Amir Yazdani | EPFL Graph Search

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Chemical characterization of organic aerosols with a focus on biomass burning and mid-infrared spectroscopy

Amir Yazdani

Aerosols play a significant role in the atmosphere through affecting the radiative budget, cloud condensation nuclei activity, and visibility. They also cause adverse health effects leading to premature deaths. A major fraction of aerosols is organic matter (OM), which has a complex composition, is produced by various sources, and undergoes chemical transformation in the atmosphere, making its full characterization difficult. Biomass burning has become a major source of OM with an increasing effect due to more frequent large wildfires around the world resulting partly from climate change.In this thesis, Fourier transform infrared spectroscopy (FTIR) has been employed in six different projects as the main tool to characterize OM from various sources especially biomass burning in order to understand its formation and evolution in the atmosphere. This technique has been used to analyze the functional group (FG) composition of OM, specific spectral profiles from different sources, and biomass burning marker signatures in a nondestructive and cost-efficient manner. FTIR has also been combined with other analytical techniques. First, we used the spectral profiles in the aliphatic CH region of IR spectra to extract information about the molecular structure of atmospheric organic aerosols in terms of their mean carbon number and molecular weight. We found that urban, rural, and biomass burning aerosols have distinct mean molecular weights and carbon numbers. FTIR and aerosol mass spectrometry (AMS) were used in the second project to characterize burning aerosols in an environmental simulation chamber. We showed the agreement of these two instruments in terms of the OM mass concentration and elemental ratios (H:C, and O:C). We found that AMS spectra contained functional group information that agreed with that of FTIR even for moderately aged aerosols. In the third project, we used univariate and multivariate statistics to combine FTIR and AMS measurements to better understand and interpret the complex AMS mass spectra in terms of the FG composition and to estimate the high-time-resolution FG composition of combustion aerosols during the course of aging. In another environmental chamber study (the fourth project), we developed a procedure to estimate the often-neglected evolution of primary biomass burning aerosols during daytime and nighttime chemical agings. We found that at least 15 % of the primary aerosol mass undergoes chemical transformation at relatively short time scales in the order of a day in the atmosphere. Organic biomass burning markers were among the fastest decaying species, making the identification of aged biomass burning aerosols in the atmosphere challenging. In the fifth project, we used the biomass burning marker signatures in the FTIR spectra for the first time to identify and quantify atmospheric samples affected by wood smoke. The FTIR-based identification method agreed well with those using ion chromatography and satellite observations. This method, which is one of the few scalable to large air pollution monitoring networks, was applied to around 20,000 filters collected across the US in 2015 to estimate the impact of biomass burning. In the sixth project, we used FTIR to characterize OM emitted from different cookstoves and fuels. We found similarities between the spectra of unburned fuels and OM emissions and measured high abundances of aromatics and polycyclic aromatic hydrocarbons in the particulate emissions.

EPFL2022

Chemical evolution of primary and secondary biomass burning aerosols during daytime and nighttime

Athanasios Nenes, Satoshi Takahama, Amir Yazdani

Fine particulate matter (PM) affects visibility, climate, and public health. Biomass burning (BB) in the forms of residential wood burning, wildfires, and prescribed burning is a major source of primary and secondary organic matter (OM, an important fraction fine PM), and brown and black carbon (BrC and BC). The contribution of BB to the atmospheric fine PM is only expected to increase in the foreseeable future. Recent studies have highlighted the enhancement in the biomass burning organic aerosol (bbOA) concentrations with aging and reported on the chemical composition of the secondary biomass burning organic aerosol (bbSOA) formed under different conditions. However, the chemical processing of the primary biomass burning organic aerosol (bbPOA) with aging is not well characterized. This chemical processing can potentially alter the chemical composition of bbOA drastically and render its identification and quantification in the atmosphere difficult. We used aerosol mass spectrometry (AMS) and Fourier transform infrared spectroscopy (FTIR) as two complementary methods to quantify the bbPOA aging in this study. AMS measures the bulk composition of OM with a relatively high temporal resolution but provides limited parent compound information due to the extensive fragmentation. FTIR, carried out on PTFE filter samples, provides detailed information about the functional group composition of the OM and certain bbOA makers at the expense of a relatively low temporal resolution. In a series of aging experiments at the Center for Studies of Air Qualities and Climate Change (C-STACC), primary emissions from wood and pellet stoves were injected into an environmental simulation chamber. Primary emissions were aged using hydroxyl and nitrate radicals simulating the atmospheric day-time and night-time oxidation. A high-resolution time-of-flight (HR-TOF) AMS was used to identify the composition of non-refractory PM1. PM1 was also collected on PTFE filters before and after aging for the off-line FTIR analysis. AMS and FTIR agreed well in terms of the measured bbOA mass concentrations, elemental ratios, and the evolution of biomass burning tracers. We developed a procedure to quantify the bbPOA aging using AMS and FTIR. Using AMS, we found that up to 17 % of the bbPOA mass underwent some form of transformation with aging. This transformation was more intense under day-time conditions. FTIR detected a more extensive oxidation (up to two times that of AMS), suggesting a substantial processing of bbPOA, and revealing the limitations of AMS to capture bbPOA aging due to the extensive fragmentation. Different bbOA-related ion fragments were observed to decay at different rates under different conditions (e.g., oxidants and relative humidity). These different decay rates can potentially be used to identify the extent of bbPOA aging in the atmosphere. The bbSOA formed during the daytime oxidation was dominated by acid contributions, resembling certain atmospheric biomass burning samples. The unique, acid-dominated FTIR spectrum of bbSOA can potentially be used as another indicator of the aged bbOA in the atmosphere.

2022

Fragment ion-functional group relationships in organic aerosols using aerosol mass spectrometry and mid-infrared spectroscopy

Nikunj Dudani, Satoshi Takahama, Amir Yazdani

Aerosol mass spectrometry (AMS) and mid-infrared spectroscopy (MIR) are two analytical methods for characterizing the chemical composition of OM. While AMS provides high-temporal-resolution bulk measurements, the extensive fragmentation during the electron impact (EI) ionization makes the characterization of OM components limited. The analysis of aerosols collected on PTFE filters using MIR, on the other hand, provides functional group (FG) information with reduced sample alteration but results in a relatively low temporal resolution. In this work, we compared and combined MIR and AMS measurements for several environmental chamber experiments to achieve a better understanding of the AMS spectra and the OM chemical evolution by aging. Fresh emissions of wood and coal burning were injected into an environmental simulation chamber and aged with hydroxyl and nitrate radicals. A high-resolution time-of-flight (HR-TOF) AMS measured the bulk chemical composition of fine PM. Fine aerosols were also sampled on PTFE filters before and after aging for the offline MIR analysis. After comparing AMS and MIR bulk measurements, we used multivariate statistics to identify the influential functional groups contributing to AMS OM mass for different aerosol sources and aging processes. We also identified the key mass fragments resulting from each functional group for the complex OM generated from biomass and fossil fuel combustion. Finally, we developed a statistical model that enables estimation of the high-time-resolution functional group composition of OM using collocated AMS and MIR measurements. Using this approach, AMS spectra can be used to interpolate the functional group measurements by MIR, allowing us to better understand the evolution of OM during the aging process.

European Geosciences Union2022