Total Organic Aerosol Concentration Research Articles

Seasonal characteristics of emission, distribution, and radiative effect of marine organic aerosols over the western Pacific Ocean: an investigation with a coupled regional climate aerosol model

Abstract. Organic aerosols from marine sources over the western Pacific Ocean of East Asia were investigated using an online coupled regional chemistry–climate model RIEMS-Chem for the entire year 2014. Model evaluation against a wide variety of observations from research cruises and in situ measurements demonstrated a good skill of the model in simulating temporal variation and spatial distribution of particulate matter with aerodynamic diameter less than 2.5 and 10 µm (PM2.5 and PM10), black carbon (BC), organic carbon (OC), sodium, and aerosol optical depth (AOD) in the marine atmosphere. The inclusion of marine organic aerosols improved model performance on OC concentration by reducing model biases of up to 20 %. The regional and annual mean near-surface marine organic aerosol (MOA) concentration was estimated to be 0.27 µg m−3, with the maximum in spring and the minimum in winter, and contributed 26 % of the total organic aerosol concentration on average over the western Pacific. Marine primary organic aerosol (MPOA) accounted for the majority of marine organic aerosol (MOA) mass, and the MPOA concentration exhibited the maximum in autumn and the minimum in summer, whereas marine secondary organic aerosol (MSOA) was approximately 1–2 orders of magnitude lower than MPOA, having a distinct summer maximum and a winter minimum. MOA induced a direct radiative effect (DREMOA) of −0.27 W m−2 and an indirect radiative effect (IREMOA) of −0.66 W m−2 at the top of the atmosphere (TOA) in terms of annual and oceanic average over the western Pacific, with the highest seasonal mean IREMOA up to −0.94 W m−2 in spring. IREMOA was stronger than, but in a similar magnitude to, the IRE due to sea salt aerosol on average, and it was approximately 9 % of the IRE due to anthropogenic aerosols in terms of annual mean over the western Pacific. This ratio increased to 19 % in the northern parts of the western Pacific in autumn. This study reveals an important role of MOA in perturbing cloud properties and shortwave radiation fluxes in the western Pacific of East Asia.

Response of organic aerosol to Delhi's pollution control measures over the period 2011–2018

Some of the world's highest air pollution episodes occur in Delhi, India and studies have shown particulate matter (PM) is the leading air pollutant to cause adverse health effects on Delhi's population. It is therefore vital to chart sources of PM over long time periods to effectively identify trends, particularly as multiple air quality mitigation measures have been implemented in Delhi over the past 10 years but remain unevaluated. An automated offline aerosol mass spectrometry (AMS) method has been developed which has enabled high-throughput analysis of PM filters. This novel offline-AMS method uses an organic solvent mix of acetone and water to deliver high extraction recoveries of organic aerosol (OA) (95.4 ± 8.3%). Positive matrix factorisation (PMF) source apportionment was performed on the OA fraction extracted from PM10 filter samples collected in Delhi in 2011, 2015 and 2018 to provide snapshots of the responses of OA to changes in sources in Delhi. The nine factors of OA resolved by PMF group into four primary source categories: traffic, cooking, coal-combustion and burning-related (solid fuel or open burning). Burning-related OA made the largest contribution during the winter and post-monsoon, when total OA concentrations were at their highest. Annual mean burning-related OA concentrations declined by 47% between 2015 and 2018, likely associated with the 2015 ban on open waste burning and controls and incentives to reduce crop-residue burning. Compositional analysis of OA factors shows municipal waste burning tracers still present in 2018, indicating further scope to reduce burning-related OA. The closure of the two coal power stations, along with initiatives to decrease coal use in industry, businesses, and residential homes, resulted in a significant decrease (87%) in coal-combustion OA. This corresponds to a 17% reduction in total OA, which shows the effectiveness of these measures in reducing PM10. Increases in traffic OA appear to have been offset by the introduction of the Bharat stage emissions standards for vehicles as the increases do not reflect the rapid increase in registered vehicles. However, daytime restrictions on heavy goods vehicles (HGVs) entering the city is linked to large increases in PM10 during the winter and post-monsoon, likely because the large influx of diesel-engine HGVs during the early mornings and evenings is timed with a particularly low planetary boundary layer height that enhances surface concentrations.

Simulation of the evolution of biomass burning organic aerosol with different volatility basis set schemes in PMCAMx-SRv1.0

Abstract. A source-resolved three-dimensional chemical transport model, PMCAMx-SR (Particulate Matter Comprehensive Air-quality Model with extensions – Source Resolved), was applied in the continental US to investigate the contribution of the various components (primary and secondary) of biomass burning organic aerosol (bbOA) to organic aerosol levels. Two different schemes based on the volatility basis set were used for the simulation of the bbOA during different seasons. The first is the default scheme of PMCAMx-SR, and the second is a recently developed scheme based on laboratory experiments of the bbOA evolution. The simulations with the alternative bbOA scheme predict much higher total bbOA concentrations when compared with the base case ones. This is mainly due to the high emissions of intermediate-volatility organic compounds (IVOCs) assumed in the alternative scheme. The oxidation of these compounds is predicted to be a significant source of secondary organic aerosol. The impact of the other parameters that differ in the two schemes is low to negligible. The monthly average maximum predicted concentrations of the alternative bbOA scheme were approximately an order of magnitude higher than those of the default scheme during all seasons. The performance of the two schemes was evaluated against observed total organic aerosol concentrations from several measurement sites across the US. The results were different for the different seasons examined. The default scheme performed better during July and September, while the alternative scheme performed a little better during April. These results illustrate the uncertainty of the corresponding predictions and the need to quantify the emissions and reactions of IVOCs from specific biomass sources and to better constrain the total (primary and secondary) bbOA levels.

Aircraft Measurements of Physicochemical Evolution of Atmospheric Aerosols in Air Pollution Plumes over a Megacity and Suburban Areas

As part of the Megacity Air Pollution Studies (MAPS)-Seoul campaign, three types of research flights were conducted over the Seoul Metropolitan Area (SMA) from May till June 2015 to measure the spatial distribution of a pollution plume near a power plant and petrochemical complex, the vertical profiles of pollutants on the western coast of Korea, and the pollutant distribution in the SMA. The pollution plume (~0–700 m) was highly concentrated and dominated by organic aerosol (OA), which very likely oxidized in the plume, as it showed slightly less oxidation near the source and significantly less oxidation at altitudes above the plume. One vertical profile displayed transitions in concentration and changes in the dominant components, suggesting that the particle sources and/or processing differed above ~1000 m; below 1000 m, where the total mass and OA concentrations were high, sulfate and likely transport sources predominated. The other profile, which was assessed during a separate flight, exhibited sharp increases in the OA number concentration and mean diameter, less oxidized organic content, and higher organic and nitrate concentrations above 1400 m, indicating high-altitude transport and a cleaner boundary layer. Finally, flights investigating the distributions of pollutants in the central, upwind, and downwind SMA regions generally detected high levels of oxidation downwind as well as different aerosol masses between the low and high altitudes. This study highlights the necessity of understanding the complex vertical structures of particle layers, such as those identified in and around the SMA, in order to facilitate the adoption of efficient air quality control strategies and enhance air quality forecasting.

Source apportionment of organic aerosol and ozone and the effects of emission reductions

Formation of organic aerosol (OA) and ozone were simulated for the greater Houston area in September 2013 and 2028 (summer conditions) using the Comprehensive Air Quality Model with Extensions (CAMx). We also simulated the background concentrations in the absence of US anthropogenic emissions. The 1.5-dimensional volatility basis set scheme was used to represent formation of primary and secondary OA (POA and SOA). The path-integral method for source apportionment was successfully extended to OA formation using the same numerical approach previously used for ozone and then applied to apportion the anthropogenic increment of OA and ozone to six emission source categories. The anthropogenic increment is the difference between the 2013 or 2028 simulation and the background simulation. Averaged over 21 air monitoring sites in Houston, point sources make the largest contribution to monthly average SOA and OA (25%–37%, depending on year) and to monthly maximum 8-h ozone (21%–26%). Onroad and then nonroad sources are the next most important in 2013, but their contributions diminish by 2028. The sources’ contributions to SOA correlate much more strongly with their NOx emissions than with their emissions of organic precursor species. Anthropogenic emissions enhance OA formation from biogenic emissions, and this enhancement is 46% and 48% of the total OA concentration in Houston in 2013 and 2028, respectively. Point sources make the largest contribution to the enhancement. Additional results for a larger regional area covering Texas and nearby states are also presented.

Simulation of atmospheric organic aerosol using its volatility–oxygen-content distribution during the PEGASOS 2012 campaign

Abstract. A lot of effort has been made to understand and constrain the atmospheric aging of the organic aerosol (OA). Different parameterizations of the organic aerosol formation and evolution in the two-dimensional volatility basis set (2D-VBS) framework are evaluated using ground and airborne measurements collected in the 2012 Pan-European Gas AeroSOls-climate interaction Study (PEGASOS) field campaign in the Po Valley (Italy). A number of chemical aging schemes are examined, taking into account various functionalization and fragmentation pathways for biogenic and anthropogenic OA components. Model predictions and measurements, both at the ground and aloft, indicate a relatively oxidized OA with little average diurnal variation. Total OA concentration and O : C ratios are reproduced within experimental error by a number of chemical aging schemes. Anthropogenic secondary OA (SOA) is predicted to contribute 15–25 % of the total OA, while SOA from intermediate volatility compound oxidation contributes another 20–35 %. Biogenic SOA (bSOA) contributions varied from 15 to 45 % depending on the modeling scheme. Primary OA contributed around 5 % for all schemes and was comparable to the hydrocarbon-like OA (HOA) concentrations derived from the positive matrix factorization of the aerosol mass spectrometer (PMF-AMS) ground measurements. The average OA and O : C diurnal variation and their vertical profiles showed a surprisingly modest sensitivity to the assumed vaporization enthalpy for all aging schemes. This can be explained by the interplay between the partitioning of the semi-volatile compounds and their gas-phase chemical aging reactions.

Emission of intermediate, semi and low volatile organic compounds from traffic and their impact on secondary organic aerosol concentrations over Greater Paris

Exhaust particle emissions are mostly made of black carbon and/or organic compounds, with some of these organic compounds existing in both the gas and particle phases. Although emissions of volatile organic compounds (VOC) are usually measured at the exhaust, emissions in the gas phase of lower volatility compounds (POAvapor) are not. However, these gas-phase emissions may be oxidised after emission and enhance the formation of secondary organic aerosols (SOA). They are shown here to contribute to most of the SOA formation in Central Paris. POAvapor emissions are usually estimated from primary organic aerosol emissions in the particle phase (POA). However, they could also be estimated from VOC emissions for both gasoline and diesel vehicles using previously published measurements from chamber measurements. Estimating POAvapor from VOC emissions and ageing exhaust emissions with a simple model included in the Polyphemus air-quality platform compare well to measurements of SOA formation performed in chamber experiments. Over Greater Paris, POAvapor emissions estimated using POA and VOC emissions are compared using the HEAVEN bottom-up traffic emissions model. The impact on the simulated atmospheric concentrations is then assessed using the Polyphemus/Polair3D chemistry-transport model. Estimating POAvapor emissions from VOC emissions rather than POA emissions lead to lower emissions along motorway axes (between −50% and −70%) and larger emissions in urban areas (up to between +120% and +140% in Central Paris). The impact on total organic aerosol concentrations (gas plus particle) is lower than the impact on emissions: between −8% and 25% along motorway axes and in urban areas respectively. Particle-phase organic concentrations are lower when POAvapor emissions are estimated from VOC than POA emissions, even in Central Paris where the total organic aerosol concentration is higher, because of different assumptions on the emission volatility distribution, stressing the importance of characterizing not only the emission strength, but also the emission volatility distribution.

Mixing times of organic molecules within secondary organic aerosol particles: a global planetary boundary layer perspective

Abstract. When simulating the formation and life cycle of secondary organic aerosol (SOA) with chemical transport models, it is often assumed that organic molecules are well mixed within SOA particles on the timescale of 1 h. While this assumption has been debated vigorously in the literature, the issue remains unresolved in part due to a lack of information on the mixing times within SOA particles as a function of both temperature and relative humidity. Using laboratory data, meteorological fields, and a chemical transport model, we estimated how often mixing times are < 1 h within SOA in the planetary boundary layer (PBL), the region of the atmosphere where SOA concentrations are on average the highest. First, a parameterization for viscosity as a function of temperature and RH was developed for α-pinene SOA using room-temperature and low-temperature viscosity data for α-pinene SOA generated in the laboratory using mass concentrations of ∼ 1000 µg m−3. Based on this parameterization, the mixing times within α-pinene SOA are < 1 h for 98.5 % and 99.9 % of the occurrences in the PBL during January and July, respectively, when concentrations are significant (total organic aerosol concentrations are > 0.5 µg m−3 at the surface). Next, as a starting point to quantify how often mixing times of organic molecules are < 1 h within α-pinene SOA generated using low, atmospherically relevant mass concentrations, we developed a temperature-independent parameterization for viscosity using the room-temperature viscosity data for α-pinene SOA generated in the laboratory using a mass concentration of ∼ 70 µg m−3. Based on this temperature-independent parameterization, mixing times within α-pinene SOA are < 1 h for 27 and 19.5 % of the occurrences in the PBL during January and July, respectively, when concentrations are significant. However, associated with these conclusions are several caveats, and due to these caveats we are unable to make strong conclusions about how often mixing times of organic molecules are < 1 h within α-pinene SOA generated using low, atmospherically relevant mass concentrations. Finally, a parameterization for viscosity of anthropogenic SOA as a function of temperature and RH was developed using sucrose–water data. Based on this parameterization, and assuming sucrose is a good proxy for anthropogenic SOA, 70 and 83 % of the mixing times within anthropogenic SOA in the PBL are < 1 h for January and July, respectively, when concentrations are significant. These percentages are likely lower limits due to the assumptions used to calculate mixing times.

Evaluation of European air quality modelled by CAMx including the volatility basis set scheme

Abstract. Four periods of EMEP (European Monitoring and Evaluation Programme) intensive measurement campaigns (June 2006, January 2007, September–October 2008 and February–March 2009) were modelled using the regional air quality model CAMx with VBS (volatility basis set) approach for the first time in Europe within the framework of the EURODELTA-III model intercomparison exercise. More detailed analysis and sensitivity tests were performed for the period of February–March 2009 and June 2006 to investigate the uncertainties in emissions as well as to improve the modelling of organic aerosol (OA). Model performance for selected gas phase species and PM2.5 was evaluated using the European air quality database AirBase. Sulfur dioxide (SO2) and ozone (O3) were found to be overestimated for all the four periods, with O3 having the largest mean bias during June 2006 and January–February 2007 periods (8.9 pbb and 12.3 ppb mean biases respectively). In contrast, nitrogen dioxide (NO2) and carbon monoxide (CO) were found to be underestimated for all the four periods. CAMx reproduced both total concentrations and monthly variations of PM2.5 for all the four periods with average biases ranging from −2.1 to 1.0 µg m−3. Comparisons with AMS (aerosol mass spectrometer) measurements at different sites in Europe during February–March 2009 showed that in general the model overpredicts the inorganic aerosol fraction and underpredicts the organic one, such that the good agreement for PM2.5 is partly due to compensation of errors. The effect of the choice of VBS scheme on OA was investigated as well. Two sensitivity tests with volatility distributions based on previous chamber and ambient measurements data were performed. For February–March 2009 the chamber case reduced the total OA concentrations by about 42 % on average. In contrast, a test based on ambient measurement data increased OA concentrations by about 42 % for the same period bringing model and observations into better agreement. Comparison with the AMS data at the rural Swiss site Payerne in June 2006 shows no significant improvement in modelled OA concentration. Further sensitivity tests with increased biogenic and anthropogenic emissions suggest that OA in Payerne was affected by changes in emissions from residential heating during the February–March 2009 whereas it was more sensitive to biogenic precursors in June 2006.

Gas–particle partitioning and hydrolysis of organic nitrates formed from the oxidation of <i>α</i>-pinene in environmental chamber experiments

Abstract. Gas–particle partitioning and hydrolysis of organic nitrates (ON) influences their role as sinks and sources of NOx and their effects on the formation of tropospheric ozone and organic aerosol (OA). In this work, organic nitrates were formed from the photo-oxidation of α-pinene in environmental chamber experiments under different conditions. Particle-phase ON hydrolysis rates, consistent with observed ON decay, exhibited a nonlinear dependence on relative humidity (RH): an ON decay rate of 2 day−1 was observed when the RH ranged between 20 and 60 %, and no significant ON decay was observed at RH lower than 20 %. In experiments when the highest observed RH exceeded the deliquescence RH of the ammonium sulfate seed aerosol, the particle-phase ON decay rate was as high as 7 day−1 and more variable. The ON gas–particle partitioning was dependent on total OA concentration and temperature, consistent with absorptive partitioning theory. In a volatility basis set, the ON partitioning was consistent with mass fractions of [0 0.11 0.03 0.86] at saturation mass concentrations (C*) of [1 10 100 1000] µg m−3.

Water uptake by organic aerosol and its influence on gas/particle partitioning of secondary organic aerosol in the United States

Abstract Organic aerosol (OA) is at least partly hygroscopic, i.e., water partitions into the organic phase to a degree determined by the relative humidity (RH), the organic chemical composition, and the particle size. This organic-phase water increases the aerosol mass and provides a larger absorbing matrix while decreasing its mean molecular weight, which can encourage additional condensation of semi-volatile organic compounds. Most regional and global atmospheric models account for water uptake by inorganic salts but do not explicitly account for organic-phase water and its subsequent impact on gas/particle partitioning of semi-volatile OA. In this work, we incorporated the organic-phase water model described by Pankow et al. (2015) into the UCD/CIT air quality model to simulate water uptake by OA and assessed its influence on total OA mass concentrations. The model was run for one summer month over two distinct regions: South Coast Air Basin (SoCAB) surrounding Los Angeles, California and the eastern United States (US). In SoCAB where the OA was dominated by non-hygroscopic primary OA (POA), there was very little organic-phase water uptake (0.1–0.2 μg m−3) and consequently very little enhancement (or growth) in total OA concentrations (OA + organic-phase water): a 3% increase in total OA mass was predicted for a 0.1 increase in relative humidity. In contrast, in the eastern US where secondary OA (SOA) from biogenic sources dominated the OA, substantial organic-phase water uptake and enhancement in total OA concentrations was predicted, even in urban locations. On average, the model predicted a 20% growth in total OA mass for a 0.1 increase in relative humidity; the growth was equivalent to a 250 nm particle with a hygroscopicity parameter (κ) of 0.15. Further, for the same relative humidity, the exact extent of organic-phase water uptake and total OA enhancement was found to be dependent on the particle mixing state. When the source-oriented mixing state of aerosols was considered, generally, less organic-phase water uptake was predicted than when simple internal mixing approximations were made. Overall, the results indicated that organic-phase water can significantly influence predicted total OA concentrations under certain conditions. Regional models applied in areas with high humidity and significant SOA formation should include calculations for organic-phase water in order to capture this effect.

Characterization of primary and secondary wood combustion products generated under different burner loads

Abstract. Residential wood burning contributes to the total atmospheric aerosol burden; however, large uncertainties remain in the magnitude and characteristics of wood burning products. Primary emissions are influenced by a variety of parameters, including appliance type, burner wood load and wood type. In addition to directly emitted particles, previous laboratory studies have shown that oxidation of gas-phase emissions produces compounds with sufficiently low volatility to readily partition to the particles, forming considerable quantities of secondary organic aerosol (SOA). However, relatively little is known about wood burning SOA, and the effects of burn parameters on SOA formation and composition are yet to be determined. There is clearly a need for further study of primary and secondary wood combustion aerosols to advance our knowledge of atmospheric aerosols and their impacts on health, air quality and climate. For the first time, smog chamber experiments were conducted to investigate the effects of wood loading on both primary and secondary wood combustion products. Products were characterized using a range of particle- and gas-phase instrumentation, including an aerosol mass spectrometer (AMS). A novel approach for polycyclic aromatic hydrocarbon (PAH) quantification from AMS data was developed and results were compared to those from GC-MS analysis of filter samples. Similar total particle mass emission factors were observed under high and average wood loadings; however, high fuel loadings were found to generate significantly higher contributions of PAHs to the total organic aerosol (OA) mass compared to average loadings. PAHs contributed 15 ± 4% (mean ±2 sample standard deviations) to the total OA mass in high-load experiments, compared to 4 ± 1% in average-load experiments. With aging, total OA concentrations increased by a factor of 3 ± 1 for high load experiments compared to 1.6 ± 0.4 for average-load experiments. In the AMS, an increase in PAH and aromatic signature ions at lower m / z values, likely fragments from larger functionalized PAHs, was observed with aging. Filter samples also showed an increase in functionalized PAHs in the particles with aging, particularly oxidized naphthalene species. As PAHs and their oxidation products are known to have deleterious effects on health, this is a noteworthy finding to aid in the mitigation of negative wood burning impacts by improving burner operation protocols.

Gas/particle partitioning of total alkyl nitrates observed with TD‐LIF in Bakersfield

Limitations in the chemical characterization of tropospheric organic aerosol (OA) continue to impede attempts to fully understand its chemical sources and sinks. To assess the role of organic nitrates in OA, we used a new thermal dissociation‐laser induced fluorescence‐based (TD‐LIF) technique to obtain a high‐time‐resolution record of total aerosol organic nitrates (hereafter ΣANsaer) at the Bakersfield, CA supersite during the 2010 CalNex campaign. The TD‐LIF measurements compare well with Fourier transform infrared measurements from collocated filter samples. These measurements show that ΣANs are a ubiquitous component of the OA with the –ONO2 subunit comprising on average 4.8% of the OA mass. Scaling this fraction by an estimate of the organic backbone mass yields an estimate that 17–23% of OA molecules contain nitrate functional groups. Measurements of both total ΣAN (gas + aerosol) and ΣANaer show that on average 21% of ΣANs are in the condensed phase, suggesting atmospheric organic nitrates have similar volatilities to analogous non‐nitrate oxidized organic compounds. The fraction of ΣAN that is in the condensed phase increases with total OA concentration, providing direct evidence from the atmosphere that absorptive partitioning into OA has some control over the ΣAN phase partitioning. The specific molecular identity of the ΣAN is incompletely understood. Both biogenic hydrocarbons and long chain alkanes are calculated to be significant sources of low volatility nitrates in Bakersfield, and ultra performance liquid chromatography coupled to an electrospray ionization high‐resolution quadrupole time‐of‐flight mass spectrometer measurements confirm the existence of particulate nitrooxy organosulfates derived from gas‐phase oxidation of both isoprene and monoterpenes.

A functional group oxidation model (FGOM) for SOA formation and aging

Abstract. Secondary organic aerosol (SOA) formation from a volatile organic compound (VOC) involves multiple generations of oxidation that include functionalization and fragmentation of the parent carbon backbone and likely particle-phase oxidation and/or accretion reactions. Despite the typical complexity of the detailed molecular mechanism of SOA formation and aging, a relatively small number of functional groups characterize the oxidized molecules that constitute SOA. Given the carbon number and set of functional groups, the volatility of the molecule can be estimated. We present here a functional group oxidation model (FGOM) that represents the process of SOA formation and aging. The FGOM contains a set of parameters that are to be determined by fitting of the model to laboratory chamber data: total organic aerosol concentration, and O : C and H : C atomic ratios. The sensitivity of the model prediction to variation of the adjustable parameters allows one to assess the relative importance of various pathways involved in SOA formation. An analysis of SOA formation from the high- and low-NOx photooxidation of four C12 alkanes (n-dodecane, 2-methylundecane, hexylcyclohexane, and cyclododecane) using the FGOM is presented, and comparison with the statistical oxidation model (SOM) of Cappa et al. (2013) is discussed.

Aerosol mass spectrometric analysis of the chemical composition of non-refractory PM1 samples from school environments in Brisbane, Australia

Long-term exposure to vehicle emissions has been associated with detrimental health effects. Children are amongst the most susceptible group and schools represent an environment where they can experience significant exposure to vehicle emissions. However, there are limited studies on children's exposure to vehicle emissions in schools. The aim of this study was to quantify the concentration of organic aerosol (OA) and in particular, vehicle emissions that children are exposed to during school hours. Therefore an Aerodyne compact time-of-flight aerosol mass spectrometer (TOF-AMS) was deployed at five urban schools in Brisbane, Australia. TOF-AMS enabled the chemical composition of the non-refractory (NR-PM1) to be analysed with a high temporal resolution to assess the concentration of vehicle emissions and other OA components during school hours. The organic fraction at each school comprised the majority of NR-PM1. Primary emissions were found to dominate the OA at only one school which had an O:C ratio of 0.17, due to fuel powered gardening equipment used near the TOF-AMS. A significant source of the OA at two of the schools was aged vehicle emissions from nearby highways. More oxidised OA was observed at the remaining two schools, which also recorded strong biomass burning influences. In general, the diurnal cycle of the total OA concentration varied between schools and was found to be at a minimum during school hours. The major organic component that school children were exposed to during school hours was secondary OA at all schools. Peak exposure of school children to vehicle emissions occurred during school drop-off and pick-up times. Unless a school is located near major roads, children are exposed predominately to regional secondary OA as opposed to local emissions during school hours in urban environments.

Aged organic aerosol in the Eastern Mediterranean: the Finokalia Aerosol Measurement Experiment – 2008

Abstract. Aged organic aerosol (OA) was measured at a remote coastal site on the island of Crete, Greece during the Finokalia Aerosol Measurement Experiment-2008 (FAME-2008), which was part of the EUCAARI intensive campaign of May 2008. The site at Finokalia is influenced by air masses from different source regions, including long-range transport of pollution from continental Europe. A quadrupole aerosol mass spectrometer (Q-AMS) was employed to measure the size-resolved chemical composition of non-refractory submicron aerosol (NR-PM1), and to estimate the extent of oxidation of the organic aerosol. Factor analysis was used to gain insights into the processes and sources affecting the OA composition. The particles were internally mixed and liquid. The largest fraction of the dry NR-PM1 sampled was ammonium sulfate and ammonium bisulfate, followed by organics and a small amount of nitrate. The variability in OA composition could be explained with two factors of oxygenated organic aerosol (OOA) with differing extents of oxidation but similar volatility. Hydrocarbon-like organic aerosol (HOA) was not detected. There was no statistically significant diurnal variation in the bulk composition of NR-PM1 such as total sulfate or total organic aerosol concentrations. However, the OA composition exhibited statistically significant diurnal variation with more oxidized OA in the afternoon. The organic aerosol was highly oxidized, regardless of the source region. Total OA concentrations also varied little with source region, suggesting that local sources had only a small effect on OA concentrations measured at Finokalia. The aerosol was transported for about one day before arriving at the site, corresponding to an OH exposure of approximately 4×1011 molecules cm−3 s. The constant extent of oxidation suggests that atmospheric aging results in a highly oxidized OA at these OH exposures, regardless of the aerosol source.

Isoprene oxidation by nitrate radical: alkyl nitrate and secondary organic aerosol yields

Abstract. Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289–301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O3 and 31.1 ppb NOx. We find the yield of nitrates is 70±8% from the isoprene + NO3 reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO3 is 40±20%. We find an effective rate constant for reaction of NO3 with the group of first generation oxidation products to be 7×10−14 molecule−1 cm3 s−1. At the low total organic aerosol concentration in the chamber (max=0.52 μg m−3) we observed a mass yield (ΔSOA mass/Δisoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as ΔSOA/Δisoprene2x where Δisoprene2x is the mass of isoprene which reacted twice with NO3). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C5 products of isoprene oxidation.

Development and initial application of the sesquiversion of MADRID

Organic aerosol (OA) models are typically formulated on the basis of empirical parameters or from first principles based on molecular constituents and properties. The overarching theme of sesqui‐MADRID is the representation of primary and secondary organic aerosol constituents using surrogate molecular structures. Empirical partitioning characteristics guide the selection of surrogate secondary OA (SOA). Semivolatile primary OA (POA) are represented by selective compounds of key source types. The assignment of surrogate structures allows the use of group contribution methods for the calculation of thermodynamic properties such as activity coefficients, vapor pressures, and enthalpies of vaporization. A new activity coefficient model represents the interactions among POA, SOA, and water. The inclusion of water increases the total OA concentration by increasing the absorbing medium. If activity coefficients are assumed to be unity, OA increases substantially. Treatment of nonideality is essential in OA modules. When thermodynamically favorable, two phases with different hydrophobicity are modeled. Phase separation of OA is more common when water is considered as part of the absorbing medium than when it is not. Phase separation results in a small increase in total OA, because hydrophobic and hydrophilic compounds can exist in two liquid phases rather than repel each other in a single phase. Allowing POA to evaporate reduces the fraction of POA relative to SOA. The decrease can lead to a less than proportional decrease (or an increase) in total OA due to changes in composition, activity coefficients, and partitioning of individual constituents. A three‐dimensional demonstration of sesqui‐MADRID for a typical summer day shows an overall increase in OA in the southeastern United States compared to an empirical approach. Sesqui‐MADRID predicts higher contributions from volatile and hydrophilic constituents and similar concentrations for constituents that are hydrophobic and less volatile.