Michael J. Cubison

Evolution of Organic Aerosols in the Atmosphere

Framework for Change Organic aerosols make up 20 to 90% of the particulate mass of the troposphere and are important factors in both climate and human heath. However, their sources and removal pathways are very uncertain, and their atmospheric evolution is poorly characterized. Jimenez et al. (p. 1525; see the Perspective by Andreae) present an integrated framework of organic aerosol compositional evolution in the atmosphere, based on model results and field and laboratory data that simulate the dynamic aging behavior of organic aerosols. Particles become more oxidized, more hygroscopic, and less volatile with age, as they become oxygenated organic aerosols. These results should lead to better predictions of climate and air quality. Organic aerosols are not compositionally static, but they evolve dramatically within hours to days of their formation. Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high–time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

The importance of aerosol mixing state and size-resolved composition on CCN concentration and the variation of the importance with atmospheric aging of aerosols

The study presents results form a detailed CCn study where different assumptions of CCN compositions and mixing states for a data set acquired during the MILAGRO experiment in Mexico City are evaluated and compared. The authors make five assumptions of different complexity and try to reproduce measured CCN number concentrations. They show that the assumption of an external mixture often leads to good results although it may not represent the true composition/mixing state of the aerosol but rather opposing effects cancel. The findings are useful and promise that relatively simple assumptions can be made to predict CCN number concentrations, even in pol-

Observational insights into aerosol formation from isoprene.

Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F(MAE formation)). The strong temperature dependence of F(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m(-3)) and MAE-derived organosulfates (MAE-OS; ~1 ng m(-3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m(-3)) relative to MAE-OS (<0.0005 ng m(-3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10-100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.

A Chemical Ionization High-Resolution Time-of-Flight Mass Spectrometer Coupled to a Micro Orifice Volatilization Impactor (MOVI-HRToF-CIMS) for Analysis of Gas and Particle-Phase Organic Species

We describe a new instrument, chemical ionization (CI) high-resolution time-of-flight mass spectrometer (ToFMS) coupled to a micro-orifice volatilization impactor (MOVI-HRToF-CIMS). The MOVI-HRToF-CIMS instrument is unique in that, within a compact field-deployable package, it provides (1) quantifiable molecular-level information for both gas and particle-phase organic species on timescales ranging from ≤1 s for gases to 10–60 min for particle-phase compounds that can be used to efficiently probe oxidation and secondary organic aerosol (SOA) formation mechanisms, and (2) relative volatility information of the detected compounds simultaneously estimated using the programmed thermal desorption information obtained from the MOVI. We demonstrate the capabilities of a prototype instrument using known test compounds and complex mixtures generated from the oxidation of biogenic and anthropogenic hydrocarbons. We present spectra obtained using both negative and positive ion CI with acetate (CH3C(O)O−) and protonated water clusters (H3O+·(H2O) n ), respectively, as reagent ions. The instrument has high mass resolving power (R = 5000 above m/Q 250 Th) and mass accuracy (±20 ppm) enabling estimation of compound elemental composition. Instrument sensitivity in negative ion mode was tested using formic acid as a representative gas-phase compound, and that for particle-phase compounds was tested using palmitic, azelaic, and tricarballylic acids. With a heated MOVI inlet, an ion count rate of ∼15 Hz is achieved when sampling 1 pptv (= 1 pmol/mol) of formic acid (or other monocarboxylic acids) under typical operating conditions. This sensitivity translates to detection limits less than 1 ng/m3 for carboxylic acids in the particle-phase. We also discuss the remaining challenges with this instrument to broadly characterizing gaseous and particulate oxygenated organic compounds in situ. Copyright 2012 American Association for Aerosol Research

Effects of sources and meteorology on particulate matter in the Western Mediterranean Basin: An overview of the DAURE campaign

DAURE (Determination of the Sources of Atmospheric Aerosols in Urban and Rural Environments in the Western Mediterranean) was a multidisciplinary international field campaign aimed at investigating the sources and meteorological controls of particulate matter in the Western Mediterranean Basin (WMB). Measurements were simultaneously performed at an urban-coastal (Barcelona, BCN) and a rural-elevated (Montseny, MSY) site pair in NE Spain during winter and summer. State-of-the-art methods such as 14C analysis, proton-transfer reaction mass spectrometry, and high-resolution aerosol mass spectrometry were applied for the first time in the WMB as part of DAURE. WMB regional pollution episodes were associated with high concentrations of inorganic and organic species formed during the transport to inland areas and built up at regional scales. Winter pollutants accumulation depended on the degree of regional stagnation of an air mass under anticyclonic conditions and the planetary boundary layer height. In summer, regional recirculation and biogenic secondary organic aerosols (SOA) formation mainly determined the regional pollutant concentrations. The contribution from fossil sources to organic carbon (OC) and elemental carbon (EC) and hydrocarbon-like organic aerosol concentrations were higher at BCN compared with MSY due to traffic emissions. The relative contribution of nonfossil OC was higher at MSY especially in summer due to biogenic emissions. The fossil OC/EC ratio at MSY was twice the corresponding ratio at BCN indicating that a substantial fraction of fossil OC was due to fossil SOA. In winter, BCN cooking emissions were identified as an important source of modern carbon in primary organic aerosol.

Estimating the contribution of organic acids to northern hemispheric continental organic aerosol

Using chemical ionization mass spectrometry to detect particle-phase acids (acid-CIMS) and aerosol mass spectrometry (AMS) measurements from Colorado, USA, and two studies in Hyytiala, Finland, we quantify the fraction of organic aerosol (OA) mass that is composed of molecules with acid functional groups (facid). Molecules containing one or more carboxylic acid functionality contributed approximately 29% (45-51%) of the OA mass in Colorado (Finland). Organic acid mass concentration correlates well with AMS m/z 44 (primarily CO2+), a commonly used marker for highly oxidized aerosol. Using the average empirical relationship between AMS m/z 44 and organic acids in these three studies, together with m/z 44 data from 29 continental northern hemispheric (NH) AMS datasets, we estimate that molecules containing carboxylic acid functionality constitute on average 28% (range 10-50%) of NH continental OA mass with typically higher values at rural/remote sites and during summer and lower values at urban sites and during winter.

Observations of total RONO 2 over the boreal forest: NO x sinks and HNO 3 sources

In contrast with the textbook view of remote chemistry where HNO_3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO_2 (ΣANs) from isoprene and other terpene precursors is the primary net chemical loss of nitrogen oxides over the remote continents where the concentration of nitrogen oxides is low. This then increases the prominence of questions concerning the chemical lifetime and ultimate fate of ΣANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer which show that ΣANs account for ~20% of total oxidized nitrogen and that their instantaneous production rate is larger than that of HNO3. This confirms the primary role of reactions producing ΣANs as a control over the lifetime of NO_x (NO_x = NO + NO_2) in remote, continental environments. However, HNO_3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO_3 lifetime. We investigate a range of proposed loss mechanisms that would explain the inferred lifetime of ΣANs finding that in combination with deposition, two processes are consistent with the observations: (1) rapid ozonolysis of isoprene nitrates where at least ~40% of the ozonolysis products release NO_x from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO_3 as a product. Implications of these ideas for our understanding of NO_x and NO_y budget in remote and rural locations are discussed.

Accumulation‐mode aerosol number concentrations in the Arctic during the ARCTAS aircraft campaign: Long‐range transport of polluted and clean air from the Asian continent

[1] We evaluate the impact of transport from midlatitudes on aerosol number concentrations in the accumulation mode (light‐scattering particles (LSP) with diameters >180 nm) in the Arctic during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. We focus on transport from the Asian continent. We find marked contrasts in the number concentration (NLSP), transport efficiency (TEN_LSP, the fraction transported from sources to the Arctic), size distribution, and the chemical composition of aerosols between air parcels from anthropogenic sources in East Asia (Asian AN) and biomass burning sources in Russia and Kazakhstan (Russian BB). Asian AN air had lower NLSP and TEN_LSP (25 cm �3 and 18% in spring and 6.2 cm �3 and 3.0% in summer) than Russian BB air (280 cm �3 and 97% in spring and 36 cm �3 and 7.6% in summer) due to more efficient wet scavenging during transport from East Asia. Russian BB in this spring is the most important source of accumulation‐mode aerosols over the Arctic, and BB emissions are found to be the primary source of aerosols within all the data in spring during ARCTAS. On the other hand, the contribution of Asian AN transport had a negligible effect on the accumulation‐mode aerosol number concentration in the Arctic during ARCTAS. Compared with background air, NLSP was 2.3–4.7 times greater for Russian BB air but 2.4–2.6 times less for Asian AN air in both spring and summer. This result shows that the transport of Asian AN air decreases aerosol number concentrations in the Arctic, despite the large emissions of aerosols in East Asia. The very low aerosol number concentrations in Asian AN air were caused by wet removal during vertical transport in association with warm conveyor belts (WCBs). Therefore, this cleansing effect will be prominent for air transported via WCBs from other midlatitude regions and seasons. The inflow of clean midlatitude air can potentially have an important impact on accumulation‐mode aerosol number concentrations in the Arctic. Citation: Matsui, H., et al. (2011), Accumulation‐mode aerosol number concentrations in the Arctic during the ARCTAS aircraft campaign: Long‐range transport of polluted and clean air from the Asian continent, J. Geophys. Res., 116, D20217, doi:10.1029/2011JD016189.

Highly Oxygenated Multifunctional Compounds in α-Pinene Secondary Organic Aerosol

Highly oxygenated multifunctional organic compounds (HOMs) originating from biogenic emissions constitute a widespread source of organic aerosols in the pristine atmosphere. However, the molecular forms in which HOMs are present in the condensed phase upon gas-particle partitioning remain unclear. In this study, we show that highly oxygenated molecules that contain multiple peroxide functionalities are readily cationized by the attachment of Na+ during electrospray ionization operated in the positive ion mode. With this method, we present the first identification of HOMs characterized as C8-10H12-18O4-9 monomers and C16-20H24-36O8-14 dimers in α-pinene derived secondary organic aerosol (SOA). Simultaneous detection of these molecules in the gas phase provides direct evidence for their gas-to-particle conversion. Molecular properties of particulate HOMs generated from ozonolysis and OH oxidation of unsubstituted (C10H16) and deuterated (C10H13D3) α-pinene are investigated using coupled ion mobility spectrometry with mass spectrometry. The systematic shift in the mass of monomers in the deuterated system is consistent with the decomposition of isomeric vinylhydroperoxides to release vinoxy radical isotopologues, the precursors to a sequence of autoxidation reactions that ultimately yield HOMs in the gas phase. The remarkable difference observed in the dimer abundance under O3- versus OH-dominant environments underlines the competition between intramolecular hydrogen migration of peroxy radicals and their bimolecular termination reactions. Our results provide new and direct molecular-level information for a key component needed for achieving carbon mass closure of α-pinene SOA.

Closure Between Chemical Composition and Hygroscopic Growth of Aerosol Particles During TORCH2

Aerosol physical and chemical properties have been investigated during the TORCH2 field campaign in Weybourne, UK, on the North Norfolk coast (Gysel et al., 2006). An Aerodyne aerosol mass spectrometer (Q-AMS) was employed to measure highly time and size resolved chemical composition of the non-refractory compounds in PM1. Hygroscopic growth factors (HGF) of particles with different dry sizes have been measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA). Clean air masses not exposed to contact with land over the course of several days were encountered from 11-13 and 21-23 May, during which the aerosol composition showed little variation and the particles were sulphate dominated with an organic mass fraction of 20% or less. Accordingly the HGFs were quite constant and as high as 1.6-1.8 for 60 to 217 nm particles (thick black lines in Fig. 1). Aged polluted air masses originating from the Northeastern Atlantic and transported across Ireland and mid or north England were encountered from 14-20 May, where the organic fraction was still roughly constant at ~30, 50, and 70% for dry diameters 217, 137, and 60 nm, respectively. Accordingly, the overall inorganic fraction was also roughly constant, but the respective contributions from ammonium sulphate (AS) and ammonium nitrate (AN) showed a distinct diurnal pattern with maxima of AS and AN during the day and night. The HGFs also showed a distinct diurnal pattern, which is surprising regarding the roughly constant ratio of inorganic to organic compounds. Predictions of expected HGFs, based on the sizeresolved chemical composition obtained by the AMS, were made using the Zdanovskii-Stokes-Robinson (ZSR) rule for calculating the water activity of mixtures from pure compound properties. HGFs of pure AS and AN were taken from thermodynamic models, whereas a bulk HGF of 1.20 was assumed for the organics with small reductions accounting for the Kelvin effect. The black lines with markers in Fig. 1 show that predicted growth factors agree well with measurements in the clean air mass, but there is a clear disagreement at larger sizes during the aged polluted event. However, very good agreement between measurement and AMS/ZSR prediction at any size and any time is achieved when assuming that ~60% of the particulate AN evaporated in the HTDMA (grey lines in Fig. 1). A long residence time of ~60 s in the HTDMA was chosen to ensure full equilibration with water vapour, which, in retrospect, is likely to have caused the AN artefact in the HGF measurements. This study has shown that high time and size resolution is a crucial prerequisite for a valid hygroscopicity closure. The ZSR rule along with a constant organic HGF is an adequate simplification of the thermodynamic modelling in case of aged air masses, because more detailed chemical characterisation for comprehensive thermodynamic modelling typically undermines time and size resolution.