Hygroscopicity and phase transitions were measured with deposited particles of ammonium sulfate (AS), ammonium nitrate (AN), malonic acid (MA), glutaric acid (GA), glyoxylic acid (GlyA), as well as two mixed particle systems AS-MA and AS-GA using micro-Raman spectroscopy. Hygroscopicity was presented in terms of water-to-solute mass ratios, which were obtained from the integrated area ratios of the Raman water band to a distinct solute peak. Deliquescence and crystallization were confirmed by abrupt changes in the Raman peak positions and the full-widthhalf-heights of distinct solute peaks. The results for AS, AN, MA, and GA agreed well with literature reports and model predictions. For GlyA, we detected the Raman water band at near 0% RH, indicating that the spectral technique is sensitive for hygroscopic measurements at very low RH. Additional spectral feature at ∼0% RH was also observed. In the case of more complicated ASdicarboxylic acid mixed systems, the partial phase transitions of the organic components were identified using the intensity ratios of aqueous to solid C=O peaks. AS-MA particles did not completely crystallize and gradual water uptake with increasing RH from 3% was observed. Moreover, it was found that AS-GA particles showed step-wise crystallization in which the AS fraction crystallized prior to the GA fraction. The measured water content and complete DRH of both mixed systems were consistent with the published values. The results show the utility of micro-Raman spectroscopic analysis in studying hygroscopicity and phase characterizations of the chemical species in mixed particles.

Abstract. 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.

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 < 0.2) were observed throughout the study. The average MH mode GF (1.44-1.52) over the second half of September was significantly lower than that in other periods (1.53-1.64) due to the high organic loading under the influence of the continental airstream. The average ensemble mean growth factor (GF*) of maritime aerosols was higher (1.53-1.59) than that of continental aerosols (1.45-1.48). Closure between the aerosol hygroscopicity and chemical composition was also evaluated. Over 90% of measured data and predictions based on Extended Aerosol Inorganics Model with a constant GF of the organic fraction (GF org ) of 1.18 are within 10% closure. Approximations for GF org using the fraction of m/z 44 in organic mass spectra, the oxygen-to-carbon atomic ratio, and PMF-resolved organic factors from HR-ToF-AMS measurements did not yield better closure results, likely because of the overall dominance of sulfate over the whole study period. Finally, GF org of 1 to 1.5 (with κ org up to 0.29) was found to best fit the additional water content unexplained by the inorganic species.

We studied the phase transition and hygroscopicity of particles of a mixture of two atmospherically relevant species, ammonium sulfate and adipic acid. We conducted extensive investigations on ammonium sulfate-adipic acid (AS-AA) particles deposited on a hydrophobic substrate with the weight percentage (wt%) of AA ranging from 3 to 90%. Crystallization and deliquescence were observed through an optical microscope, which enabled multiple particles to be examined simultaneously. At an initial relative humidity (RH) of 94%, which was the highest RH setting, AA solids were formed from the deposited solution droplets, leading to the presence of mixed-phase particles prior to the complete crystallization recorded between 31% and 42% RH of all mixtures. The complete crystallization RH values were close to that of pure AS, indicating that the AA solids did not promote effective heterogeneous nucleation of AS. When the RH was increased, partial deliquescence in all mixtures was observed at 80% RH, which was attributed to the dissolution of the AS fractions. Full deliquescence was only observed in 3 wt% AA particles at 91% RH. We also used Micro-Raman Spectroscopy to determine the hygroscopicity of mixtures with up to 50 wt% AA. The hygroscopic behavior of the AS fraction in the mixed particles was found to resemble that of pure AS particles. However, there was observable water uptake in the solid particles at about 70% RH in mixtures with up to 70 wt% AA. This early water uptake was more pronounced in mixtures with lower weight percentages of AA (AA less than 30 wt%).

The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and sizeresolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100 nm and 200 nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH D 10-93%. The Kappa (k) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) k at individual RHs (k RH ) and (ii) best-fit k covering the range of RHs measured (k f ) for the morehygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, k at 90% RH or above (k >90 ) fell between 0.18 and 0.48, and was within 15% of k f in 83% of the datasets. Regression analysis between k >90 or k f and AMS mass fractions showed that k was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, k RH increased as RH decreased and the average increase in k was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated k were mostly within 20% of k >90 and k f . GF predictions using the empirical correlation of k with AMS mass fractions or the ZSR estimated k were within 10% of additional measurements and hence k >90 is useful for predicting GF at lower RHs.

The effects of the polymorphism of glutaric acid (GA) particles on their deliquescence properties at room temperature were studied. The polymorphic states of GA, including the metastable alpha-form and the thermodynamically stable beta-form, were characterized by Raman spectral measurements. The deliquescence of GA particles was also studied by Raman and optical observations. It was found that the alpha-form deliquesced at 85-86% relative humidity (RH), whereas some alpha-form transformed to the beta-form rather than completing full deliquescence. The beta-form absorbed water slowly at 87-89% RH, which was higher than the deliquescence relative humidity (DRH) of the alpha-form. The water-uptake process was not completed in 5 h, but the particles deliquesced quickly (<15 min) when the RH was increased to about 90%, indicating that mass transfer was delayed in the beta-form and that a higher RH of 90% was needed to cross the kinetic barrier. We showed that polymorphic transformation alters the deliquescence properties of GA particles, which might explain the discrepancies in DRH values of GA particles (83-90%) reported in the literature. This transformation should be taken into account in future laboratory experiments and aerosol thermodynamic models.

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 diffusionbased particle humidifier, a DMA design allowing selection of particles up to 2 mm 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 very stable at 90%, within 1% RH deviation, as confirmed by GF measurements on ammonium sulfate (AS) aerosols performed on separate days. The hygroscopic properties of various pure aerosols were examined and the results agreed well with model predictions. The application of the BMI HTDMA for field measurements was also demonstrated. Two modes were resolved from the GF distributions at 90% RH and variable hygroscopic growth with changing RH was observed.

Kandelia candel plants were grown in a simulated tide-tank system in the greenhouse to determine the performance of the mangrove ecosystem in treating synthetic wastewater of various strengths (NW, 5NW and 25NW). NW had a strength similar to the natural municipal wastewater, while 5NW and 25NW had 5 and 25 times, respectively, the amount of nutrients and heavy metals as in NW. The system was flooded daily with artificial seawater to simulate the tidal regime. Synthetic wastewater was irrigated to the system three times a week for 3 months. The results of effluent characteristics showed that the removal efficiencies of nutrients and metals from the wastewater were nearly 98% (except for organic N) and 96%, respectively, in NW and 5NW tanks whereas those of 25NW tanks were 75% for nutrients, 92% for Cd, Cr, Cu and around 88% for Ni and Zn. This study demonstrated that the mangrove ecosystem had a very high capacity to retain or immobilize the nutrients and heavy metals in the wastewater suggesting mangrove wetland had inherent physical, chemical and biological properties for adsorption and utilization of nutrients and heavy metals.