Formation and Chemical Aging of Atmospheric Carbonaceous Aerosol
Creators: name: Tasoglou, Antonios
Publisher: name: Figshare
Year: 2018
Subjects: subject: 90499 Chemical Engineering not elsewhere classified
subject: FOS: Chemical engineering

description: Atmospheric aerosols can cause serious human health problems and are also affecting the energy balance of our planet contributing to climate change. Organic aerosol (OA) is the most diverse and least understood component of submicron aerosols, in part because of a wide variety of biogenic and anthropogenic sources as well as contributions from both direct emission and secondary formation in the atmosphere. Air quality models often seriously under-predict the concentration of OA in the atmosphere due mainly to our lack of understanding of the atmospheric chemical and physical processing of the emitted organic compounds. A series of experimental studies were performed to address some of the major questions regarding atmospheric OA. In the first phase of the work, the secondary organic aerosol (SOA) production during the oxidation of β-caryophyllene by ozone (O3) and hydroxyl radicals (OH) and the subsequent chemical aging of the products during reactions with OH were investigated. Experiments were conducted with ozone, hydroxyl radicals at low NOx (zero added NOx) and at high NOx (100s of ppb). The SOA mass yield at 10 μg m-3 of organic aerosol was 27% for the ozonolysis, 20% for the reaction with OH at low NOx and 38% at high NOx under dry conditions, 20oC, and ozone excess. Parameterizations of the fresh SOA yields have been developed. The average fresh SOA atomic O:C ratio varied from 0.24 to 0.34 depending on the oxidant and the NOx level, while the H:C ratio was close to 1.5 for all systems examined. An average density of 1.06±0.1 μg m-3 of the β-caryophyllene SOA was estimated. The exposure to UV-light had no effect on the β-caryophyllene SOA concentration and Aerosol Mass Spectrometer (AMS) mass spectrum. The chemical aging of the produced β-caryophyllene SOA was studied by exposing the fresh SOA to high concentrations (107 molecules cm-3) of OH for several hours. These additional reactions ii increased the SOA concentration by 15-40% and the O:C by approximately 25%. A limited number of experiments suggested that there was a significant impact of the relative humidity on the chemical aging of the SOA. The evaporation rates of β-caryophyllene SOA were quantified by using a thermodenuder allowing us to estimate the corresponding volatility distributions and effective vaporization enthalpies. In the second step the accuracy of continuous black carbon measurements of a series of commercially available instruments was assessed for biomass burning particulate matter. Black carbon-containing particles are the most strongly light absorbing aerosols in the atmosphere. They are emitted during the combustion of fossil fuels, biofuels, and biomass. Measurements of black carbon are challenging because of its semi-empirical definition based on physical properties and not chemical structure, the complex and continuously changing morphology of the corresponding particles, and the effects of other particulate components on its absorption. In this study we compare six available commercial continuous BC instruments using biomass burning aerosol. The comparison involves a Soot Particle Aerosol Mass Spectrometer (SP-AMS), a Single Particle Soot Photometer (SP2), an aethalometer, a Multiangle Absorption Photometer (MAAP), and a blue and a green photoacoustic extinctiometer (PAX). An SP-AMS collection efficiency equal to 0.35 was measured for this aerosol system. The SP-AMS was then compared to all the other commercial instruments. Two regimes of behavior were identified corresponding to high and low organic/black carbon ratio. New mass absorption cross sections (MAC) were calculated for the optical instruments for the two regimes. The new MAC values varied from 30% to 2.3 times the instrument default values depending on the instrument and the regime. This comparison of the optical instruments suggests a stronger discrepancy among the BC measurements as the organic carbon content of the BC-containing particles increases. In the next step we focused on the chemical aging of combustion emissions. Smog chamber experiments were conducted to study the changes of the physical properties and chemical composition of biomass burning particles as they evolve in the atmosphere. A Soot Particle Aerosol Mass Spectrometer (SP-AMS) and a Single Particle Soot Photometer (SP2) were used for the chemical characterization of the particles. An Aethalometer as well as a green and a blue photoacoustic extinctiometer (PAX) were used for the study of the aerosol optical properties. As the biomass burning smoke aged, exposed to UV light, ozone, or OH radicals, organic material condensed on the preexisting particles. This coating led to an increase of the absorption of the black carbon-containing particles by as much as a factor of two. The absorption enhancement of biomass burning particles due to their coating with aromatic secondary organic aerosol (SOA) was also studied. The resulting absorption enhancement was determined mainly by the changes in the SOA mass concentration and not the changes of its oxidation state. The enhancement of the absorption of the aging biomass burning particles was consistent with the predictions of a core-shell Mie theory model assuming spherical particles and non-absorbing coating. In the last phase of the work emissions from cooking activities were studied. Cooking organic aerosol (COA) is a significant fraction of the total fine aerosol in urban areas around the world. COA chemical aging experiments took place in a smog chamber in the presence of UV light or in excess of ozone. Positive matrix factorization was used to characterize the changes in the chemical composition of the COA during the chemical aging. The chemical composition of the produced aged COA was similar for both aging methods The chemical aging processes cause an increase of the organic mass and its oxidation state. The fresh COA particles have a low CCN activity but their activity increases significantly as they chemically age.