ALBERT

Description: A new capture vaporizer (CV) has been developed and used recently in the Aerodyne aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) instead of the standard vaporizer (SV) to reduce the particle bounce. It is important to characterize the CV performance in different environments. In this study, we characterized specific organic aerosols (OAs) from vehicle, cooking, biomass burning, and coal burning emissions by a time-of-flight ACSM (TOF-ACSM) with the CV. Their corresponding marker ions that have been defined in the previous SV-based analysis are still valid in the CV mass spectra. Spectra of OAs from cooking and vehicle exhaust show similarities in distinct alkyl fragments but different ratios of m∕z 55 and 57. Ions related to polycyclic aromatic hydrocarbons are present in the OA spectra obtained from burning lignite and bituminous coal but not in the spectra obtained from burning anthracite. Although the relative intensities of m∕z 60 and 73 are much lower in the CV spectra than in the SV spectra for biomass burning OA, they are still relatively greater compared with the spectra for other sources. Our data suggest an atmospheric background of f60 of below 0.03 % for CV. Moreover, we deployed the CV TOF-ACSM along with a SV AMS in urban Beijing during the winter of 2017 to characterize ambient OA with strong anthropogenic influences. The CV TOF-ACSM shows a collection efficiency (CE) of about unity. The CV and SV data show consistent mass concentrations of sulfate, nitrate, ammonium, and OA. Six OA factors are identified by the positive matrix factorization (PMF) analysis for both the CV and the SV data. The SV and CV PMF factors show good correlations in mass concentrations. The SV and CV factors related to coal combustion and cooking differ significantly in loadings, explained by the PMF uncertainty and the lack of understanding of the relative ionization efficiency and CE for primary OA. The CV factors related to secondary sources show greater loadings than the SV factors, which may be associated with the changes in signal-to-noise ratios of various ions in the PMF analysis. Our results support improved mass quantification and useful source identification by the CV for ambient particles in the polluted urban environment. The difference in factor loadings between SV and CV should be considered when interpreting or comparing the PMF results among studies.

Description: Light-absorbing organic carbon (or brown carbon, BrC) in atmospheric particles has received much attention for its potential role in global radiative forcing. While a number of field measurement campaigns have differentiated light absorption by black carbon (BC) and BrC, the chemical characteristics of BrC are not well understood. In this study, we present co-located real-time light absorption and chemical composition measurements of atmospheric particles to explore the relationship between the chemical and optical characteristics of BrC at a suburban site downwind of Guangzhou, China, from November to December 2014. BrC and BC contributions to light absorption were estimated using measurements from a seven-wavelength aethalometer, while the chemical composition of non-refractory PM1 was measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Using the absorption Ångström exponent (AAE) method, we estimated that BrC contributed 23.6 % to the total aerosol absorption at 370 nm, 18.1 % at 470 nm, 10.7 % at 520 nm, 10.7 % at 590 nm, and 10.5 % at 660 nm. Biomass burning organic aerosol (BBOA) has the highest mass absorption coefficient among sources of organic aerosols. Its contribution to total brown carbon absorption coefficient decreased but that of low-volatility oxygenated organic aerosol (LVOOA) increased with increasing wavelength, suggesting the need for wavelength-dependent light absorption analysis for BrC in association with its chemical makeup. Clear correlations of N-containing ion fragments with absorption coefficient were observed. These correlations also depended on their degrees of unsaturation/cyclization and oxygenation. While the current study relates light absorption by BrC to ion fragments, more detailed chemical characterization is warranted to constrain this relationship.

Description: Calcium- and magnesium-containing salts are important components for mineral dust and sea salt aerosols, but their physicochemical properties are not well understood yet. In this study, hygroscopic properties of eight Ca- and Mg-containing salts, including Ca(NO3)2⚫4H2O, Mg(NO3)2⚫6H2O, MgCl2⚫6H2O, CaCl2⚫6H2O, Ca(HCOO)2, Mg(HCOO)2⚫2H2O, Ca(CH3COO)2⚫H2O and Mg(CH3COO)2⚫4H2O, were investigated using two complementary techniques. A vapor sorption analyzer was used to measure the change of sample mass with relative humidity (RH) under isotherm conditions, and the deliquescence relative humidities (DRHs) for temperature in the range of 5–30 ∘C as well as water-to-solute ratios as a function of RH at 5 and 25 ∘C were reported for these eight compounds. DRH values showed large variation for these compounds; for example, at 25 ∘C DRHs were measured to be ∼ 28.5 % for CaCl2⚫6H2O and 〉95 % for Ca(HCOO)2 and Mg(HCOO)2⚫2H2O. We further found that the dependence of DRH on temperature can be approximated by the Clausius–Clapeyron equation. In addition, a humidity tandem differential mobility analyzer was used to measure the change in mobility diameter with RH (up to 90 %) at room temperature, in order to determine hygroscopic growth factors of aerosol particles generated by atomizing water solutions of these eight compounds. All the aerosol particles studied in this work, very likely to be amorphous under dry conditions, started to grow at very low RH (as low as 10 %) and showed continuous growth with RH. Hygroscopic growth factors at 90 % RH were found to range from 1.26 ± 0.04 for Ca(HCOO)2 to 1.79 ± 0.03 for Ca(NO3)2, and the single hygroscopicity parameter ranged from 0.09–0.13 for Ca(CH3COO)2 to 0.49–0.56 for Ca(NO3)2. Overall, our work provides a comprehensive investigation of hygroscopic properties of these Ca- and Mg-containing salts, largely improving our knowledge of the physicochemical properties of mineral dust and sea salt aerosols.

Description: Hygroscopicity largely affects environmental and climatic impacts of pollen grains, one important type of primary biological aerosol particles in the troposphere. However, our knowledge of pollen hygroscopicity is rather limited, and the effect of temperature in particular has rarely been explored before. In this work three different techniques, including a vapor sorption analyzer, diffusion reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission Fourier transform infrared spectroscopy (transmission FTIR) were employed to characterize six anemophilous pollen species and to investigate their hygroscopic properties as a function of relative humidity (RH, up to 95 %) and temperature (5 or 15, 25 and 37 ∘C). Substantial mass increase due to water uptake was observed for all the six pollen species, and at 25 ∘C the relative mass increase at 90 % RH, when compared to that at

Description: Particulate matter (PM) pollution on the peripheries of Chinese megacities can be as serious as in cities themselves. Given the substantial vehicular emissions in inner-city areas, the direct transport of primary PM (e.g., black carbon and primary organics) and effective formation of secondary PM from precursors (e.g., NOx and volatile organic compounds) can contribute to PM pollution in buffer zones between cities. To investigate how traffic emissions in inner-city areas impact these adjacent buffer zones, a suite of real-time instruments were deployed in Panyu, downwind from central Guangzhou, from November to December 2014. Nitrate mass fraction was higher on high-PM days, with the average nitrate-to-sulfate ratio increasing from around 0.35 to 1.5 as the PM mass concentration increased from 10 to 160 µg m−3. Particulate nitrate was strongly correlated with excess ammonium (([NH4+] ∕ [SO42−] − 1.5)  ×  [SO42−]), with higher concentrations in December than in November due to lower temperatures. The organic mass fraction was the highest across all PM1 levels throughout the campaign. While organic aerosols (OA) were dominated by secondary organic aerosols (SOA  =  semi-volatile oxygenated organic aerosols + low-volatility oxygenated organic aerosols) as a campaign average, freshly emitted hydrocarbon-like organic aerosols (HOA) contributed up to 40 % of OA during high-OA periods, which typically occurred at nighttime and contributed 23.8 to 28.4 % on average. This was due to daytime traffic restrictions on heavy-duty vehicles in Guangzhou, and HOA almost increased linearly with total OA concentration. SOA increased as odd oxygen (Ox  =  O3 + NO2) increased during the day due to photochemistry. A combination of nighttime traffic emissions and daytime photochemistry contributed to the buildup of PM in Panyu. The mitigation of PM pollution in inner-city areas by reducing vehicular traffic can potentially improve air quality in peripheral areas.

Description: Hygroscopicity is one of the most important physicochemical properties of aerosol particles and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100 %). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development.

Description: Episodes with high concentrations of particulate matter (PM) across the seasons were investigated during four one-month campaigns at a suburban site in Hong Kong. High-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements revealed that both regional transport and secondary formation contributed to high PM levels during the episodes at this site. Based on distinct meteorological conditions, episodes were categorized into three types: liquid water content (LWC), solar irradiance (IR), and long-range transport (LRT). Despite the difference in meteorological conditions, all episodes were characterized by a high fraction of sulfate (45 %–56 %) and organics (23 %–34 %). However, aerosols in LWC episodes were less aged, consisting of the lowest fraction of secondary organics aerosols (SOA) and the highest fraction of small particles. Large particles mixed internally while freshly formed small particles mixed externally in LWC episodes. Aerosols in LRT episodes, by contrast, were the most aged and consisted of the highest proportion of low-volatility oxygenated organic aerosols (LVOOA) and the lowest proportion of small particles. Both small and large particles mixed externally in LRT episodes. The highest proportion of semi-volatile oxygenated organic aerosols (SVOOA) and a medium proportion of small particles were observed in IR episodes. Both small and large particles were likely externally mixed during IR episodes. Unlike in the other two types of episodes, in IR episodes aerosols experienced the most dramatic size increase and diurnal variation, with a time lag between SVOOA and LVOOA and a gradual increase in carbon oxidation state (OSc ≈ 2 × O : C – H : C). Five out of ten episodes were of the IR type, further reflecting the importance of this type of episode. The evolution of aerosol components in one particular episode of the IR type, which exhibited a clear land-sea breeze pattern, was examined in detail. Sulfate and SOA due to photochemical aging were very efficiently produced during the course of six hours. The “fresh” SOA (SVOOA) was initially formed at a higher rate than the “aged” SOA (LVOOA). The SVOOA transformed to LVOOA at the later stage of photochemical aging. This tranformation was further supported by mass spectral analysis, which showed an increase in the most oxidized ion (CO2+) and decreases in moderately oxidized ones (C2H3O+, C3H3O+ and C3H5O+). By measuring the physical and chemical properties of PM in a highly time-resolved manner, the current study was able to demonstrate the dynamic and complex nature of PM transformation during high-PM episodes.

Description: 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.

Description: Hygroscopicity largely affects environmental and climatic impacts of pollen grains, one important type of primary biological aerosol particles in the troposphere. However, our knowledge in pollen hygroscopicity is rather limited, and especially the effect of temperature has rarely been explored before. In this work three different techniques, including a vapor sorption analyzer, diffusion reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission Fourier transform infrared spectroscopy (transmission FTIR) were employed to characterize six anemophilous pollen species and to investigate their hygroscopic properties as a function of relative humidity (RH, up to 95%) and temperature (5 or 15, 25 and 37°C). Substantial mass increase due to water uptake was observed for all the six pollen species, and at 25°C the relative mass increase at 90% RH, when compared to that at

Description: Light-absorbing organic carbon (or brown carbon, BrC) in atmospheric particles has received much attention for its potential role in global radiative forcing. While a number of field measurement campaigns have differentiated light absorption by black carbon (BC) and BrC, the chemical characteristics of BrC are not well understood. In this study, we present co-located real-time light absorption and chemical composition measurements of atmospheric particles to explore the relationship between the chemical and optical characteristics of BrC at a suburban site downwind of Guangzhou, China from November to December 2014. BrC and BC contributions to light absorption were estimated using measurements from a seven-wavelength aethalometer, while the chemical composition of non-refractory PM1 was measured with a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Using the Absorption Angstrom Exponent (AAE) method, we estimated that BrC contributed 23.6% to the total aerosol absorption at 370nm, 18.1% at 470nm, 10.7% at 520nm, 10.7% at 590nm, and 10.5% at 660nm. Biomass burning organic aerosol (BBOA) has the highest mass absorption coefficient among sources of organic aerosols. Its contribution to total brown carbon absorption coefficient decreased but that of low-volatility oxygenated organic aerosol (LVOOA) increased with increasing wavelength, suggesting the need for wavelength-dependent light absorption analysis for BrC in association with its chemical makeup. Clear correlations of N-containing ion fragments with absorption coefficient were observed. These correlations also depended on their degrees of unsaturation/cyclization and oxygenation. While the current study relates light absorption by BrC to ion fragments, more detailed chemical characterization is warranted to constrain this relationship.