Hong Kong, one of the world's 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-PM 1 ) 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 PM 1 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.

[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 PM 1 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 (D va ). 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 PM 1 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.

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.