Berto Paul Yok Long Lee

Simultaneous HTDMA and HR‐ToF‐AMS measurements at the HKUST Supersite in Hong Kong in 2011

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 (GForg) of 1.18 are within 10% closure. Approximations for GForg 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, GForg of 1 to 1.5 (with κorg up to 0.29) was found to best fit the additional water content unexplained by the inorganic species.

Analysis of organic sulfur compounds in atmospheric aerosols at the HKUST supersite in Hong Kong using HR-ToF-AMS.

Organic sulfur compounds have been identified in ambient secondary organic aerosols, but their contribution to organic mass is not well quantified. In this study, using a high-resolution time-of-flight aerosol mass spectrometer (AMS), concentrations of organic sulfur compounds were estimated based on the high-resolution fragmentation patterns of methanesulfonic acid (MSA), and organosulfates (OS), including alkyl, phenyl, and cycloalkyl sulfates, obtained in laboratory experiments. Mass concentrations of MSA and minimum mass concentrations of OS were determined in a field campaign conducted at a coastal site of Hong Kong in September 2011. MSA and OS together accounted for at least 5% of AMS detected organics. MSA is of marine origin with its formation dominated by local photochemical activities and enhanced by aqueous phase processing. OS concentrations are better correlated with particle liquid water content (LWC) than with particle acidity. High-molecular-weight OS were detected in the continental influenced period probably because they had grown into larger molecules during long-range transport or they were formed from large anthropogenic precursors. This study highlights the importance of both aqueous-phase processing and regional influence, i.e., different air mass origins, on organic sulfur compound formation in coastal cities like Hong Kong.

Sizing Characterization of the Fast-Mobility Particle Sizer (FMPS) Against SMPS and HR-ToF-AMS

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. Copyright 2013 American Association for Aerosol Research

Characteristics of submicron particulate matter at the urban roadside in downtown Hong Kong—Overview of 4 months of continuous high-resolution aerosol mass spectrometer measurements

Hong Kong, one of the worlds most densely populated cities and an international financial center, has been suffering from traffic-related air pollution. This study presents the first real-time high-resolution aerosol mass spectrometry measurements of submicron nonrefractory particulate matter (NR-PM1) at the urban roadside in Hong Kong from March to July 2013 with the aim to identify major sources, to assess local and nonlocal emissions, and to characterize trends at different time scales. Organics were dominant, with fresh primary organic aerosol representing two thirds of the total measured organics. Cooking contributions in organic aerosol were assessed directly for the first time in Hong Kong and exceeded those related to vehicles although traffic was still the major PM1 source when elemental carbon was included. These findings were supported by additional measurements including traffic data, elemental/organic carbon, and VOC data. Springtime concentrations were about double of those in summer, due to a strong seasonal transition which affected meteorological conditions and street-level circulation. Local formation of secondary species was not clearly discernible in either season. The elemental composition of organic aerosol remained stable with similar elemental ratios across the covered seasons: OM/OC: 1.49 ± 0.13, O/C: 0.25 ± 0.10, H/C: 1.68 ± 0.08 for spring and OM/OC: 1.43 ± 0.14, O/C: 0.21 ± 0.11, H/C: 1.69 ± 0.08 for summer. Diurnal changes in H/C and O/C as a result of mixing of primary organic aerosol and secondary organic aerosol were evident in the van Krevelen plot.

Comparison of daytime and nighttime new particle growth at the HKUST supersite in Hong Kong.

Particles larger than 50-100 nm in diameter have been considered to be effective cloud condensation nuclei (CCN) under typical atmospheric conditions. We studied the growth of newly formed particles (NPs) in the atmosphere and the conditions for these particles to grow beyond 50 nm at a suburban coastal site in Hong Kong. Altogether, 17 new particle formation events each lasting over 1 h were observed in 17 days during 8 Mar-28 Apr and 1 Nov-30 Dec 2011. In 12 events, single-stage growth of NPs was observed in daytime when the median mobility diameter of NPs (Dp) increased up to ∼40 nm but did not increase further. In three events, two-stage particle growth to 61-97 nm was observed at nighttime. The second stage growth was preceded by a first-stage growth in daytime when the Dp reached 43 ± 4 nm. In all these 15 events, organics and sulfuric acid were major contributors to the first-stage growth in daytime. Ammonium nitrate unlikely contributed to the growth in daytime, but it was correlated with the second-stage growth of ∼40 nm NPs to CCN sizes at nighttime. The remaining two events apparently showed second-stage growth in late afternoon but were confirmed to be due to mixing of NPs with pre-existing particles. We conclude that daytime NP growth cannot reach CCN sizes at our site, but nighttime NP growth driven by organics and NH4NO3 can.

Relative Humidity-Dependent HTDMA Measurements of Ambient Aerosols at the HKUST Supersite in Hong Kong, China

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 size-resolved 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 = 10–93%. The Kappa (κ) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) κ at individual RHs (κRH) and (ii) best-fit κ covering the range of RHs measured (κf) for the more-hygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, κ at 90% RH or above (κ>90) fell between 0.18 and 0.48, and was within 15% of κf in 83% of the datasets. Regression analysis between κ>90 or κf and AMS mass fractions showed that κ was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, κRH increased as RH decreased and the average increase in κ 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 κ were mostly within 20% of κ>90 and κf. GF predictions using the empirical correlation of κ with AMS mass fractions or the ZSR estimated κ were within 10% of additional measurements and hence κ>90 is useful for predicting GF at lower RHs. Copyright 2015 American Association for Aerosol Research

Characterization and source identification of sub-micron particles at the HKUST Supersite in Hong Kong

Particle size distribution measurements were conducted continuously at a 30-second interval using the Fast Mobility Particle Sizer (FMPS) in August, September, November and December of 2011 at a coastal background site in Hong Kong. Concurrent measurements of CO, NOx, O3, SO2 and volatile organic compounds (VOCs) were used to determine the causes of high particle number concentration (PNC) events. In all sampling months, PNC were usually higher in the evening, likely resulting from the arrival of upwind air pollutants as wind direction changed in the late afternoon. On the more polluted days, the PNC were usually higher around noon, particularly in August, similar to the diurnal trend of O3. The mode diameter at noon was smaller than in other time periods in all sampling months, further highlighting the role of secondary formation at this urban background site. A prolonged period of pollution episode occurred in late August. High PNC resulted from the arrival of pollution laden air from the PRD region or super regions. In December, new particle formation followed by subsequent growth accounted for most of the polluted days. Overall, meteorology was the most important parameter affecting particle concentrations and formation at this Hong Kong background site.