in each season of the year. The average NR-PM1 concentration of ~ 15 μg m−3 is higher than those AMS measurements made in South Korea and Japan, but lower than those in North China, the Yangtze River Delta and the nearby Pearl River Delta. The seasonal dependence of the total NR-PM1 monthly averaged concentrations was small, but that of the fractions of the species in NR-PM1 was significant. Site characteristic plays an important role in the relative fractions of species in NR-PM1 and our results are generally consistent with measurements at other non-urban sites in this regard. Detailed analyses were conducted on the AMS data in the aspects of (1) species concentrations, (2) size distributions, (3) degree of oxygenation of organics, and (4) positive matrix factorization (PMF)-resolved organic factors in a seasonal context, as well as with air mass origin from back-trajectory analysis. Sulfate had the highest fraction in NR-PM1 (> 40%), and the surrogates of secondary organic species – semi-volatile oxygenated organic aerosol (SVOOA) and low-volatility oxygenated organic aerosol (LVOOA) – prevailed (~ 80%) in the organic portion of NR-PM1. Local contributions to the organic portion of NR-PM1 at this suburban site was strongly dependent on season. The hydrocarbon-like organic aerosol (HOA) factor related to local traffic emissions contributed > 10% to organic aerosols in spring and summer but only 6–7% in autumn and winter. The cooking organic aerosol (COA) factor contributed > 10% to organic aerosols in winter. With the aid of highly time-resolved data, diurnal patterns of the degree of oxygenation of organic aerosols were used to determine the sources and formation processes of the least understood organic portion of PM. The oxygen-to-carbon atomic ratio (O : C) and average carbon oxidation state OS C) showed little variation in autumn and winter, when the long-range transport of oxidized organics dominated, whereas they peaked in the afternoon in spring and summer, when locally produced secondary organic aerosol prevailed. Air mass origin, in contrast, had a strong influence on both NR-PM1 concentrations and the fractions of species in NR-PM1. The findings of the current study provide a better understanding of the role of air mass origin in the seasonal characteristics of the PM composition and the relative importance of local vs. transported organic aerosols in this region.

The formation of secondary organic aerosol (SOA) has been widely studied in the presence of dry seed particles at low relative humidity (RH). At higher RH, seed particles can exist as dry or wet particles. Here, we investigated the formation of SOA from the photooxidation of toluene using an oxidation flow reactor under a range of OH exposures on initially wet or dry ammonium sulfate (AS) seed particles at an RH of 68 %. At an OH exposure of 4.66 × 1010 molecules cm -3 s, the ratio of the SOA yield on wet AS seeds to that on dry AS seeds was 1.31 ± 0.02. However, this ratio decreased to 1.01 ± 0.01 at an OH exposure of 5.28 × 1011 molecules cm -3 s. The decrease in the ratios of SOA yields as the increase of OH exposure may be due to the early deliquescence of initially dry AS seeds after coated by highly oxidized toluene-derived SOA. SOA formation lowered the deliquescence RH of AS and resulted in the uptake of water by both AS and SOA. Hence the initially dry AS seeds contained aerosol liquid water (ALW) soon after a large fraction of SOA formed and the SOA yield and ALW approached those of the initially wet AS seeds as OH exposure and ALW increased. However, a higher oxidation state of the SOA on initially wet AS seeds than that on dry AS seeds was observed at all levels of OH exposure. The difference in mass fractions of m/z 29, 43 and 44 of SOA mass spectra indicated that SOA formed on initially wet seeds may be enriched in earlier-generation products containing carbonyl functional groups at low OH exposures and later-generation products containing acidic functional groups at high exposures. Our results suggest that AS dry seeds soon turn to at least partially deliquesced particles during SOA formation and more studies on the interplay of SOA formation and ALW are warranted.

A Time of Flight Chemical Ionisation Mass spectrometer (CIMS) utilizing the Fast Inlet for Gas and AEROsol (FIGAERO) was deployed at a regional site 40 km north west of Beijing and successfully identified and measured 17 sulfur containing organics (SCOs = organo/nitrooxyorgano sulfates and sulfonates) with biogenic and anthropogenic precursors. The SCOs were quantified using laboratory synthesized standards of lactic acid sulfate and nitrophenol organosulfate (NP OS). The mean total (of the 17 identified by CIMS) SCO particle mass concentration was 207 ± 106 ng m−3 and had a maxima of 542 ng m−3, although contributed to only 2 ± 2.8 % of the organic aerosol (OA). SCO contribution to submicron mass (PM1) indicates a dominant secondary production of SCO due to the low contribution of SCOs to PM1 during periods of high mass loading. The CIMS identified a persistent gas phase presence of SCOs in the ambient air, which was further supported by post campaign vapour pressure measurements of NP OS. An increase in RH appeared to promote partitioning to the particle phase whereas higher temperatures favored higher gas phase concentrations. On average 12 % of the total SCOs were observed in the gas phase with C10H16NSO7 having just 5 % and IEPOX OS having 44 % on average in the gas phase. Biogenic emissions contributed to only 19 % of total SCOs detected. C10H16NSO7, an alpha-pinene derived SCO, representing the highest fraction (10 %) followed by an isoprene-derived SCO. Anthropogenic SCOs with PAH and aromatic precursors dominated the SCO mass loading (51 % total SCOs) with C11H11SO7, derived from methyl napthalene oxidation, contributing to 40 ng m−3 and 0.3 % of the OA mass. Biomass burning was also identified as a potential anthropogenic and biogenic source of SCOs, based on nitrophenol (NP) and acetonitrile time series via secondary production of NP OS. Gas and particle phase measurements of glycolic acid suggest that partitioning towards the particle phase promotes glycolic acid sulfate production, contrary to the current formation mechanism suggested in the literature. Highly oxidised multifunctional organic compounds (HOMS) and RO2 radical diurnal profiles, as measured by the iodide ToF-CIMS, are similar to that of total SCOs, supporting results that indicate HOMS are able to play a role in SCO production. Anthropogenic related SCOs correlated well with benzene, although their abundance depended highly on the photochemical age of the air mass, tracked using the ratio between pinonic acid and its oxidation product, acting as a qualitative photochemical clock. The HSO4.H2SO4− cluster measured by the CIMS was utilized as a qualitative marker for acidity and provides further evidence that the production rate of total SCOs is efficient in highly acidic aerosols with high SO42− and organic content. This dependency becomes more complex when observing individual SCOs due to variability of specific VOC precursors.

Probing a bullfrog retina with spatially uniform light pulses of correlated stochastic intervals, we calculate the mutual information between the spiking output at the ganglion cells measured with multi-electrode array (MEA) and the interval of the stimulus at a time shift later. The time-integrated information from the output about the future stimulus is maximized when the mean interval of the stimulus is within the dynamic range of the well-established anticipative phenomena of omitted-stimulus responses for the retina. The peak position of the mutual information as a function of the time shift is typically negative considering the processing delay of the retina. However, the peak position can become positive for long enough correlation time of the stimulus when the pulse intervals are generated by a Hidden Markovian model (HMM). This is indicative of a predictive behavior of the retina which is possible only when the hidden variable of the HMM can be recovered from the history of the stimulus for a prediction of its future. We verify that stochastic intervals of the same mean, variance, and correlation time do not result in the same predictive behavior of the retina when they are generated by an Ornstein-Uhlenbeck (OU) process, which is strictly Markovian.

We present electrospray surface-enhanced Raman spectroscopy (ES-SERS) as a new approach to measuring the surface chemical compositions of atmospherically relevant particles. The surface-sensitive SERS is realized by electrospraying Ag nanoparticle aerosols over analyte particles. Spectral features at v(SO42−), v(C–H) and v(O–H) modes were observed from the normal Raman and SERS measurements of laboratory-generated supermicron particles of ammonium sulfate (AS), AS mixed with succinic acid (AS ∕ SA) and AS mixed with sucrose (AS ∕ sucrose). SERS measurements showed strong interaction (or chemisorption) between Ag nanoparticles and surface aqueous sulfate [SO42−] with [SO42−]AS ∕ sucrose > [SO42−]AS ∕ SA > [SO42−]AS. Enhanced spectra of the solid AS and AS ∕ SA particles revealed the formation of surface-adsorbed water on their surfaces at 60 % relative humidity. These observations of surface aqueous sulfate and adsorbed water demonstrate a possible role of surface-adsorbed water in facilitating the dissolution of sulfate from the bulk phase into its water layer(s). Submicron ambient aerosol particles collected in Hong Kong exhibited non-enhanced features of black carbon and enhanced features of sulfate and organic matter (carbonyl group), indicating an enrichment of sulfate and organic matter on the particle surface.

Mass concentration based particle size distributions measured by a high-resolution aerosol mass spectrometer were systematically analyzed to assess long and short-term temporal characteristics of ambient particle size distributions sampled at a typical urban environment close to emission sources and a suburban coastal site representing a regional and local pollution receptor location in Hong Kong. Measured distributions were bimodal and deconvoluted into submodes which were analyzed for day-to-day variations and diurnal variations. Traffic and cooking emissions at the urban site contributed substantially to particle mass in both modes, while notable decreases in mass median diameters were limited to the morning rush hour. Inorganic particle components displayed varying diurnal behavior, including nocturnal nitrate formation and daytime photochemical formation evident in both modes. Suburban particle size distributions exhibited notable seasonal disparities with differing influence of local formation, particularly in spring and summer, and transport which dominated in the fall season leading to notably higher sulfate and organic accumulation mode particle concentrations. Variations in particle mixing state were evaluated by comparison of inter-species mass median diameter trends at both measurement sites. Internal mixing was prevalent in the accumulation mode in spring at the urban site, while greater frequency of time periods with external mixing of particle populations comprising different fractions of organic constituents was observed in summer. At the suburban site, sulfate and nitrate in the accumulation mode more frequently exhibited differing particle size distributions in all seasons signifying a greater extent of external mixing. At the urban site, periods of greater submicron inorganic mass concentrations were more likely to be caused by increases in both Aitken and accumulation mode particle mass in summer, while at the suburban receptor location organic and nitrate Aitken mode particle mass contributed more regularly to higher total submicron species mass concentrations in most seasons (spring, summer and winter).

The adverse health effect of acidic ultrafine particles (AUFPs) has been widely recognized in scientific societies. These particles mainly deposit on the surface by diffusion and so far there is no mature method for the measurement of airborne AUFPs. The purpose of this study was to develop a diffusion sampler (DS) with iron nanofilm detectors to effectively measure the number concentration and size distribution of airborne AUFPs in indoor and outdoor environments. The developed DS was made of stainless steel with a flat and rectangular channel with 1.0 mm height, 50 mm width, and 500 mm length. The iron nanofilm detectors were deployed on rectangular recesses inside the sampler at three different locations along the length of the channel to collect the ultrafine particles. The exposed detectors were then scanned using an atomic force microscope (AFM) to numerate and distinguish the AUFPs from the nonacidic UFPs. Prior to sampling, the semi-empirical equations for the diffusive deposition efficiency of pa...

The cloud condensation nuclei (CCN) properties of atmospheric aerosols were measured on 1–30 May 2011 at the HKUST (Hong Kong University of Science and Technology) Supersite, a coastal site in Hong Kong. Size-resolved CCN activation curves, the ratio of number concentration of CCN (NCCN) to aerosol concentration (NCN) as a function of particle size, were obtained at supersaturation (SS) = 0.15, 0.35, 0.50, and 0.70% using a DMT (Droplet Measurement Technologies) CCN counter (CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk size-integrated NCCN ranged from ~500 cm−3 at SS = 0.15% to ~2100 cm−3 at SS = 0.70%, and the mean bulk NCCN / NCN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (D50) at SS = 0.15, 0.35, 0.50, and 0.70% were 116, 67, 56, and 46 nm, respectively. The corresponding average hygroscopic parameters (κCCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κCCN can be attributed to the increase in organic to inorganic volume ratio as particle size decreases, as measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The κCCN correlates reasonably well with κAMS_SR based on size-resolved AMS measurements: κAMS_SR = κorg × forg + κinorg × finorg, where forg and finorg are the organic and inorganic volume fractions, respectively, κorg = 0.1 and κinorg = 0.6, with a R2 of 0.51. In closure analysis, NCCN was estimated by integrating the measured size-resolved NCN for particles larger than D50 derived from κ assuming internal mixing state. Estimates using κAMS_SR show that the measured and predicted NCCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κAMS was calculated from bulk PM1 AMS measurements of particles because PM1 was dominated by particles of 200 to 500 nm in diameter, which had a larger inorganic fraction than those of D50 (particle diameter < 200 nm). A constant κ = 0.33 (the average value of κAMS_SR over the course of campaign) was found to give an NCCN prediction within 12% of the actual measured values. We also compared NCCN estimates based on the measured average D50 and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. NCCN appears to be relatively more sensitive to the mixing state and hygroscopicity at a high SS = 0.70% and a low SS = 0.15%, respectively.

The Tandem Differential Mobility Analyzer (TDMA) technique coupled with aerosol humidification has been widely used for studying aerosol hygroscopicity. In this study, we evaluate the performance of a commercial Humidified TDMA (BMI HTDMA, Model 3002) with respect to DMA sizing, relative humidity (RH) control, and growth factor (GF) measurements. Unique features of this particular HTDMA include a diffusion-based particle humidifier, a DMA design allowing selection of particles up to 2 μm diameter at only 5600 volts, and the ability to study the complete deliquescence and efflorescence cycle. The sizing agreement between DMA 1 and 2 was within 2% over the 35 to 500 nm diameter range. The measured TDMA responses agreed well with theoretical calculations. The RH control and stability were tested at a suburban field site in Hong Kong. The system achieved RH equilibrium in less than 4 min when changing the RH set point. With indoor temperature changes of less than 1°C per hour, the RH control of the system was...

This paper presents the results of simultaneous aerosol hygroscopicity and composition measurements using a Humidified Tandem Differential Mobility Analyzer (HTDMA) and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) at the HKUST Supersite, located at a coastal suburban site in Hong Kong. Growth factors (GFs) at 90% relative humidity and the size-resolved composition of ambient aerosols at dry electrical mobility diameters of 75, 100, 150, and 200 nm were measured in May, September, and November 2011. Bimodal GF distributions with a dominant more hygroscopic (MH) mode and a small fraction of the nonhygroscopic or less hygroscopic mode (number fraction

Most aerosol chemical characterization studies to date involve bulk particle analysis. The surface chemical and physical properties of aerosol particles have rarely been analyzed, despite the particles’ potential health impacts and interactions with gas in the atmosphere. Aerosol particles ranging from 0.056 to 10 μm in size collected using a 10-stage impactor sampler from a busy walkway in a downtown area of Hong Kong were analyzed using X-ray photoelectron spectroscopy (XPS), a technique providing both elemental and chemical state information about the particle surfaces. Six key elements were detected: nitrogen (N), sulfur (S), calcium (Ca), silicon (Si), oxygen (O), and carbon (C). Carbon was the dominant species on the surfaces of all particles regardless of their sizes. A higher carbon concentration was found on the surfaces of the 0.056–0.32 μm particles. The N, Si, Ca, and O concentrations were higher on the surface of the 3.2–10 μm particles than in the smaller size fractions. Sulfur was mainly fo...

The chemical characteristics of organic aerosol (OA) are still poorly constrained. Here we present observation results of the degree of oxygenation of OA based on high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) measurements made at a coastal site in Hong Kong from late April to the end of May in 2011. Two foggy periods and one hazy period were chosen for detailed analysis to compare the changes in the degree of oxygenation of OA due to different processes. Using HR-ToF-AMS measured inorganic species as input, the Extended Aerosol Inorganic Model (E-AIM) predicted a fine-particle liquid water content (LWCfp) up to 85 μg m−3 during the foggy days. Particle concentration as measured by HR-ToF-AMS was up to 60 μg m−3 during the hazy days and up to 30 μg m−3 during the foggy days. The degree of oxygenation of OA, as indicated by several parameters including the fraction of m/z 44 in organic mass spectra (f44), the elemental ratio of oxygen to carbon (O : C), and the carbon oxidation state (OSc), was evaluated against the odd oxygen (Ox) concentration, LWCfp, ionic strength (IS), and in situ pH (pHis). Observations suggest that the high concentration of OA (on average 11 μg m−3) and the high degree of oxygenation (f44 = 0.15, O : C = 0.51, and OSc = −0.31) during the hazy period were mainly due to gas-phase oxidation. During the foggy periods with low photochemical activities, the degree of oxygenation of OA was almost as high as that on the hazy days, and significantly higher than that during non-foggy/non-hazy days. However, the OA evolved quite differently in the two foggy periods. The first foggy period in late April saw a larger LWCfp and a lower Ox concentration and the OA were made up of ~ 20 % semi-volatile oxygenated organic aerosol (SVOOA) as resolved by positive matrix factorization (PMF). In the second foggy period in mid-May, higher Ox concentration and lower LWCfp were observed, and the OA were found to contain > 50 % low-volatility oxygenated organic aerosols (LVOOA). An examination of the particle characteristics (pHis, IS, and LWCfp) suggests that partitioning may have been the dominating process through which oxygenated species were incorporated into the particle phase during the first foggy period, while oxidation in the aqueous phase dominated over gas-phase processes during the second foggy period.

Particle size distributions are of profound interest in the study of ambient aerosols. Electrostatic classification using the Scanning Mobility Particle Sizer (SMPS) and more recently the Fast-Mobility Particle Sizer (FMPS) is the most commonly employed approach to establish particle size distributions for submicron particles in field and laboratory applications. The FMPS enables fast size distribution measurements on a timescale of seconds but has been speculated to underestimate particle size. Aerosol mass spectrometry has emerged as another well-accepted method for size-resolved compositional aerosol analysis with particle sizing being accomplished by flight time separation over a specified flight path under vacuum conditions. In this work, we characterized the particle sizing performance of an FMPS against simultaneous measurements with an Aerodyne Aerosol Mass Spectrometer (AMS) and an SMPS by sampling ambient particles, as well as polydisperse and monodisperse particles from aqueous inorganic salt solutions in the size range from 50 nm to 450 nm. The particle size measurements by AMS and SMPS produced similar results, while the FMPS significantly underestimated particle size by 40–50%. The discrepancy was observed in all studied ambient and laboratory-generated aerosols and appeared to be largely independent of the sampled species. The observations suggest that it is crucial to evaluate the sizing performance of the FMPS against other instruments to ensure an adequate accuracy of the particle size measurements. In this study, a simple postcorrection method for the FMPS measurements was applied, which was able to successfully reduce the initial underestimation.

[1] An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) has been employed in a field sampling campaign at a suburban coastal site in Hong Kong in springtime 2011 to provide insights into the size-resolved chemical composition of nonrefractory submicron aerosol species. This is the first time that such detailed real-time measurements have been made in Hong Kong. The total nonrefractory PM1 was dominated by sulfate (51.0%) and organics (28.2%) with considerable acidity (average in situ pH = 0.95) and a characteristic bimodal particle size distribution with peaks at 200 and 570 nm of vacuum aerodynamic diameter (Dva). Source apportionment of organic aerosol yielded three characteristic aerosol fractions (hydrocarbon-like organic aerosol, semivolatile organic aerosol and low-volatile organic aerosol) with distinct temporal patterns and distributions in different particle size regions. The influence of air mass origin on species concentrations, particle size distributions and elemental ratios was investigated using backtrajectory analysis. Larger particle diameters, greater fractions of oxygenated organic aerosol and higher organic-to-carbon ratios were observed during coastal and continental air mass influence. Three major pollution events with elevated nonrefractory PM1 concentrations were observed in the sampling period, which were related to distinct meteorological and circulatory conditions. Accumulation and redistribution of local and regional pollutants were notable in a period of strong land-sea breeze over the Pearl River Delta region, with considerable photochemical activity and particle aging. Increased fractions of oxygenated organic aerosol were apparent in foggy conditions, illustrating the importance of aqueous phase oxidation processes in a cooler and more humid time period.

The exchange reaction of ammonia in (NH4)2SO4 with an amine and the corresponding reverse reaction of amines in aminium sulfates with ammonia were investigated using an electrodynamic balance coupled with a Raman spectrometer. The temporal changes in particle mass, chemical composition, and phase state were simultaneously monitored. When the salt particles were in an aqueous state at elevated relative humidities (RHs), the exchange of ammonia/amine vapors in the particle phase was reversible. The exchange rates of aqueous particles were in general higher than those of their corresponding solid counterparts. An aqueous phase was essential for the effective displacement of ammonia and amines. Aminium salts in different phase states and with different evaporation characteristics showed remarkably different reaction behaviors in ammonia vapor. The less compact amorphous aminium sulfate solids were more susceptible to ammonia exchange than the crystalline solids. The aminium salts in a liquid state exhibited substantial amine evaporation at3% RH and formed acidic bisulfate. Under ammonia exposure, these acidic aminium droplets underwent both neutralization and displacement reactions. Stable solid salts containing ammonium, aminium, sulfate, and bisulfate were formed and hindered further reactions. The result suggests that ambient aminium sulfates may be acidic. Overall, the phase states of the ammonium and aminium salt particles crucially determine the heterogeneous reaction rates and final product properties and identities.

Particulate matter (PM) pollution on the peripheries of rapidly expanding megacities in China can be as serious as in the cities due to direct emission and transport of primary PM from cities and effective formation of secondary PM. To investigate the emission and formation of PM on the periphery of Guangzhou (a megacity in southern China), a suite of real-time instruments were deployed at Panyu, downwind of Guangzhou, for PM measurements from November to December 2014. Dominated by organics, PM1 (particles with diameter less than 1 μm) concentrations in Panyu were higher (average ~ 55.4 μg/m3) than those in nearby cities such as Hong Kong and Shenzhen. Five sources for organic aerosols (OA) were resolved by positive matrix factorization (PMF) analysis with the multilinear engine (ME-2). These sources are hydrocarbon-like organic aerosol (HOA), cooking organic aerosol (COA), biomass burning related organic aerosol (BBOA), as well as semi-volatile oxygenated organic aerosol (SVOOA) and low-volatile oxygenated organic aerosol (LVOOA). The use of the COA mass spectrum obtained in our earlier study at a urban site in Hong Kong as a constraining factor in ME-2 produced the most interpretable results for the Panyu dataset. Freshly emitted HOA contributed 40 % to the high concentrations of organics at night. The mass concentration of SOA (SVOOA + LVOOA) continuously increased as odd oxygen (Ox = O3 + NO2) increased during daytime, attributable to the secondary production of PM facilitated by photochemistry. The SOA-to-Ox ratio was higher than those reported in previous studies in North America (covering the period from spring to summer), indicating efficient photochemical production of SOA even in late autumn and early winter at this subtropical downwind site. The efficient SOA formation during daytime was likely fueled by the sufficient SOA precursors in the atmosphere. The large input of NOx, which tracked well with HOA from automobile emissions, resulted in the significant formation of nitrate in both daytime and nighttime. Strong correlations between particulate nitrate and excess ammonium ([NH4+]/[SO42−] − 1.5) × [SO42−]) were observed. Higher partitioning of nitrate into the gas phase was found in November than in December, likely due to the lower temperatures in December. Results from this study suggest that there is much room to mitigate the PM pollution in urbanized areas such as Guangzhou, as well as their peripheries, by reductions in traffic-related pollutants.

Cooking is an important source of volatile organic compounds (VOCs) and a potential source of secondary organic aerosol (SOA) both indoors and outdoors. In this study, VOC emissions from heating corn oil and stir-frying spices (i.e. garlic, ginger, myrcia and zanthoxylum piperitum (Sichuan pepper)) were characterized using an on-line membrane inlet vacuum ultraviolet single-photon ionization time-of-flight mass spectrometer (VUV-SPI-TOFMS). VOC emissions from heating corn oil were dominated by aldehydes, which were enhanced by factors of one order of magnitude when stir-frying spices. Stir-frying any of the spices studied generated large amounts of methylpyrrole (m/z 81). In addition, stir-frying garlic produced abundant dihydrohydroxymaltol (m/z 144) and diallyldisulfide (DADS) (m/z 146), while stir-frying ginger, myrcia and zanthoxylum piperitum produced abundant monoterpenes (m/z 136) and terpenoids (m/z 152, 154). SOA formed from emissions of stir-frying spices through reactions with excess ozone in a flow reactor as well as primary organic aerosol (POA) emissions were characterized using a scanning mobility particle sizer (SMPS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). Stir-frying garlic and ginger generated similar POA concentrations to those from heating corn oil while stir-frying myrcia and zanthoxylum piperitum generated double the amount of emissions. No SOA was observed from stir-frying garlic and ginger. The rates of SOA production from stir-frying myrcia and zanthoxylum piperitum were 1.8μgmin-1gspice-1 and 8.7μgmin-1gspice-1, equivalent to 13.4% and 53.1% of their own POA emission rates, respectively. Therefore, the contribution of stir-frying spices to ambient organic aerosol levels is likely dominated by POA. The rates of total terpene emission from stir-frying myrcia and zanthoxylum piperitum were estimated to be 5.1μgmin-1gspice-1 and 24.9μgmin-1gspice-1, respectively. Our results suggest that stir-frying spices could be an important source of terpenes in indoor environments in Hong Kong, at least during cooking.

Cooking emissions can potentially contribute to secondary organic aerosol (SOA) but remain poorly understood. In this study, formation of SOA from gas-phase emissions of five heated vegetable oils (i.e. corn, canola, sunflower, peanut and olive oils) was investigated in a potential aerosol mass (PAM) chamber. Experiments were conducted at 19–20 ºC and 65–70 % RH. The characterization instruments included a scanning mobility particle sizer (SMPS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). The efficiency of SOA production, in ascending order, was peanut oil, olive oil, canola oil, corn oil and sunflower oil. The major SOA precursors from heated cooking oils were related to the content of mono-unsaturated fat and omega-6 fatty acids in cooking oils. The average production rate of SOA, after aging at an OH exposure of 1.7 × 1011 molecules cm−3 s, was 1.35 ± 0.30 µg min−1, three orders of magnitude lower compared with emission rates of fine particulate matter (PM2.5) from heated cooking oils in previous studies. The mass spectra of cooking SOA highly resemble field-derived COA (cooking-related organic aerosol) in ambient air, with R2 ranging from 0.74 to 0.88, suggesting that COA might not be entirely primary in origin. The average carbon oxidation state (OSc) of SOA was −1.51–−0.81, falling in the range between ambient hydrocarbon-like organic aerosol (HOA) and semi-volatile oxygenated organic aerosol (SV-OOA), indicating that SOA in these experiments was lightly oxidized.

Using a hidden Markov model (HMM) that describes the position of a damped stochastic harmonic oscillator as a stimulus input to a data processing system, we consider the optimal response of the system when it is targeted to predict the coming stimulus at a time shift later. We quantify the predictive behavior of the system by calculating the mutual information (MI) between the response and the stimulus of the system. For a passive sensor, the MI typically peaks at a negative time shift considering the processing delay of the system. Using an iterative approach of maximum likelihood for the predictive response, we show that the MI can peak at a positive time shift, which signifies the functional behavior of active prediction. We find the phenomena of active prediction in bullfrog retinas capable of producing omitted stimulus response under periodic pulse stimuli, by subjecting the retina to the same HMM signals encoded in the pulse interval. We confirm that active prediction requires some hidden information to be recovered and utilized from the observation of past stimulus by replacing the HMM with a Ornstein–Uhlenbeck process, which is strictly Markovian, and showing that no active prediction can be observed.