Carbonyl compounds and their OH reactivities in outdoor and indoor environments
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Abstract
Carbonyl compounds including aldehydes and ketones are a major chemical category of oxygenated volatile organic compounds (VOCs) in the atmosphere, which play an important role in atmospheric chemistry. They can be directly released from biogenic and anthropogenic emissions and also be produced via secondary photochemical oxidation. The formation and loss processes of carbonyls are related to tropospheric ozone formation and can influence the formation of secondary organic aerosols. Due to the various lifetimes of carbonyls, they can affect the outside air quality locally, regionally and globally. Moreover, carbonyls can be also present in indoor air. As people spend most of their time indoors and exposure to some carbonyls can cause adverse health effects, it is important to characterize indoor sources of these compounds. Indoor carbonyls can be emitted from furnishings/materials but also from humans themselves with the latter source gaining more and more prominence only in the recent decade. With the presence of indoor oxidants transported from outside or generated indoors, carbonyls can be also be generated as secondary products via oxidation. They can contribute significantly to the total loss of OH radicals, which can be used to reveal the total airborne load of reactive gases.
To better understand the role of carbonyls in air chemistry outdoors and indoors, this doctoral project conducted VOC and total OH reactivity measurements in the marine atmosphere over the Arabian Peninsula (outdoor scenario) and in a controlled human occupied climate chamber (indoor scenario). Proton-transfer-reaction mass spectrometry (PTR-MS), a soft chemical ionization technique, was the major tools used throughout the project.
For the measurements performed over the Arabian Peninsula, abundant carbonyls observed over the polluted regions were found to be related to strong photochemical oxidation, biomass burning events, and as direct emissions from oil/gas industries and marine transportation. Model-measurement comparisons indicated important missing sources of acetaldehyde in the current model. By including an oceanic source, the model simulation improved significantly for most of the areas, especially for remote areas. The total OH reactivity measured over the polluted regions was comparable to urban areas with severe air pollution. Oxygenated VOCs (mostly carbonyls) accounted for the largest fraction of the total reactivity over the region where strong photochemistry occurred. The OH reactivity can be fully attributed to measured trace gases in half of the areas including the most polluted regions. It can also help to identify different chemical regimes regarding regional ozone formation and loss.
Concerning the indoor human emissions characterization, a study was conducted in a climate chamber under controlled conditions. VOCs and total OH reactivity were measured for whole-body, breath and dermal emissions with the effect of ozone. Acetone was found to be the most emitted carbonyl compound as well as VOC for breath-only and whole-body emissions. In the
presence of ozone, the total emission rate (amount per person) of dermal emissions increased dramatically with the increase largely contributed by oxygenated VOCs, mainly carbonyls that were skin ozonolysis products. Similar species distribution was observed for whole-body emissions under ozone-free and ozone-present conditions. Emission rates obtained from the VOC measurements can be used in future indoor air models to better simulate the air chemistry of occupied environments. The total OH reactivity was applied for the first time indoors. The measured OH reactivity of four people in the ventilated chamber was comparable to the reactivity measured in cities. It agreed well with the calculated reactivity obtained from measured individual species, indicating major reactive compounds released by humans were well characterized. Ozone induced skin lipid chemistry increased the reactivity of human emissions substantially with the elevation caused mostly by carbonyl compounds.