Emissions and secondary formation of air pollutants in urban environments - chemical characteristics using on-line mass spectrometry

Abstract

Atmospheric particulate matter (PM) and trace gases influence air quality and climate through direct or indirect effects and have detrimental impacts on human health. The highly variable emission sources in urban environments have resulted in considerable variations of air pollution levels and posed challenges in developing effective air pollution control strategies. Anthropogenic activities, such as traffic and cooking, generate abundant particulate and gaseous species, which can form secondary pollutants via complex atmospheric processes. The success to improve air quality depends on an actual reduction in both emissions and the subsequent formation of secondary pollutants. Therefore, it is particularly important to characterize the emissions and secondary formation of air pollutants from representative sources in urban environments.

This thesis presents results from independent field and laboratory measurements, where traffic emissions from heavy-duty trucks (HDTs) and marine ships and cooking emissions from heated animal fats and their potential to form secondary pollutants were studied. State of the art on-line mass spectrometric techniques, including high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS), high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), proton transfer reaction mass spectrometry (PTR-MS) and single particle aerosol mass spectrometer (SPAMS), were selectively used in the measurements.

We began with the investigation of HDT emissions by extractive sampling of the individual HDT plumes under real-world traffic uphill driving conditions at an urban roadside site in Gothenburg, Sweden. The emissions from Euro VI compliant HDTs were compared to older classes, represented by Euro V. The reduction of primary emission factors of CIMS detected species was evident (~ 90%) and in line with a reduction of 28–97% for the typical regulated pollutants (particle mass, particle number (PN), black carbon (BC), NOx and CO). Secondary production of most gas and particle phase compounds, e.g., nitric acid, organic acids and carbonyls, after photochemical aging in an oxidation flow reactor (OFR) exceeded the primary emissions (EF_Aged/EF_Fresh ratio ≥ 2). Byproducts from urea-selective catalytic reduction (SCR) systems had both primary and secondary sources. In general, the adoption of Euro VI has a significant positive effect on emissions in real-world traffic and should be promoted in urban air quality management.

In addition to emissions from HDTs, we further characterized marine ship emissions in real-time using a SPAMS at a site in urban Guangzhou. Ship emitted single particles, identified using vanadium as a chemical indicator, contain organic carbon (OC), elemental carbon (EC), metals, sulfate and nitrate. More than 95% of ship emitted particles were sulfate-containing particles and the relative peak areas (RPAs) of sulfate and vanadium in the hourly average mass spectra of ship emitted particles were highly correlated (R²=0.85), suggesting the potential contribution of ship emissions to sulfate production in coastal cities. Our analysis also reveals the impacts of the use of different blends of low sulfur fuel in PM characteristics.

Furthermore, cooking contributions in organic aerosol (OA) can exceed those from vehicles in some urban environments (e.g., urban Hong Kong). We conducted laboratory studies to examine the emissions of primary OA (POA) and non-methane organic gases (NMOGs) and oxidized OA formation using an OFR for three animal fats (i.e., lard, beef and chicken fats) heated at different temperatures. Positive matrix factorization (PMF) revealed that oxidized OA formed together with POA loss after photochemical aging, suggesting the conversion of some POA to oxidized OA. The maximum oxidized OA production rates from heated animal fats, occurring under OH exposures (OHexp) of 8.3–15 × 10¹⁰ molecules cm⁻³ s, ranged from 8.9 to 24.7 μg min⁻¹, 1.6–14.5 times of the initial POA emission rates.

This thesis delivers chemical characterization of both fresh and aged emissions from representative anthropogenic sources (traffic and cooking) in urban environments using advanced on-line mass spectrometric techniques. Overall, the current emission standards or regulations have resulted in significant reductions in the emissions of targeted pollutants from traffic sources, e.g., particle mass, PN, BC, NO_x and CO emissions from HDTs and sulfate emissions from ships. In line with the typical regulated pollutants, the gaseous compounds measured by HR-ToF-CIMS from HDT emissions were also reduced evidently. However, cooking emissions, which were rich with abundant PM and NMOGs, remain a concern as it was not well regulated. Furthermore, once emitted into the atmosphere, the emissions will be subjected to atmospheric processing and aging. The secondary pollutants produced from both traffic and cooking activities are non-negligible in urban areas, i.e., secondary PM formation resulting from both sources can exceed their primary emissions.

Date of Award	4 Jan 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chak Keung CHAN (Supervisor)