Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer

Particulate organic nitrate (pON) can be a major part of secondary organic aerosol (SOA) and is commonly quantified by indirect means from aerosol mass spectrometer (AMS) data. However, pON quantification remains challenging. Here, we set out to quantify and characterize pON in the boreal forest, through direct field observations at Station for Measuring Ecosystem Atmosphere Relationships (SMEAR) II in Hyytia''la'', Finland, and targeted single precursor laboratory studies. We utilized a long time-of-flight AMS (LToF-AMS) for aerosol chemical characterization, with a particular focus to identify CxHyOzN+ ("CHON+") fragments. We estimate that during springtime at SMEAR II, pON (including both the organic and nitrate part) accounts for similar to 10% of the particle mass concentration (calculated by the NO+/NO2+ method) and originates mainly from the NO3 radical oxidation of biogenic volatile organic compounds. The majority of the background nitrate aerosol measured is organic. The CHON+ fragment analysis was largely unsuccessful at SMEAR II, mainly due to low concentrations of the few detected fragments. However, our findings may be useful at other sites as we identified 80 unique CHON+ fragments from the laboratory measurements of SOA formed from NO3 radical oxidation of three pON precursors (beta-pinene, limonene, and guaiacol). Finally, we noted a significant effect on ion identification during the LToF-AMS high-resolution data processing, resulting in too many ions being fit, depending on whether tungsten ions (W+) were used in the peak width determination. Although this phenomenon may be instrument-specific, we encourage all (LTOF-) AMS users to investigate this effect on their instrument to reduce the possibility of incorrect identifications.Peer reviewe

The second ACTRIS inter-comparison (2016) for Aerosol Chemical Speciation Monitors (ACSM) : Calibration protocols and instrument performance evaluations

AbstractThis work describes results obtained from the 2016 Aerosol Chemical Speciation Monitor (ACSM) intercomparison exercise performed at the Aerosol Chemical Monitor Calibration Center (ACMCC, France). Fifteen quadrupole ACSMs (Q_ACSM) from the European Research Infrastructure for the observation of Aerosols, Clouds and Trace gases (ACTRIS) network were calibrated using a new procedure that acquires calibration data under the same operating conditions as those used during sampling and hence gets information representative of instrument performance. The new calibration procedure notably resulted in a decrease in the spread of the measured sulfate mass concentrations, improving the reproducibility of inorganic species measurements between ACSMs as well as the consistency with co-located independent instruments. Tested calibration procedures also allowed for the investigation of artifacts in individual instruments, such as the overestimation of m/z 44 from organic aerosol. This effect was quantified by the m/z (mass-to-charge) 44 to nitrate ratio measured during ammonium nitrate calibrations, with values ranging from 0.03 to 0.26, showing that it can be significant for some instruments. The fragmentation table correction previously proposed to account for this artifact was applied to the measurements acquired during this study. For some instruments (those with high artifacts), this fragmentation table adjustment led to an ?overcorrection? of the f44 (m/z 44/Org) signal. This correction based on measurements made with pure NH4NO3, assumes that the magnitude of the artifact is independent of chemical composition. Using data acquired at different NH4NO3 mixing ratios (from solutions of NH4NO3 and (NH4)2SO4) we observe that the magnitude of the artifact varies as a function of composition. Here we applied an updated correction, dependent on the ambient NO3 mass fraction, which resulted in an improved agreement in organic signal among instruments. This work illustrates the benefits of integrating new calibration procedures and artifact corrections, but also highlights the benefits of these intercomparison exercises to continue to improve our knowledge of how these instruments operate, and assist us in interpreting atmospheric chemistry.Peer reviewe

ACTRIS ACSM intercomparison – Part 2: Intercomparison of ME-2 organic source apportionment results from 15 individual, co-located aerosol mass spectrometers

Chemically resolved atmospheric aerosol data sets from the largest intercomparison of the Aerodyne aerosol chemical speciation monitors (ACSMs) performed to date were collected at the French atmospheric supersite SIRTA. In total 13 quadrupole ACSMs (Q-ACSM) from the European ACTRIS ACSM network, one time-of-flight ACSM (ToF-ACSM), and one high-resolution ToF aerosol mass spectrometer (AMS) were operated in parallel for about 3 weeks in November and December~2013. Part 1 of this study reports on the accuracy and precision of the instruments for all the measured species. In this work we report on the intercomparison of organic components and the results from factor analysis source apportionment by positive matrix factorisation (PMF) utilising the multilinear engine 2 (ME-2). Except for the organic contribution of mass-to-charge ratio m/z 44 to the total organics (f44), which varied by factors between 0.6 and 1.3 compared to the mean, the peaks in the organic mass spectra were similar among instruments. The m/z 44 differences in the spectra resulted in a variable f44 in the source profiles extracted by ME-2, but had only a minor influence on the extracted mass contributions of the sources. The presented source apportionment yielded four factors for all 15 instruments: hydrocarbon-like organic aerosol (HOA), cooking-related organic aerosol (COA), biomass burning-related organic aerosol (BBOA) and secondary oxygenated organic aerosol (OOA). ME-2 boundary conditions (profile constraints) were optimised individually by means of correlation to external data in order to achieve equivalent / comparable solutions for all ACSM instruments and the results are discussed together with the investigation of the influence of alternative anchors (reference profiles). A comparison of the ME-2 source apportionment output of all 15 instruments resulted in relative standard deviations (SD) from the mean between 13.7 and 22.7 % of the source's average mass contribution depending on the factors (HOA: 14.3 ± 2.2 %, COA: 15.0 ± 3.4 %, OOA: 41.5 ± 5.7 %, BBOA: 29.3 ± 5.0 %). Factors which tend to be subject to minor factor mixing (in this case COA) have higher relative uncertainties than factors which are recognised more readily like the OOA. Averaged over all factors and instruments the relative first SD from the mean of a source extracted with ME-2 was 17.2 %.JRC.H.2-Air and Climat

ACTRIS ACSM intercomparison – Part 1: Reproducibility of concentration and fragment results from 13 individual Quadrupole Aerosol Chemical Speciation Monitors (Q-ACSM) and consistency with co-located instruments

As part of the European ACTRIS project, the first large Quadrupole Aerosol Chemical Speciation Monitor (Q-ACSM) intercomparison study was conducted in the region of Paris for 3 weeks during the late-fall – early-winter period (November–December 2013). The first week was dedicated to the tuning and calibration of each instrument, whereas the second and third were dedicated to side-by-side comparison in ambient conditions with co-located instruments providing independent information on submicron aerosol optical, physical, and chemical properties. Near real-time measurements of the major chemical species (organic matter, sulfate, nitrate, ammonium, and chloride) in the non-refractory submicron aerosols (NR-PM1) were obtained here from 13 Q-ACSM. The results show that these instruments can produce highly comparable and robust measurements of the NR-PM1 total mass and its major components. Taking the median of the 13 Q-ACSM as a reference for this study, strong correlations (r2 > 0.9) were observed systematically for each individual Q-ACSM across all chemical families except for chloride for which three Q-ACSMs showing weak correlations partly due to the very low concentrations during the study. Reproducibility expanded uncertainties of Q-ACSM concentration measurements were determined using appropriate methodologies defined by the International Standard Organization (ISO 17025, 1999) and were found to be 9, 15, 19, 28, and 36 % for NR-PM1, nitrate, organic matter, sulfate, and ammonium, respectively. However, discrepancies were observed in the relative concentrations of the constituent mass fragments for each chemical component. In particular, significant differences were observed for the organic fragment at mass-to-charge ratio 44, which is a key parameter describing the oxidation state of organic aerosol. Following this first major intercomparison exercise of a large number of Q-ACSMs, detailed intercomparison results are presented, along with a discussion of some recommendations about best calibration practices, standardized data processing, and data treatment.JRC.H.2-Air and Climat

An Aggregation-Based Routing Protocol for Structured Peer to Peer Overlay Networks

International audienceStructured peer-to-peer (P2P) overlay networks provide a scalable object location and routing substrate for large scale distributed applications. However, due to the great number of nodes of such systems, message complexity of their routing protocol may considerably increase network traffic and average node hops of a message. This paper presents a novel Pastry-based routing protocol for structured P2P systems, which is specially suitable for handling per node multiple message routing requests. Our protocol exploits message aggregation and implements a multi-slice mechanism which multiplexes the sending of aggregated messages. Experimental results on top of PeerSim show that our protocol can reduce the average number of node hops messages, and thus, the global traffic and load of the network

ECHO: Efficient Complex Query over DHT overlays

International audienceIn this article we propose ECHO, a novel and lightweight solution that efficiently supports range queries over a ring-like Distributed Hash Table (DHT) structure. By implementing a tree-based index structure and an effective query routing strategy, ECHO provides low-latency and low-overhead query searches by exploiting the Tabu Search principle. Load balancing is also improved reducing the traditional bottleneck problems arising in upper level nodes of tree-based index structures such as PHT. Furthermore, ECHO copes with DHT churn problems as its index exploits logical information as opposed to static reference cache approaches or replication techniques.The performance evaluation results obtained using PeerSim simulator show that ECHO achieves efficient performance compared other solutions such as the PHT strategy and its optimized version which includes a query cache

TPT-C: A Heuristic-Based Cache to Improve Range Queries over DHTs

Distributed Hash Tables (DHTs) provide the substrate to build large scale distributed applications over Peer-to-Peer networks. A major limitation of DHTs is that they only support exact-match queries. In order to offer range queries over a DHT it is necessary to build additional indexing structures. Prefix-based indexes, such as Prefix Hash Tree (PHT), are interesting approaches for building distributed indexes on top of DHTs. Nevertheless, the lookup operation of these indexes usually generates a high amount of unnecessary traffic overhead which degrades system performance by increasing response time. In this paper, we propose a novel distributed cache system called Tabu Prefix Table (TPT-C), aiming at improving the performance of the Prefix-trees. We have implemented our solution over PHT, and the results confirm that our approach outperforms traditional existing cache solutions for prefix-tree structures.Les tables de hachage distribuées (DHT pour Distributed Hash Table) sont un support efficace pour développer des applications à grande échelle sur des réseaux pair-à-pair. La principale limitation des DHT est leur faible pouvoir d'expression. Elles ne supportent que des requêtes pour localiser une information à partir de son identifiant exact. Pour offrir des requêtes par intervalle, il est nécessaire d'ajouter aux DHT des structures d'indexation. Les index par préfix tel que l'arbre de hachage par préfix (PTH pour Prefix Hash Tree) sont une approche intéressante pour construire des index sur des DHT. Cependant, les opérations sur ces index génèrent généralement un trafic important qui dégrade les performances du système et augmente les temps de réponse des requêtes. Dans cet article, nous proposons un nouveau cache distribué appelé Tabu Prefix Table (TPT-C) pour améliorer les performances des arbres de préfix. Nous avons développé notre solution sur PHT et les résultats confirment que notre approche améliore les solutions de cache proposées précédemment sur PHT

An Aggregation-Based Routing Protocol for Structured Peer to Peer Overlay Networks

International audienceStructured peer-to-peer (P2P) overlay networks provide a scalable object location and routing substrate for large scale distributed applications. However, due to the great number of nodes of such systems, message complexity of their routing protocol may considerably increase network traffic and average node hops of a message. This paper presents a novel Pastry-based routing protocol for structured P2P systems, which is specially suitable for handling per node multiple message routing requests. Our protocol exploits message aggregation and implements a multi-slice mechanism which multiplexes the sending of aggregated messages. Experimental results on top of PeerSim show that our protocol can reduce the average number of node hops messages, and thus, the global traffic and load of the network

A Tabu Based Cache to Improve Latency and Load Balancing on Prefix Trees

International audienceDistributed Hash Tables (DHTs) provide the substrate to build large scale distributed applications over Peer-to-Peer networks. A major limitation of DHTs is that they only support exact-match queries. In order to offer range queries over a DHT it is necessary to build additional indexing structures. Prefix-based indexes, such as Prefix Hash Tree (PHT), are interesting approaches for building distributed indexes on top of DHTs. Nevertheless, the lookup operation of these indexes usually generates a high amount of unnecessary traffic overhead which degrades system performance by increasing response time. In this paper, we propose a novel distributed cache system called Tabu Prefix Table Cache (TPT-C), aiming at improving the performance of the Prefix-trees. We have implemented our solution over PHT, and the results confirm that our searching approach reduces up to a 70% the search latency and traffic overhead

An Aggregation-Based Routing Protocol for Structured Peer to Peer Overlay Networks

International audienceStructured peer-to-peer (P2P) overlay networks provide a scalable object location and routing substrate for large scale distributed applications. However, due to the great number of nodes of such systems, message complexity of their routing protocol may considerably increase network traffic and average node hops of a message. This paper presents a novel Pastry-based routing protocol for structured P2P systems, which is specially suitable for handling per node multiple message routing requests. Our protocol exploits message aggregation and implements a multi-slice mechanism which multiplexes the sending of aggregated messages. Experimental results on top of PeerSim show that our protocol can reduce the average number of node hops messages, and thus, the global traffic and load of the network