9 research outputs found
Physical properties and processes of secondary organic aerosol and its constituents
No full textAtmospheric aerosol particles are involved in several important processes including the formation of clouds and precipitation. A considerable fraction of the ambient aerosol mass consists of organic compounds of both primary and secondary origin. These organic compounds are often semi-volatile and susceptible to oxidation which makes the organic aerosol a dynamic system, both chemically and physically. Once an aerosol is formed or released into the atmosphere, several processes will begin to alter its chemical and physical properties.
The focus of the work presented in this thesis has been to use experimental methods to characterise single aerosol components and more complex experimental systems, involving the formation and processing of secondary organic aerosol (SOA). The volatility of aerosol particles, e.g. the evaporation rate of the particles upon heating, can provide information of several important properties. From an aerosol consisting only of one pure compound it is possible to derive physical quantities like saturation vapour pressure and enthalpy of evaporation. In more complex systems like a secondary organic aerosol the volatility can give information about changes in composition, state of oxidation and degree of internal or external mixing.
With the use of a volatility tandem differential mobility analyser (VTDMA), the saturation vapour pressures and enthalpies of evaporation have been determined for pure compounds that are known constituents of ambient aerosol samples i.e. nine carboxylic acids. Two of them were cyclic, pinic and pinonic acid and seven of them were straight chain dicarboxylic acids with number of carbons ranging from C4 to C10. These properties were in addition evaluated for the aminium nitrates of mono-, di-, and trimethylamine, ethyl- and monoethanolamine. The calculated saturation vapour pressures for the carboxylic acids were in the range of 10-6 to 10-3 Pa and the determined enthalpies of evaporation ranged from 83 to 161 kJ mol-1. The corresponding values for the aminium nitrates were for the calculated saturation vapour pressures approximately 10-4 Pa and for the enthalpies of evaporation 54 to 72 kJ mol-1.
The VTDMA system has also been utilised to characterise SOA formed in the AIDA and SAPHIRE smog chambers from the ozonolysis of α-pinene and limonene and the change in the SOA thermal properties during OH radical induced ageing. Further, the effect of elevated ozone concentration and radical chemistry on SOA formed from limonene ozonolysis in the G-FROST laminar flow reactor was investigated. In addition, to compare with vapour pressures of aminium nitrates SOA generated from photooxodation of alkyl amines have been characterised in the EUPHORE smog chamber.
The calculated vapour pressures of all the investigated pure compounds in this work characterise them to be in the semi-volatile organic compound (SVOC) category; hence they will be present both in the gaseous and condensed phase in the atmosphere. This implied that all these compounds will be susceptible for gas phase OH radical oxidation that was demonstrated to be an important process for the complex mixtures investigated in the smog chamber facilities. The OH chemistry was also influencing the volatility of aerosol produced in G-FROST by ozonlysis. Regarding photooxodation of amines the aerosols produced under high NOx conditions initially mimicked the pure salts but was efficiently transformed by oxidation into an aerosol with similar volatility properties as observed in the terpene oxidation experiments
Formation, ageing and thermal properties of secondary organic aerosol
No full textIn order to properly represent and predict the effects of aerosol in climate systems, an accurate description of their formation and properties is needed. This thesis describes work done to increase the knowledge of processes and properties of atmospherically relevant secondary organic aerosol (SOA) from both biogenic and anthropogenic origin. The common theme for these projects is the use of a Volatility Tandem Differential Mobility Analyser (VTDMA) setup, which in combination with other observations has generated insight into both detailed chemical mechanisms and physical processes that eventually could be suitable for testing in air quality or climate models. During the course of this work, the experimental facility
the Gothenburg Flow Reactor for Oxidation Studies at low Temperatures (G-FROST) and
the VTDMA setup, as well as a corresponding data evaluation methodology, have been
improved and refined.
Thermal properties could be linked to both formation and ageing processes of SOA. Using
a VTDMA setup, where the thermal characteristics of SOA were measured at a range of
evaporation temperatures, a sigmoidal fit to the data enabled parameterisation of their
volatility properties. The parameters extracted were e.g. the temperature corresponding to
a volume fraction remaining of 0.5 (TVFR0.5) and the slope factor (SVFR), which are measures
of the general volatility and the volatility distribution of the condensed phase products,
respectively. A higher TVFR0.5 indicates lower volatility, while an increase of SVFR states a
broader distribution of vapour pressures. The response of these parameters from changes in
experimental conditions could be linked to processes occurring both in the gaseous and the
condensed phase. In photo-chemical experiments, the change in TVFR0.5 and SVFR could be
described using the OH dose.
The gas phase processes were found to be very important for SOA ageing, driven mainly by
OH radical exposure in the outdoor chamber SAPHIR. However, processes in the condensed
phase, such as plausible non oxidative ageing processes and non-liquid behaviour of SOA
particles, were also observed.
Detailed studies of ozonolysis of the boreal forest monoterpenes β-pinene and limonene were
enabled by precise control of reaction conditions using the G-FROST. The experimental
findings in response to e.g. water and radical conditions emphasized the difference in
ozonolysis reaction paths between endo- and exocyclic compounds. The results support
the recently suggested decomposition of the stabilized Criegee Intermediate via the
hydroperoxide channel in ozonolysis of β-pinene
Size‐resolved particle emission factors for individual ships
No full textIn these experiments size‐resolved emission factors for particle number (EFPN) and mass (EFPM) have been determined for 734 individual ship passages for real‐world dilution. The method used is an extractive sampling method of the passing ship plumes where particle number/mass and CO2 were measured with high time resolution (1 Hz). The measurements were conducted on a small island located in the entrance to the port of Gothenburg (N57.6849, E11.838), the largest harbor in Scandinavia. This is an emission control area (ECA) and in close vicinity to populated areas. The average EFPN and EFPM were 2.55 \ub1 0.11
7 1016 (kg fuel)−1 and 2050 \ub1 110 mg (kg fuel)−1, respectively. The determined EF for ships with multiple passages showed a great reproducibility. Size‐resolved EFPN were peaking at small particle sizes ∼35 nm. Smaller particle sizes and hence less mass were observed by a gas turbine equipped ship compared to diesel engine equipped ships. On average 36 to 46% of the emitted particles by number were non‐volatile and 24% by mass (EFPN 1.16 \ub1 0.19
7 1016 [kg fuel]−1 and EFPM 488 \ub1 73 mg [kg fuel]−1, respectively). This study shows a great potential to gain large data‐sets regarding ship emission determining parameters that can improve current dispersion modeling for health assessments on local and regional scales. The global contributions of total and non‐volatile particle mass from shipping using this extensive data‐set from an ECA were estimated to be at least 0.80 Tgy−1 and 0.19 Tgy−1
Photochemical smog in China: scientific challenges and implications for air-quality policies
No full textSevere air-pollution events in many parts of China pose a major threat to health and ecosystems [1]. China’s air pollution is concentrated to economically developed areas, such as Beijing–Tianjin– Hebei (BTH) and Pearl–River–Delta (PRD) [2,3]. The situation has received considerable attention in international and national media, including its secondary societal and economic impacts such as lowered productivity, reduced investments and loss of professionals who have the choice of residing elsewhere. Large efforts are today underway from the government to improve the situation by measures to reduce primary emissions (see Airborne Pollution Prevention and Control Action Plan (2013–2017), available at http://www.gov.cn). This will also affect secondary pollutants such as ozone (O3) and particulate matter (PM) but how and to what magnitude are uncertain. The photochemically induced secondary pollutants will add to any severe local urban air pollution but require a very different approach for abatements. In this perspective view, we will address the complexity of photochemical smog while acknowledging the urge for similar descriptions on local urban air pollution as described elsewhere [1–4].Swedish Research Council [639-2013-6917]; National Basic Research Program from Ministry of Science and Technology, China [2013CB228503]; National Natural Science Foundation of China [91544214, 91544226]; Hong Kong Research Grants Council [C-5022-14G]SCI(E)中国科学引文数据库(CSCD)EDITORIAL MATERIAL4401-403
Volatility of secondary organic aerosol during OH radical induced ageing
No full textThe aim of this study was to investigate oxidation of SOA formed from ozonolysis of <i>&alpha;</i>-pinene and limonene by hydroxyl radicals. This paper focuses on changes of particle volatility, using a Volatility Tandem DMA (VTDMA) set-up, in order to explain and elucidate the mechanism behind atmospheric ageing of the organic aerosol. The experiments were conducted at the AIDA chamber facility of Karlsruhe Institute of Technology (KIT) in Karlsruhe and at the SAPHIR chamber of Forchungzentrum Jülich (FZJ) in Jülich. A fresh SOA was produced from ozonolysis of <i>&alpha;</i>-pinene or limonene and then aged by enhanced OH exposure. As an OH radical source in the AIDA-chamber the ozonolysis of tetramethylethylene (TME) was used while in the SAPHIR-chamber the OH was produced by natural light photochemistry. A general feature is that SOA produced from ozonolysis of <i>&alpha;</i>-pinene and limonene initially was rather volatile and becomes less volatile with time in the ozonolysis part of the experiment. Inducing OH chemistry or adding a new portion of precursors made the SOA more volatile due to addition of new semi-volatile material to the aged aerosol. The effect of OH chemistry was less pronounced in high concentration and low temperature experiments when lower relative amounts of semi-volatile material were available in the gas phase. Conclusions drawn from the changes in volatility were confirmed by comparison with the measured and modelled chemical composition of the aerosol phase. Three quantified products from the <i>&alpha;</i>-pinene oxidation; pinonic acid, pinic acid and methylbutanetricarboxylic acid (MBTCA) were used to probe the processes influencing aerosol volatility. A major conclusion from the work is that the OH induced ageing can be attributed to gas phase oxidation of products produced in the primary SOA formation process and that there was no indication on significant bulk or surface reactions. The presented results, thus, strongly emphasise the importance of gas phase oxidation of semi- or intermediate-volatile organic compounds (SVOC and IVOC) for atmospheric aerosol ageing
A transition of atmospheric emissions of particles and gases from on-road heavy-duty trucks
No full textThe transition, in extent and characteristics, of atmospheric emissions caused by the modernization of the heavy-duty on-road fleet was studied utilizing roadside measurements. Emissions of particle number (PN), particle mass (PM), black carbon (BC), nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbon (HC), particle size distributions, and particle volatility were measured from 556 individual heavy-duty trucks (HDTs). Substantial reductions in PM, BC, NOx, CO, and to a lesser extent PN were observed from Euro III to Euro VI HDTs by 99 %, 98 %, 93 %, and 57 % for the average emission factors of PM, BC, NOx, and CO, respectively. Despite significant total reductions in NOx emissions, the fraction of NO2 in the NOx emissions increased continuously from Euro IV to Euro VI HDTs. Larger data scattering was evident for PN emissions in comparison to solid particle number (SPN) for Euro VI HDTs, indicating a highly variable fraction of volatile particle components. Particle size distributions of Euro III to enhanced environmentally friendly vehicle (EEV) HDTs were bimodal, whereas those of Euro VI HDTs were nucleation mode dominated. High emitters disproportionately contributed to a large fraction of the total emissions with the highest-emitting 10 % of HDTs in each pollutant category being responsible for 65 % of total PM, 70 % of total PN, and 44 % of total NOx emissions. Euro VI HDTs, which accounted for 53 % of total kilometres driven by Swedish HDTs, were estimated to only contribute to 2 %, 6 %, 12 %, and 47 % of PM, BC, NOx, and PN emissions, respectively. A shift to a fleet dominated by Euro VI HDTs would promote a transition of atmospheric emissions towards low PM, BC, NOx, and CO levels. Nonetheless, reducing PN, SPN, and NO2 emissions from Euro VI HDTs is still important to improve air quality in urban environments
Chlorine oxidation of VOCs at a semi-rural site in Beijing:Significant chlorine liberation from ClNO<sub>2</sub> and subsequent gas- A nd particle-phase Cl-VOC production
No full textNitryl chloride (ClNO2) accumulation at night acts as a significant reservoir for active chlorine and impacts the following day's photochemistry when the chlorine atom is liberated at sunrise. Here, we report simultaneous measurements of N2O5 and a suite of inorganic halogens including ClNO2 and reactions of chloride with volatile organic compounds (Cl-VOCs) in the gas and particle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation mass spectrometer (FIGAERO-ToF-CIMS) during an intensive measurement campaign 40km northwest of Beijing in May and June 2016. A maximum mixing ratio of 2900ppt of ClNO2 was observed with a mean campaign nighttime mixing ratio of 487ppt, appearing to have an anthropogenic source supported by correlation with SO2, CO and benzene, which often persisted at high levels after sunrise until midday. This was attributed to such high mixing ratios persisting after numerous e-folding times of the photolytic lifetime enabling the chlorine atom production to reach 2.3 × 105moleculescm-3 from ClNO2 alone, peaking at 09:30LT and up to 8.4 × 105moleculescm-3 when including the supporting inorganic halogen measurements. Cl-VOCs were observed in the particle and gas phases for the first time at high time resolution and illustrate how the iodide ToF-CIMS can detect unique markers of chlorine atom chemistry in ambient air from both biogenic and anthropogenic sources. Their presence and abundance can be explained via time series of their measured and steady-state calculated precursors, enabling the assessment of competing OH and chlorine atom oxidation via measurements of products from both of these mechanisms and their relative contribution to secondary organic aerosol (SOA) formation.</p
Saturation Vapor Pressures and Transition Enthalpies of Low-Volatility Organic Molecules of Atmospheric Relevance: From Dicarboxylic Acids to Complex Mixtures
No full textThere are a number of techniques that can be used that differ in terms of whether they fundamentally probe the equilibrium and the temperature range over which they can be applied. The series of homologous, straight-chain dicarboxylic acids have received much attention over the past decade given their atmospheric relevance, commercial availability, and low saturation vapor pressures, thus making them ideal test compounds. Uncertainties in the solid-state saturation vapor pressures obtained from individual methodologies are typically on the order of 50-100%, but the differences between saturation vapor pressures obtained with different methods are approximately 1-4 orders of magnitude, with the spread tending to increase as the saturation vapor pressure decreases. Some of the dicarboxylic acids can exist with multiple solid-state structures that have distinct saturation vapor pressures. Furthermore, the samples on which measurements are performed may actually exist as amorphous subcooled liquids rather than solid crystalline compounds, again with consequences for the measured saturation vapor pressures, since the subcooled liquid phase will have a higher saturation vapor pressure than the crystalline solid phase. Compounds with equilibrium vapor pressures in this range will exhibit the greatest sensitivities in terms of their gas to particle partitioning to uncertainties in their saturation vapor pressures, with consequent impacts on the ability of explicit and semiexplicit chemical models to simulate secondary organic aerosol formation
Secondary organic aerosol reduced by mixture of atmospheric vapours
No full textSecondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene ‘scavenges’ hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production)
