The tandem differential mobility analyser (HTDMA) technique is used to detect size changes of submicron particles after a treatment such as exposure to high relative humidity (RH). Measured diameter ...growth factor distributions must be inverted, because they are only a skewed and smoothed integral transform of the actual growth factor probability density function (GF-PDF). We introduce a new approach, TDMAinv, representing the inverted GF-PDF as a piecewise linear function. Simulated measurements are used to prove the concept. Measurements of an aerosol with a bimodal GF-PDF show that TDMAinv provides equivalent information to TDMAfit, the most widely used inversion algorithm. The major advantage of TDMAinv is that convergence of the inversion is robust and independent of the initial guess. This makes TDMAinv a reliable tool to analyse large TDMA data sets. A methodology is also demonstrated for analysis of TDMA data in cases where the dominant fraction of selected particles is doubly or triply charged.
The budget of atmospheric secondary organic aerosol (SOA) is very uncertain, with recent estimates suggesting a global source of between 12 and 1820 Tg (SOA) a−1. We used a dataset of aerosol mass ...spectrometer (AMS) observations from 34 different surface locations to evaluate the GLOMAP global chemical transport model. The standard model simulation (which included SOA from monoterpenes only) underpredicted organic aerosol (OA) observed by the AMS and had little skill reproducing the variability in the dataset. We simulated SOA formation from biogenic (monoterpenes and isoprene), lumped anthropogenic and lumped biomass burning volatile organic compounds (VOCs) and varied the SOA yield from each precursor source to produce the best overall match between model and observations. We assumed that SOA is essentially non-volatile and condenses irreversibly onto existing aerosol. Our best estimate of the SOA source is 140 Tg (SOA) a−1 but with a large uncertainty range which we estimate to be 50–380 Tg (SOA) a−1. We found the minimum in normalised mean error (NME) between model and the AMS dataset when we assumed a large SOA source (100 Tg (SOA) a−1) from sources that spatially matched anthropogenic pollution (which we term antropogenically controlled SOA). We used organic carbon observations compiled by Bahadur et al. (2009) to evaluate our estimated SOA sources. We found that the model with a large anthropogenic SOA source was the most consistent with these observations, however improvement over the model with a large biogenic SOA source (250 Tg (SOA) a−1) was small. We used a dataset of 14C observations from rural locations to evaluate our estimated SOA sources. We estimated a maximum of 10 Tg (SOA) a−1 (10 %) of the anthropogenically controlled SOA source could be from fossil (urban/industrial) sources. We suggest that an additional anthropogenic source is most likely due to an anthropogenic pollution enhancement of SOA formation from biogenic VOCs. Such an anthropogenically controlled SOA source would result in substantial climate forcing. We estimated a global mean aerosol direct effect of −0.26 ± 0.15 Wm−2 and indirect (cloud albedo) effect of −0.6+0.24−0.14 Wm−2 from anthropogenically controlled SOA. The biogenic and biomass SOA sources are not well constrained with this analysis due to the limited number of OA observations in regions and periods strongly impacted by these sources. To further improve the constraints by this method, additional OA observations are needed in the tropics and the Southern Hemisphere.
We present a new and considerably extended parameterization of the thermodynamic activity coefficient model AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) at ...room temperature. AIOMFAC combines a Pitzer-like electrolyte solution model with a UNIFAC-based group-contribution approach and explicitly accounts for interactions between organic functional groups and inorganic ions. Such interactions constitute the salt-effect, may cause liquid-liquid phase separation, and affect the gas-particle partitioning of aerosols. The previous AIOMFAC version was parameterized for alkyl and hydroxyl functional groups of alcohols and polyols. With the goal to describe a wide variety of organic compounds found in atmospheric aerosols, we extend here the parameterization of AIOMFAC to include the functional groups carboxyl, hydroxyl, ketone, aldehyde, ether, ester, alkenyl, alkyl, aromatic carbon-alcohol, and aromatic hydrocarbon. Thermodynamic equilibrium data of organic-inorganic systems from the literature are critically assessed and complemented with new measurements to establish a comprehensive database. The database is used to determine simultaneously the AIOMFAC parameters describing interactions of organic functional groups with the ions H+, Li+, Na+, K+, NH4+, Mg2+, Ca2+, Cl−, Br−, NO3−, HSO4−, and SO42−. Detailed descriptions of different types of thermodynamic data, such as vapor-liquid, solid-liquid, and liquid-liquid equilibria, and their use for the model parameterization are provided. Issues regarding deficiencies of the database, types and uncertainties of experimental data, and limitations of the model, are discussed. The challenging parameter optimization problem is solved with a novel combination of powerful global minimization algorithms. A number of exemplary calculations for systems containing atmospherically relevant aerosol components are shown. Amongst others, we discuss aqueous mixtures of ammonium sulfate with dicarboxylic acids and with levoglucosan. Overall, the new parameterization of AIOMFAC agrees well with a large number of experimental datasets. However, due to various reasons, for certain mixtures important deviations can occur. The new parameterization makes AIOMFAC a versatile thermodynamic tool. It enables the calculation of activity coefficients of thousands of different organic compounds in organic-inorganic mixtures of numerous components. Models based on AIOMFAC can be used to compute deliquescence relative humidities, liquid-liquid phase separations, and gas-particle partitioning of multicomponent mixtures of relevance for atmospheric chemistry or in other scientific fields.
The wet removal of black carbon aerosol (BC) in the atmosphere is a crucial factor in determining its atmospheric lifetime and thereby the vertical and horizontal distributions, dispersion on local ...and regional scales, and the direct, semi-direct and indirect radiative forcing effects. The in-cloud scavenging and wet deposition rate of freshly emitted hydrophobic BC will be increased on acquisition of more-hydrophilic components by coagulation or coating processes. The lifetime of BC is still subject to considerable uncertainty for most of the model inputs, which is largely due to the insufficient constraints on the BC hydrophobic-to-hydrophilic conversion process from observational field data. This study was conducted at a site along UK North Norfolk coastline, where the BC particles were transported from different regions within Western Europe. A hygroscopicity tandem differential mobility analyser (HTDMA) was coupled with a single particle soot photometer (SP2) to measure the hygroscopic properties of BC particles and associated mixing state in real time. In addition, a Soot Particle AMS (SP-AMS) measured the chemical compositions of additional material associated with BC particles. The ensemble of BC particles persistently contained a less-hygroscopic mode at a growth factor (gf) of around 1.05 at 90% RH (dry diameter 163 nm). Importantly, a more-hygroscopic mode of BC particles was observed throughout the experiment, the gf of these BC particles extended up to ~1.4–1.6 with the minimum between this and the less hygroscopic mode at a gf ~1.25, or equivalent effective hygroscopicity parameter κ ~0.1. The gf of BC particles (gfBC) was highly influenced by the composition of associated soluble material: increases of gfBC were associated with secondary inorganic components, and these increases were more pronounced when ammonium nitrate was in the BC particles; however the presence of secondary organic matter suppressed the gfBC below that of pure inorganics. The Zdanovskii-Stokes-Robinson (ZSR) mixing rule captures the hygroscopicity contributions from different compositions within ±30% compared to the measured results, however is subject to uncertainty due to the complex morphology of BC component and potential artefacts associated with semivolatile particles measured with the HTDMA. This study provides detailed insights on BC hygroscopicity associated with its mixing state, and the results will importantly constrain the microphysical mixing schemes of BC as used by a variety of high level models. In particular, this provides direct evidence to highlight the need to consider ammonium nitrate ageing of BC particles because this will result in particles becoming hydrophilic on much shorter timescales than for sulphate formation, which is often the only mechanism considered.
In the atmosphere nighttime removal of volatile organic compounds is initiated to a large extent by reaction with the nitrate radical (NO3) forming organic nitrates which partition between gas and ...particulate phase. Here we show based on particle phase measurements performed at a suburban site in the Netherlands that organic nitrates contribute substantially to particulate nitrate and organic mass. Comparisons with a chemistry transport model indicate that most of the measured particulate organic nitrates are formed by NO3 oxidation. Using aerosol composition data from three intensive observation periods at numerous measurement sites across Europe, we conclude that organic nitrates are a considerable fraction of fine particulate matter (PM1) at the continental scale. Organic nitrates represent 34% to 44% of measured submicron aerosol nitrate and are found at all urban and rural sites, implying a substantial potential of PM reduction by NOx emission control.
Key Points
Particulate organic nitrate is ubiquitous in Europe
The 34 to 44 percent of fine particulate nitrate is organic
Nighttime chemistry is a dominant source of particulate organic nitrates
The effect of nanogel colloidal and dissolved organic matter <0.2 μm, secreted by marine biota, on the hygroscopic growth and droplet activation behaviour of the primary marine aerosol was studied. ...Seawater proxies were prepared by the combination of artificial seawater devoid of marine organics and natural seawater enriched in organic exudate released by laboratory-grown phytoplankton cultures, as described in a companion paper. The primary aerosol was produced by bubble bursting, using a plunging multijet system as an aerosol generator. The aerosol generated from seawater proxies enriched with marine exudate presented organic volume fractions on the order of 8–37%, as derived by applying a simple mixing rule. The hygroscopic growth and cloud condensation nuclei (CCN) activity of the marine organics-enriched particles where 9–17% and 5–24% lower, respectively, than those of the aerosol produced from artificial seawater devoid of exudate. Experiments in a companion paper indicated that the cloud nuclei formation could be enhanced in diatom bloom areas because of the increase in the primary particle production induced by marine organics. The experiments in the present study, however, indicate that the impacts of such an enhancement would be counteracted by the reduction in the CCN activity of the primary particles enriched in marine organics. The extent of the effect of the biogenic matter on the particle behaviour was dependent on the seawater organic concentration and type of algal exudate. Aerosol produced from seawater proxies containing diatomaceous exudate presented higher hydrophobicity and lower CCN activity than those enriched with nanoplankton exudate. The organic fraction of the particles was found to correlate with the seawater organic concentration, without observing saturation of the particle organic mass fraction even for unrealistically high organic matter concentration in seawater. These findings are indicative that discrepancies on the composition of the primary aerosol between different studies could partly be explained by the difference in the nature and concentration of the organic matter in the source seawater employed. Consistently across the experiments, theoretical analysis based on the Köhler model predicted a reduction in the primary marine aerosol CCN activity upon the incorporation of marine organics into the particle composition. This effect is consequence of the replacement of small inorganic sea salt molecules by large molar mass organic molecules, together with a moderate suppression of the surface tension at the point of activation of 5–0.5%, which leads to a dominance of the reduction in the dissolved solute in the Raoult term.
Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on ...HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.
This paper investigates the physical and chemical characteristics of biomass burning aerosol over West Africa using data from the UK Facility for Airborne Atmospheric Measurements aircraft. ...Measurements of biomass burning aerosol were made during the Dust and Biomass‐burning Experiment (DABEX) and Dust Outflow and Deposition to the Ocean (DODO) field experiments in January and February 2006. Layers of aged biomass burning aerosols were observed throughout the region, and fresh biomass burning aerosols were encountered during the penetration of smoke plumes at low altitudes. Vertical profiles of aerosol properties across the region are shown. Measurements from an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q‐AMS) show changes in chemical composition between fresh and aged biomass burning aerosols, over a region spanning thousands of kilometers. These data represent the first time that continental‐scale variability in biomass burning aerosol composition has been observed. However, an almost linear relationship between organic aerosol mass concentration and carbon monoxide concentration was observed across the region. A net carbon loss occurs over the aerosol lifetime in the region owing to a combination of chemical processing and repartitioning of organic mass to the gas phase. Evolution of the number size distribution was observed, with coagulation concluded to be the dominant process involved, a finding supported by coagulation box modeling. Regional‐scale emission ratios for organic (0.041) and black carbon (0.0072) with respect to CO have been estimated over West Africa, one of the largest sources of biomass burning globally. Biomass burning emissions from the West African Sahel are poorly represented in the literature, and these results represent important continental‐scale emissions. They are in good agreement with literature values from other regions.
The formation of secondary organic aerosol (SOA) indoors is one of the many consequences of the rich and complex chemistry that occurs therein. Given particulate matter has well documented health ...effects, we need to understand the mechanism for SOA formation indoors and its resulting composition. This study evaluates some uncertainties that exist in quantifying gas-to-particle partitioning of SOA-forming compounds using an indoor detailed chemical model. In particular, we investigate the impacts of using different methods to estimate compound vapour pressures as well as simulating the formation of highly oxygenated organic molecules (HOM) via auto-oxidation on SOA formation indoors. Estimation of vapour pressures for 136 α-pinene oxidation species by six investigated methods led to standard deviations of 28–216%. Inclusion of HOM formation improved model performance across three of the six assessed vapour pressure estimation methods when comparing against experimental data, particularly when the NO2 concentration was relatively high. We also explored the predicted SOA composition using two product classification methods, the first assuming the molecule is dominated by one functionality according to its name, and the second accounting for the fractional weighting of each functional group within a molecule. The SOA composition was dominated by the HOM species when the NO2-to-α-terpineol ratio was high for both product classification methods, as these conditions promoted formation of the nitrate radical and hence formation of HOM monomers. As the NO2-to-α-terpineol ratio decreased, peroxides and acids dominated the simple classification, whereas for the fractional classification, carbonyl and alcohol groups became more important.
•SOA concentration predictions are highly dependent on selected vapour pressure method used.•Highly oxygenated organic molecules form an important component of indoor SOA.•Composition of indoor SOA is highly complex and depends on indoor conditions.
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