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Investigation of Particle Effective Density, Size Distribution and Dose Metric of Ambient Aerosol


The exposure to particulate matter (PM) has strong adverse health effects, such as aggravated asthma, decrease in lung function, and increase respiratory symptoms. PM is complex and dynamic in its chemical composition and physical mixing state. The properties of PM have not been thoroughly characterized, and the mechanism of PM causing adverse effect has not been well understood. This study focuses on characterizing physical properties of urban background air particles by analyzing the particle spectra, particle effective density, black carbon, and active surface area along with chemical characterization using AMS. The effective density profiles were used to estimate the respiratory system-deposited ambient particle mass according to lung deposition fraction curve reported by ICRP (1994).

The first study was to determine size-resolved effective densities of ambient aerosols in Riverside, CA for four 7-day periods during 2015 – 2016. The mass of size selected particles (50, 70, 101 and 152 nm) were measured to determine the effective density for particles. A catalytic stripper (CS) in alternating mode was used to remove volatile compounds on the aerosol before density measurements. Aerosol non-refractory composition measurement was conducted in June 2016 campaign to understand the effect of chemical composition on particle density. The average particle densities for all measurement campaigns with CS bypassing mode (BP) were 1.17 g/cm3 at 50 nm and 1.25 – 1.28 g/cm3 at 70, 101 and 152 nm. The average density after CS conditioning (CS mode) showed a decreasing trend from 1.22 g/cm3 to 1.04 g/cm3 withincreasing selected size, and a mass fractal dimension (Df) of 2.85. Both the BP and CS mode particles showed the lowest effective density from 6:00 AM to 9:00 AM and highest density from 11:00 AM to 3:00 PM. The diurnal variation of density became more profound as particle size increases for both sampling modes. The variation was found to be more intense for the CS mode compared than the BP mode. To confirm this observation, organic aerosol and ammonium nitrate mass in the similar size range were tested simultaneously with BP mode. The results showed effective density measurements correlated well with positively (R2=0.78) and negatively (R2=0.62). This study provided an update to the aerosol density profiles of a well-known receptor site (Riverside, CA) and investigated the transformation of particles in different seasons. The obtained effective density profiles were used in the proceeding study in order to effectively estimate the respiratory-deposited ambient aerosol mass.

In the second study, the size distributions and effective densities of ambient particles measured in September 2015 and June 2016 were used to calculate time-dependent lung deposited particulate matter (PM) mass using lung deposition fraction curve reported by ICRP (1994). The particle active surface area, black carbon (BC) mass, particle number (PN), solid particle number (SPN), and suspended PM mass (from particle size distributions and effective densities) were also obtained to investigate correlations with PM dose. Non-refractory organic and inorganic mass measured by an aerosol mass spectrometer (mAMS) provided additional information in relation to dose versus chemical nature of particles.

Ambient particle size distributions showed strong diurnal variations during the sampling period. While the lung deposited PM mass fraction (0.32-0.36) did not vary much with time, lung deposited and suspended PM mass in the ambient air showed similar trend. Deposition in the alveolar region was the highest (0.3-1.0 μg/m3), which was followed by the nasal region (0.16 – 0.83 μg/m3) and tracheobronchial region (0.05 – 0.27 μg/m3). All three regions peaked at 12:00 PM – 6:00 PM and reached the lowest value near midnight.

One of the goals of this study was to consider the most prominent ambient PM measurement method in consideration to health and lung deposition. Suspended PM mass had the highest correlation with respiratory doses, which was followed by organics measured using mAMS, active surface area, and BC mass. It is also found that particle hygroscopicity did not affect correlations between metrics and lung deposited PM mass. By taking into account the accessibility and cost of these measurements, we propose the particle active surface area and BC mass to be considered when evaluating/monitoring the health effects caused by PM.

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