Effect of aqueous-phase processing on aerosol chemistry and size distributions in Fresno, California, during wintertime

2012 ◽  
Vol 9 (3) ◽  
pp. 221 ◽  
Author(s):  
Xinlei Ge ◽  
Qi Zhang ◽  
Yele Sun ◽  
Christopher R. Ruehl ◽  
Ari Setyan
Keyword(s):  
Aqueous Phase ◽  
Fine Particles ◽  
Organic Aerosol ◽  
Size Distributions ◽  
Aerosol Chemistry ◽  
Aerosol Composition ◽  
Secondary Aerosol ◽  
Organic Species ◽  
Secondary Species ◽  
Aerosol Properties

Environmental contextAqueous-phase processes in fogs and clouds can significantly alter atmospheric fine particles with consequences for climate and human health. We studied the influence of fog and rain on atmospheric aerosol properties, and show that aqueous-phase reactions contribute to the production of secondary aerosol species and change significantly the composition and microphysical properties of aerosols. In contrast, rains effectively remove aerosols and reduce their concentrations. AbstractSubmicrometre aerosols (PM1) were characterised in situ with a high resolution time-of-flight aerosol mass spectrometer and a scanning mobility particle sizer in Fresno, CA, from 9 to 23 January 2010. Three dense fog events occurred during the first week of the campaign whereas the last week was influenced by frequent rain events. We thus studied the effects of aqueous-phase processing on aerosol properties by examining the temporal variations of submicrometre aerosol composition and size distributions. Rains removed secondary species effectively, leading to low loadings of PM1 dominated by primary organic species. Fog episodes, however, increased the concentrations of secondary aerosol species (sulfate, nitrate, ammonium and oxygenated organic aerosol). The size distributions of these secondary species, which always showed a droplet mode peaking at ~500 nm in the vacuum aerodynamic diameter, increased in mode size during fog episodes as well. In addition, the oxygen-to-carbon ratio of oxygenated organic species increased in foggy days, indicating that fog processing likely enhances the production of secondary organic aerosol as well as its oxidation degree. Overall, our observations show that aqueous-phase processes significantly affect submicrometre aerosol chemistry and microphysics in the Central Valley of California during winter, responsible for the production of secondary inorganic and organic aerosol species and the formation of droplet mode particles, thus altering the climatic and health effects of ambient aerosols in this region.

scholarly journals Aerosol composition and sources during the Chinese Spring Festival: fireworks, secondary aerosol, and holiday effects

2015 ◽  
Vol 15 (11) ◽  
pp. 6023-6034 ◽  
Author(s):  
Q. Jiang ◽  
Y. L. Sun ◽  
Z. Wang ◽  
Y. Yin
Keyword(s):  
Chinese Spring ◽  
Fine Particles ◽  
Nitrogen Monoxide ◽  
Primary Source ◽  
Organic Aerosol ◽  
Aerosol Composition ◽  
Secondary Aerosol ◽  
Particulate Pollution ◽  
Spring Festival ◽  
The Impact

Abstract. Aerosol particles were characterized by an Aerodyne aerosol chemical speciation monitor along with various collocated instruments in Beijing, China, to investigate the role of fireworks (FW) and secondary aerosol in particulate pollution during the Chinese Spring Festival of 2013. Three FW events, exerting significant and short-term impacts on fine particles (PM2.5), were observed on the days of Lunar New Year, Lunar Fifth Day, and Lantern Festival. The FW were shown to have a large impact on non-refractory potassium, chloride, sulfate, and organics in submicron aerosol (PM1), of which FW organics appeared to be emitted mainly in secondary, with its mass spectrum resembling that of secondary organic aerosol (SOA). Pollution events (PEs) and clean periods (CPs) alternated routinely throughout the study. Secondary particulate matter (SPM = SOA + sulfate + nitrate + ammonium) dominated the total PM1 mass on average, accounting for 63–82% during nine PEs in this study. The elevated contributions of secondary species during PEs resulted in a higher mass extinction efficiency of PM1 (6.4 m2 g-1) than during CPs (4.4 m2 g-1). The Chinese Spring Festival also provides a unique opportunity to study the impact of reduced anthropogenic emissions on aerosol chemistry in the city. Primary species showed ubiquitous reductions during the holiday period with the largest reduction being in cooking organic aerosol (OA; 69%), in nitrogen monoxide (54%), and in coal combustion OA (28%). Secondary sulfate, however, remained only slightly changed, and the SOA and the total PM2.5 even slightly increased. Our results have significant implications for controlling local primary source emissions during PEs, e.g., cooking and traffic activities. Controlling these factors might have a limited effect on improving air quality in the megacity of Beijing, due to the dominance of SPM from regional transport in aerosol particle composition.

scholarly journals Size-resolved characterization of organic aerosol in the North China Plain: new insights from high resolution spectral analysis

2021 ◽  
Author(s):  
Weiqi Xu ◽  
Chun Chen ◽  
Yanmei Qiu ◽  
Conghui Xie ◽  
Yunle Chen ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
North China Plain ◽  
North China ◽  
Fine Particles ◽  
Large Fraction ◽  
Organic Aerosol ◽  
Size Distributions ◽  
The North ◽  
The Impact

Organic aerosol (OA), a large fraction of fine particles, has a large impact on climate radiative forcing and human health, and the impact depends strongly on size distributions. Here we...

scholarly journals Seasonal variations in the high time-resolved aerosol composition, sources, and chemical process of background submicron particles in North China Plain

2020 ◽  
Author(s):  
Jiayun Li ◽  
Liming Cao ◽  
Wenkang Gao ◽  
Lingyan He ◽  
Yingchao Yan ◽  
...  
Keyword(s):  
Aqueous Phase ◽  
Seasonal Variations ◽  
North China Plain ◽  
North China ◽  
Organic Aerosol ◽  
Submicron Particles ◽  
Western China ◽  
Aerosol Composition ◽  
Oxidation Degree ◽  
Four Seasons

Abstract. For the first time in the North China Plain (NCP), we investigated the seasonal variations of submicron particles (NR-PM1) and its chemical composition at a background mountain station using Aerodyne high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). The averaged NR-PM1 were highest in autumn (15.1 μg m−3) and lowest in summer (12.4 μg m−3), with the abundance of more nitrate in spring (34 %), winter (31 %), and autumn (34 %), and elevated organics (40 %) and sulfate (38 %) proportion in summer. The submicron particles were almost neutralized by excess ammonium in all four seasons except summer, when the aerosol particles appeared to be slightly acidic. The size distribution of all PM1 species showed a consistent accumulation mode peaked at approximately 600–800 nm (dva), indicating the highly aged and internally mixed nature of the background aerosols, which further supported by the source appointment using multilinear engine (ME-2) and significant contributions of aged secondary organic aerosol (SOA) in organic aerosol (OA) were resolved in all seasons (> 77 %), especially in summer (95 %). The oxidation degree and evolution process of OAs in the four seasons were further investigated, and enhanced carbon oxidation state (−0.45–0.10), O / C (0.54–0.75) and OM / OC (1.86–2.13) ratios compared with urban studies were observed, with the highest oxidation degree of which appeared in summer, likely due to the relatively stronger photochemical processing which dominated the processes of both less oxidized OA (LO-OOA) and more oxidized OA (MO-OOA) formations. Aqueous-phase processing also contributed to the SOA formation but prevailed in autumn and winter and the role of which to MO-OOA and LO-OOA also varied in different seasons. In addition, compared with the urban atmosphere, LO-OOA formation in the background atmosphere exhibited more regional characteristics, as photochemical and aqueous-phase processing enhanced during the transport in summer and autumn, respectively. Furthermore, the backward trajectories analysis showed that higher submicron particles were associated with air mass for short distance transported from the southern regions in four seasons, while the long-range transport from Inner Mongolia (west and north regions) also contributed to the summer particle pollutions in the background areas of NCP. Our results illustrate the background particles in NCP are influenced significantly by aging processing and transport, and the more neutralized state of submicron particles with the abundance of nitrate compared with those in the background atmosphere in southern and western China, highlighting the regional reductions in emissions of nitrogen oxide and ammonia are critical for remedying the increased occurrence of nitrate-dominated haze event in the NCP.

scholarly journals Aerosol composition, oxidation properties, and sources in Beijing: results from the 2014 Asia-Pacific Economic Cooperation summit study

2015 ◽  
Vol 15 (23) ◽  
pp. 13681-13698 ◽  
Author(s):  
W. Q. Xu ◽  
Y. L. Sun ◽  
C. Chen ◽  
W. Du ◽  
T. T. Han ◽  
...  
Keyword(s):  
Air Pollution ◽  
Organic Aerosol ◽  
Economic Cooperation ◽  
Asia Pacific ◽  
Local Source ◽  
Size Distributions ◽  
Aerosol Composition ◽  
Asia Pacific Economic Cooperation ◽  
Emission Controls ◽  
Oxidation Properties

Abstract. The mitigation of air pollution in megacities remains a great challenge because of the complex sources and formation mechanisms of aerosol particles. The 2014 Asia-Pacific Economic Cooperation (APEC) summit in Beijing serves as a unique experiment to study the impacts of emission controls on aerosol composition, size distributions, and oxidation properties. Herein, a high-resolution time-of-flight aerosol mass spectrometer was deployed in urban Beijing for real-time measurements of size-resolved non-refractory submicron aerosol (NR-PM1) species from 14 October to 12 November 2014, along with a range of collocated measurements. The average (±σ) PM1 was 41.6 (±38.9) μg m−3 during APEC, which was decreased by 53 % compared with that before APEC. The aerosol composition showed substantial changes owing to emission controls during APEC. Secondary inorganic aerosol (SIA: sulfate + nitrate + ammonium) showed significant reductions of 62–69 %, whereas organics presented much smaller decreases (35 %). The results from the positive matrix factorization of organic aerosol (OA) indicated that highly oxidized secondary organic aerosol (SOA) showed decreases similar to those of SIA during APEC. However, primary organic aerosol (POA) from cooking, traffic, and biomass-burning sources were comparable to those before APEC, indicating the presence of strong local source emissions. The oxidation properties showed corresponding changes in response to OA composition. The average oxygen-to-carbon level during APEC was 0.36 (±0.10), which is lower than the 0.43 (±0.13) measured before APEC, demonstrating a decrease in the OA oxidation degree. The changes in size distributions of primary and secondary species varied during APEC. SIA and SOA showed significant reductions in large accumulation modes with peak diameters shifting from ~ 650 to 400 nm during APEC, whereas those of POA remained relatively unchanged. The changes in aerosol composition, size distributions, and oxidation degrees during the aging processes were further illustrated in a case study of a severe haze episode. Our results elucidated a complex response of aerosol chemistry to emission controls, which has significant implications that emission controls over regional scales can substantially reduce secondary particulates. However, stricter emission controls for local source emissions are needed for further mitigating air pollution in the megacity of Beijing.

scholarly journals Three-dimensional factorization of size-resolved organic aerosol mass spectra from Mexico City

2012 ◽  
Vol 5 (1) ◽  
pp. 195-224 ◽  
Author(s):  
I. M. Ulbrich ◽  
M. R. Canagaratna ◽  
M. J. Cubison ◽  
Q. Zhang ◽  
N. L. Ng ◽  
...  
Keyword(s):  
High Resolution ◽  
Size Distribution ◽  
Mexico City ◽  
Matrix Model ◽  
Mass Spectra ◽  
Organic Aerosol ◽  
Size Distributions ◽  
Aerosol Composition ◽  
Composition Data ◽  
Vector Matrix

Abstract. A size-resolved submicron organic aerosol composition dataset from a high-resolution time-of-flight mass spectrometer (HR-ToF-AMS) collected in Mexico City during the MILAGRO campaign in March 2006 is analyzed using 3-dimensional (3-D) factorization models. A method for estimating the precision of the size-resolved composition data for use with the factorization models is presented here for the first time. Two 3-D models are applied to the dataset. One model is a 3-vector decomposition (PARAFAC model), which assumes that each chemical component has a constant size distribution over all time steps. The second model is a vector-matrix decomposition (Tucker 1 model) that allows a chemical component to have a size distribution that varies in time. To our knowledge, this is the first report of an application of 3-D factorization models to data from fast aerosol instrumentation, and the first application of this vector-matrix model to any ambient aerosol dataset. A larger number of degrees of freedom in the vector-matrix model enable fitting real variations in factor size distributions, but also make the model susceptible to fitting noise in the dataset, giving some unphysical results. For this dataset and model, more physically meaningful results were obtained by partially constraining the factor mass spectra using a priori information and a new regularization method. We find four factors with each model: hydrocarbon-like organic aerosol (HOA), biomass-burning organic aerosol (BBOA), oxidized organic aerosol (OOA), and a locally occurring organic aerosol (LOA). These four factors have previously been reported from 2-dimensional factor analysis of the high-resolution mass spectral dataset from this study. The size distributions of these four factors are consistent with previous reports for these particle types. Both 3-D models produce useful results, but the vector-matrix model captures real variability in the size distributions that cannot be captured by the 3-vector model. A tracer m/z-based method provides a useful approximation for the component size distributions in this study. Variation in the size distributions is demonstrated in a case study day with a large secondary aerosol formation event, in which there is evidence for the coating of HOA-containing particles with secondary species, shifting the HOA size distribution to larger particle sizes. These 3-D factorizations could be used to extract size-resolved aerosol composition data for correlation with aerosol hygroscopicity, cloud condensation nuclei (CCN), and other aerosol impacts. Furthermore, other fast and chemically complex 3-D datasets, including those from thermal desorption or chromatographic separation, could be analyzed with these 3-D factorization models. Applications of these models to new datasets requires careful construction of error estimates and appropriate choice of models that match the underlying structure of those data. Factorization studies with these 3-D datasets have the potential to provide further insights into organic aerosol sources and processing.

scholarly journals Aerosol Chemistry Resolved by Mass Spectrometry: Linking Field Measurements of Cloud Condensation Nuclei Activity to Organic Aerosol Composition

2016 ◽  
Vol 50 (20) ◽  
pp. 10823-10832 ◽  
Author(s):  
Alexander L. Vogel ◽  
Johannes Schneider ◽  
Christina Müller-Tautges ◽  
Gavin J. Phillips ◽  
Mira L. Pöhlker ◽  
...  
Keyword(s):  
Mass Spectrometry ◽  
Cloud Condensation Nuclei ◽  
Field Measurements ◽  
Organic Aerosol ◽  
Condensation Nuclei ◽  
Aerosol Chemistry ◽  
Aerosol Composition

scholarly journals Three-dimensional factorization of size-resolved organic aerosol mass spectra from Mexico City

2011 ◽  
Vol 4 (4) ◽  
pp. 4561-4630 ◽  
Author(s):  
I. M. Ulbrich ◽  
M. R. Canagaratna ◽  
M. J. Cubison ◽  
Q. Zhang ◽  
N. L. Ng ◽  
...  
Keyword(s):  
High Resolution ◽  
Size Distribution ◽  
Mexico City ◽  
Matrix Model ◽  
Mass Spectra ◽  
Organic Aerosol ◽  
Size Distributions ◽  
Aerosol Composition ◽  
Composition Data ◽  
Vector Matrix

Abstract. A size-resolved submicron organic aerosol composition dataset from a high-resolution time-of-flight mass spectrometer (HR-ToF-AMS) collected in Mexico City during the MILAGRO campaign in March 2006 is analyzed using 3-dimensional (3-D) factorization models. A method for estimating the precision of the size-resolved composition data for use with the factorization models is presented here for the first time. Two 3-D models are applied to the dataset. One model is a 3-vector decomposition (PARAFAC model), which assumes that each chemical component has a constant size distribution over all time steps. The second model is a vector-matrix decomposition (Tucker 1 model) that allows a chemical component to have a size distribution that varies in time. To our knowledge, this is the first report of an application of 3-D factorization models to data from fast aerosol instrumentation; it is also the first application of this vector-matrix model to any ambient aerosol dataset. A larger number of degrees of freedom in the vector-matrix model enable fitting real variations in factor size distributions, but also make the model susceptible to fitting noise in the dataset, giving some unphysical results. For this dataset and model, more physical results were obtained by partially constraining the factor mass spectra using a priori information and a new regularization method. We find four factors with each model: hydrocarbon-like organic aerosol (HOA), biomass-burning organic aerosol (BBOA), oxidized organic aerosol (OOA), and a locally occurring organic aerosol (LOA). These four factors have previously been reported from 2-dimensional factor analysis of the high-resolution mass spectral dataset from this study. The size distributions of these four factors are consistent with previous reports for these particle types. Both 3-D models produce useful results, but the vector-matrix model captures real variability in the size distributions that cannot be captured by the 3-vector model. A tracer m/z-based method provides a useful approximation for the component size distributions in this study. Variation in the size distributions is demonstrated in a case study day with a large secondary aerosol formation event, in which there is evidence for the coating of HOA-containing particles with secondary species, shifting the HOA size distribution to larger particle sizes. These 3-D factorizations could be used to extract size-resolved aerosol composition data for correlation with aerosol hygroscopicity, cloud condensation nuclei (CCN), and other aerosol impacts. Furthermore, application of these 3-D factorization models to other fast and chemically complex 3-D datasets, including those from thermal desorption or chromatographic separation, has the potential to provide further insights into organic aerosol sources and processing.

scholarly journals Aerosol composition and the contribution of SOA formation over Mediterranean forests

2017 ◽  
Author(s):  
Evelyn Freney ◽  
Karine Sellegri ◽  
Mounir Chrit ◽  
Kouji Adachi ◽  
Joel Brito ◽  
...  
Keyword(s):  
Gas Phase ◽  
Organic Aerosol ◽  
Oxidation Products ◽  
Dust Particles ◽  
Large Contribution ◽  
Mediterranean Forests ◽  
Aerosol Composition ◽  
Tem Analysis ◽  
Organic Species ◽  
Inorganic Species

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas phase measurements were deployed aboard the SAFIRE ATR-42 research aircraft in summer 2014. The present study focuses on the 4 flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by C-ToF-AMS instrument were dominated by organic species (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulphate aerosols. The contribution from the anthropogenic nitrate and black carbon (BC) concentrations, measured by an SP2, never contributed to more than 5 % each to the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) were coherent with the C-ToF-AMS instrument, showing a large contribution of externally mixed organic aerosol and externally mixed sulphate particles. Externally mixed organic aerosols, were equally identified with S signals, which may suggest the presence of organo-sulphates. Measurements of refractory species from TEM analysis showed a significant contribution of both sea salt and dust particles depending on the air mass trajectory. The organic aerosol measured by the C-ToF-AMS contained only evidence of oxidised organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic/inorganic matrices separated the oxidised organic aerosol into a more oxidised organic aerosol (MOOA), and a less oxidised organic aerosol (LOOA). The MOOA component is associated with inorganics species and had higher O : C ratios than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a PTR-MS, such as isoprene and its oxidation products (methylvinylketone (MVK), methacroleine (MACR), and isoprene hydroxyhydroperoxides (ISOPOOH)). Despite a significantly high mixing ratio of isoprene (2– ppbV) and oxidation products (0.6 and 1.2 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX) within the aerosol organic mass spectrum (m / z 53 and m / z 82) were very weak, suggesting that isoprene SOA may be formed through a non-IEPOX route here, or with different precursors without clear mass spectral signatures in the C-ToF-AMS. This was corroborated through simulations performed with the Polyphemus model showing that 60 to 80 % of SOA originated from biogenic precursors: about 15 to 32 % isoprene (non-IEPOX) SOA, 10 % sesquiterpenes SOA and 35 to 40 % monoterpenes SOA). A total of 20 to 34 % was attributed to purely anthropogenic precursors (aromatics and intermediate/semi volatile compounds).

Summertime and wintertime atmospheric processes of secondary aerosol in Beijing

2020 ◽  
Author(s):  
Jing Duan ◽  
Rujin Huang ◽  
Chunshui Lin ◽  
Haiyan Ni ◽  
Meng Wang
Keyword(s):  
Aqueous Phase ◽  
Liquid Water ◽  
Fine Particles ◽  
Photochemical Oxidation ◽  
Formation Mechanisms ◽  
Phase Reaction ◽  
Aerosol Formation ◽  
Secondary Aerosol ◽  
Atmospheric Processes ◽  
Sulfate Formation

<p>Secondary aerosol constitutes a large fraction of fine particles in urban air of China. However, its formation mechanisms and atmospheric processes remain largely uncertain despite considerable studies in recent years. To elucidate the seasonal variations of fine particles composition and secondary aerosol formation, an Aerodyne quadrupole aerosol chemical speciation monitor (Q-ACSM) combined with other online instruments were used to characterize the submicron particulate matter (diameter < 1 μm, PM<sub>1</sub>) in Beijing during summer and winter 2015. Our results suggest that the photochemical oxidation was the major pathway for sulfate formation during summer, whereas aqueous-phase reaction became an important process for sulfate formation during winter. High concentration of nitrate (17% of the PM<sub>1</sub> mass) was found during winter explained by enhanced gas-to-particle partitioning at low temperature, while high nitrate concentration (19%) was also observed under the conditions of high relative humidity (RH) during summer likely due to the hydrophilic property of NH<sub>4</sub>NO<sub>3</sub> and hydrolysis of N<sub>2</sub>O<sub>5</sub>. As for SOA formation, photochemical oxidation perhaps played an important role for summertime oxygenated OA (OOA) formation and wintertime less oxidized OOA (LO-OOA) formation. The wintertime more oxidized OOA (MO-OOA) showed a good correlation with aerosol liquid water content (ALWC), indicating more important contribution of aqueous-phase processing than photochemical production to MO-OOA. Meanwhile, the dependence of LO-OOA and the mass ratio of LO-OOA to MO-OOA on atmospheric oxidative tracer (i.e., O<sub>x</sub>) both degraded when RH were greater than 60%, suggesting that RH or aerosol liquid water may also affect the LO-OOA formation.</p>

scholarly journals Sources and processes that control the submicron organic aerosol composition in an urban Mediterranean environment (Athens): a high temporal-resolution chemical composition measurement study

2019 ◽  
Vol 19 (2) ◽  
pp. 901-919 ◽  
Author(s):  
Iasonas Stavroulas ◽  
Aikaterini Bougiatioti ◽  
Georgios Grivas ◽  
Despina Paraskevopoulou ◽  
Maria Tsagkaraki ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Biomass Burning ◽  
Volatile Component ◽  
Organic Aerosol ◽  
High Temporal Resolution ◽  
Organic Fraction ◽  
Aerosol Composition ◽  
Secondary Aerosol ◽  
Submicron Aerosol ◽  
Combustion Sources

Abstract. Submicron aerosol chemical composition was studied during a year-long period (26 July 2016–31 July 2017) and two wintertime intensive campaigns (18 December 2013–21 February 2014 and 23 December 2015–17 February 2016), at a central site in Athens, Greece, using an Aerosol Chemical Speciation Monitor (ACSM). Concurrent measurements included a particle-into-liquid sampler (PILS-IC), a scanning mobility particle sizer (SMPS), an AE-33 Aethalometer, and ion chromatography analysis on 24 or 12 h filter samples. The aim of the study was to characterize the seasonal variability of the main submicron aerosol constituents and decipher the sources of organic aerosol (OA). Organics were found to contribute almost half of the submicron mass, with 30 min resolution concentrations during wintertime reaching up to 200 µg m−3. During winter (all three campaigns combined), primary sources contributed about 33 % of the organic fraction, and comprised biomass burning (10 %), fossil fuel combustion (13 %), and cooking (10 %), while the remaining 67 % was attributed to secondary aerosol. The semi-volatile component of the oxidized organic aerosol (SV-OOA; 22 %) was found to be clearly linked to combustion sources, in particular biomass burning; part of the very oxidized, low-volatility component (LV-OOA; 44 %) could also be attributed to the oxidation of emissions from these primary combustion sources. These results, based on the combined contribution of biomass burning organic aerosol (BBOA) and SV-OOA, indicate the importance of increased biomass burning in the urban environment of Athens as a result of the economic recession. During summer, when concentrations of fine aerosols are considerably lower, more than 80 % of the organic fraction is attributed to secondary aerosol (SV-OOA 31 % and LV-OOA 53 %). In contrast to winter, SV-OOA appears to result from a well-mixed type of aerosol that is linked to fast photochemical processes and the oxidation of primary traffic and biogenic emissions. Finally, LV-OOA presents a more regional character in summer, owing to the oxidation of OA over the period of a few days.

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