Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component ...of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.
It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by ...terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about -0.2 to -0.9 W m(-2). The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.
Despite the recent decrease in pollution events in Chinese urban areas, the World Health Organization air quality guideline values are still exceeded. Observations from monitoring networks show a ...stronger decrease of organic aerosol directly emitted to the atmosphere relative to secondary organic aerosol (SOA) generated from oxidation processes. Here, the uptake of water‐soluble gas‐phase oxidation products is reported as a major SOA contribution to particulate pollution in Beijing, triggered by the increase of aerosol liquid water. In pollution episodes, this pathway is enough to explain the increase in SOA mass, with formaldehyde, acetaldehyde, glycolaldehyde, formic acid, and acetic acid alone explaining 15%–25% of the SOA increase. Future mitigation strategies to reduce non‐methane volatile organic compound emissions should be considered to reduce organic particulate pollution in China.
Plain Language Summary
In the rapidly developing Chinese economy, air pollution from particulate matter (PM) is a major human health risk factor. We show that secondary organic aerosol (SOA) generated from oxidation processes represent 50%–80% of the organic PM in Beijing. We find that non‐equilibrium dissolution of C1−C2 carbonyl compounds to particles is a major pathway of SOA formation during pollution events. These compounds are ubiquitous products in the chemical oxidation of hydrocarbons; thus, the reduction of a single volatile organic compound precursor would not reduce the organic PM, but rather a broad reduction of the organic reactivity is required.
Key Points
Secondary organic aerosol generated from oxidation processes dominates organic particulate pollution in Beijing
Non‐equilibrium dissolution of carbonyl compounds to particles is a major pathway of SOA formation during haze episodes
A broad reduction of the gas‐phase organic reactivity is required to reduce secondary organic aerosol formation in haze events
An intercomparison of different aerosol chemical characterization techniques has been performed as part of a chamber study of biogenic secondary organic aerosol (BSOA) formation and aging at the ...atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). Three different aerosol sampling techniques – the aerosol collection module (ACM), the chemical analysis of aerosol online (CHARON) and the collection thermal-desorption unit (TD) were connected to proton transfer reaction time-of-flight mass spectrometers (PTR-ToF-MSs) to provide chemical characterization of the SOA. The techniques were compared among each other and to results from an aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS). The experiments investigated SOA formation from the ozonolysis of β-pinene, limonene, a β-pinene–limonene mix and real plant emissions from Pinus sylvestris L. (Scots pine). The SOA was subsequently aged by photo-oxidation, except for limonene SOA, which was aged by NO3 oxidation. Despite significant differences in the aerosol collection and desorption methods of the PTR-based techniques, the determined chemical composition, i.e. the same major contributing signals, was found by all instruments for the different chemical systems studied. These signals could be attributed to known products expected from the oxidation of the examined monoterpenes. The sampling and desorption method of ACM and TD provided additional information on the volatility of individual compounds and showed relatively good agreement. Averaged over all experiments, the total aerosol mass recovery compared to an SMPS varied within 80 ± 10, 51 ± 5 and 27 ± 3 % for CHARON, ACM and TD, respectively. Comparison to the oxygen-to-carbon ratios (O : C) obtained by AMS showed that all PTR-based techniques observed lower O : C ratios, indicating a loss of molecular oxygen either during aerosol sampling or detection. The differences in total mass recovery and O : C between the three instruments resulted predominantly from differences in the field strength (E∕N) in the drift tube reaction ionization chambers of the PTR-ToF-MS instruments and from dissimilarities in the collection/desorption of aerosols. Laboratory case studies showed that PTR-ToF-MS E∕N conditions influenced fragmentation which resulted in water and further neutral fragment losses of the detected molecules. Since ACM and TD were operated in higher E∕N than CHARON, this resulted in higher fragmentation, thus affecting primarily the detected oxygen and carbon content and therefore also the mass recovery. Overall, these techniques have been shown to provide valuable insight on the chemical characteristics of BSOA and can address unknown thermodynamic properties such as partitioning coefficient values and volatility patterns down to a compound-specific level.
Air quality measurements usually consist of ground-based instrumentation at fixed locations. However, vertical profiles of pollutants are of interest for understanding processes, distribution, ...dilution and concentration. Therefore, a multicopter system has been developed to investigate the vertical distribution of the concentration of aerosol particles, black carbon, ozone, nitrogen oxides (NOx) and carbon monoxide and the meteorological parameters of temperature and humidity. This article presents the requirements by different users, the setup of the quadrocopter system, the instrumentation and the results of first applications. The vertical distribution of particulate matter next to a highway was strongly related to atmospheric stratification, with different concentrations below and above the temperature inversion present in the morning. After the qualification phase described in this article, two identically equipped multicopters will be used upwind and downwind of line or diffuse sources such as highways or urban areas to quantify the influence of their emissions on the local air quality.
Reaction rate coefficients and product yields in the gas phase reaction of O3 with the short‐chained alkenes ethene, propene, 1‐butene, isobutene, (E)‐butene, and (Z)‐butene were determined by ...Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR). In a first set of experiments, reaction rate coefficients were acquired in an absolute reaction rate study from the measured concentration time profiles of ozone and the alkenes with side reactions being suppressed by adding a radical scavenger. The rate coefficients obtained agree well with literature data; for all but one alkene, the deviation was less than 10%. In a second set of experiments, OH yields were derived from the additional alkene turnover in the absence of a radical scavenger. In contrast to other studies, the OH yields determined in the dry chamber (propene, 0.10 ± 0.07; 1‐butene, 0.00 ± 0.08, isobutene, 0.30 ± 0.14; (Z)‐butene, 0.18 ± 0.09; and (E)‐butene, 0.70 ± 0.12) differed from the yields obtained under humid conditions (propene, 0.30 ± 0.08; 1‐butene, 0.30 ± 0.09; isobutene, 0.80 ± 0.10; (Z)‐butene, 0.40 ± 0.05; and (E)‐butene, 0.60 ± 0.12). The only exception was ethene ozonolysis, where no OH production was observed. HO2 yields (propene, 1.50 ± 0.75; 1‐butene, 1.60 ± 0.80; and isobutene, 2.00 ± 1.00) estimated from the additional ozone turnover compared to the experiments where radicals were not scavenged are reported here for the first time. Furthermore, the yields of the stable ozonolysis products CO, acetaldehyde, and formaldehyde were acquired by monitoring the concentration time profile of the respective compound.
Secondary organic aerosol (SOA) is known to form from a variety of anthropogenic and biogenic precursors. Current estimates of global SOA production vary over 2 orders of magnitude. Since no direct ...measurement technique for SOA exists, quantifying SOA remains a challenge for atmospheric studies. The identification of biogenic SOA (BSOA) based on mass spectral signatures offers the possibility to derive source information of organic aerosol (OA) with high time resolution. Here we present data from simulation experiments. The BSOA from tree emissions was characterized with an Aerodyne quadrupole aerosol mass spectrometer (Q-AMS). Collection efficiencies were close to 1, and effective densities of the BSOA were found to be 1.3 ± 0.1 g/cm3. The mass spectra of SOA from different trees were found to be highly similar. The average BSOA mass spectrum from tree emissions is compared to a BSOA component spectrum extracted from field data. It is shown that overall the spectra agree well and that the mass spectral features of BSOA are distinctively different from those of OA components related to fresh fossil fuel and biomass combustions. The simulation chamber mass spectrum may potentially be useful for the identification and interpretation of biogenic SOA components in ambient data sets.
A chamber study was carried out to investigate the stable carbon isotopic composition (δ13C) of secondary organic aerosol (SOA) formed from ozonolysis of β‐pinene. β‐Pinene (600 ppb) with a known ...δ13C value (−30.1‰) and 500 ppb ozone were injected into the chamber in the absence of light and the resulting SOA was collected on preheated quartz fiber filters. Furthermore, δ13C values of the gas‐phase β‐pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC‐IRMS). β‐Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of β‐pinene with ozone was measured to be 1.0026 2.6 ± 1.5‰). The δ13C value of total secondary organic aerosol was very similar to that of its precursor (average = −29.6 ± 0.2‰) independent of experiment time. Nopinone, one of the major oxidation products of β‐pinene, was found in both the gas and aerosol phases. The gas‐phase nopinone was heavier than the initial β‐pinene by 1.3‰ but lighter than the corresponding aerosol‐phase nopinone. On average, the gas‐phase nopinone was lighter by 2.3‰ than the corresponding aerosol‐phase nopinone. The second product found in the SOA was detected as acetone, but it desorbed from the filter at a higher temperature than nopinone, which indicates that it is a pyrolysis product. The acetone showed a much lower δ13C (−36.6‰) compared to the initial β‐pinene δ13C.
At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for ...the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10^sup 6^ cm^sup -3^) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10^sup 6^ cm^sup -3^, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 - 2) ×10^sup 6^ cm^sup -3^ higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO^sub x^ concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.PUBLICATION ABSTRACT
Secondary 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).
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