Spatial and seasonal variability of measured anthropogenic non-methane hydrocarbons in urban atmospheres: Implication on emission ratios
Introduction
Volatile organic compounds (VOC) are important chemical constituents of urban air pollution, as precursors of tropospheric ozone (O3) (Seinfeld, 1989) and secondary organic aerosols (SOA) (e.g. Odum et al., 1997; Kroll and Seinfeld, 2008). Several papers have shown the importance of VOC emissions on both ozone and SOA formation through air quality modeling studies (e.g. Zhang et al., 2004, Hodzic et al., 2009, Coll et al., 2010, Kim et al., 2011). Some VOC, such as benzene and 1,3-butadiene, are classified as toxic air pollutants and are subject to regulations (US EPA, 2008; Directive 98/70/CE).
Non-methane Hydrocarbons (NMHC), a subset of VOC, are emitted by both anthropogenic and biogenic sources. While on a global scale VOC emissions are dominated by biogenic sources (Guenther et al., 2006), in most urban areas the anthropogenic emissions are the principal contributor to NMHC sources. NMHC can be separated into several classes including alkanes associated with the incomplete combustion of tailpipe emissions and fuel evaporation from vehicles and gas stations (Watson et al., 2001); aromatics found as components of vehicle exhaust, fuel evaporation and solvent use; alkenes and alkynes mainly associated with combustion processes. Note that lighter alkanes (C2–C4) are components of natural gas and liquefied petroleum gas (LPG) (Harley et al., 1992, Blake and Rowland, 1995, Chen et al., 2001).
The development of emission inventories is a first step in regulating toxic air pollutants and developing strategies for controlling ozone and SOA pollution. They are essential for various applications such as determination of major air pollutant sources, establishment of emission trends over time with the implementation of emission reduction strategies, input for air quality modeling. However, high uncertainties persist with emission inventories due to the diversity and multiplicity of sources (e.g. Carmichael et al., 2003, Ma et al., 2006, Urbanski et al., 2011, Zhao and Wang, 2009, Zhao et al., 2011) and the evaluation of emission inventories still remains an important and critical task. In the last decade, numerous papers have evaluated emission inventories using satellite observations (e.g. Martin et al., 2003, Martin et al., 2006, Lin et al., 2010) or in-situ measurements (e.g. Vautard et al., 2003, Hodzic et al., 2005, Liousse et al., 2010, Gaimoz et al., 2011). Recent studies have determined urban emission ratios (ER) of several NMHC versus inert species such as carbon monoxide or acetylene (C2H2) from ambient concentrations, and successfully used these ambient ratios to evaluate emission database (Baker et al., 2007, Warneke et al., 2007, Gentner et al., 2009, Coll et al., 2010, Russo et al., 2010, Borbon et al., 2013a). These studies point to strong disagreement between emission inventories and observations, especially regarding the dominating sources. Contrary to observations, emission inventories suggest that motor vehicle emissions is no longer the dominating source in northern mid-latitude urban areas (Borbon et al., 2013a, Gaimoz et al., 2011, Niedojadlo et al., 2007) and solvent use related activities could be quite significant. Ambient concentrations used in these studies are typically collected during intensive field projects over short periods e.g. one month in summer, when photochemical pollution is maximum. One pending question is to what extent these ambient emission ratios reflect seasonal changes in emissions as suggested by temporally resolved inventories.
This work analyzes the spatial and seasonal variability of the urban NMHC composition by using observations collected by the French air quality networks between 2001 and 2010 in four French urban areas (Paris megacity, Grenoble, Marseille and Strasbourg). The objective is to (1) compare the NMHC composition between French urban areas, (2) to show how this composition changes with season and (3) how this seasonal change affects the emission ratios. The timescale of interest is the season. Analysis of multi-year trends of NMHC emission ratios which is not the purpose of this paper is described in a companion paper (Borbon et al., 2013b). First, we test the consistency of the urban composition for a wide range of NMHC as suggested by Parrish et al. (1998). In particular, these tests detect the potential influence of nearby sources that could locally affect the NMHC distribution but are not representative of the regional urban NMHC emission mixture. Second, we show the implication of the seasonality of NMHC urban composition on the values of urban emission ratios. Datasets are described in Section 2. In Section 3, the quality of the data is assessed which is a first prerequisite when using routine NMHC observations. Section 4 examines the spatial and seasonal composition of urban NMHC regarding source signature for the four cities investigated. In Section 5, emission ratios are derived from present ambient data. Conclusions are given in Section 6.
Section snippets
Data set
In France, a long-term monitoring program for NMHC, launched by ADEME (Agence de la Maîtrise de l'Energie et de l'Environnement) and the French Ministry of the Environment started in 2001 and was operated by four local French Air Quality Monitoring Network (AASQA). Usually, 38 compounds belonging to the European ozone precursor priority list (Kotzias et al., 1995) are measured: 15 alkanes, 11 alkenes, 11 aromatics and acetylene. In this paper, we analyze hourly measurements of 31 NMHC performed
Consistency tests
In order to insure the quality of AASQA data, the measuring system integrates a systematic quality assurance (QA) and quality control (QC) procedure as described in Badol et al. (2004). This procedure mostly consists of regular checks (every two weeks) of NMHC response coefficients based on the use of certified calibration standards and control charts based on chromatographic retention time.
However, the effort required to perform data quality procedure throughout the duration of the monitoring
Urban NMHC composition
In this section, we examine the spatial and temporal composition of urban NMHC at the four monitoring sites on filtered datasets. Only data between 2002 and 2004 when all stations were still operational (Table 1) are used for this initial comparison. First, the variability of NMHC urban composition is compared between the different sites. Second, the seasonal variability of NMHC urban composition between the summer and winter seasons is discussed. Fig. 3 depicts the percentage (in volume)
NMHC urban emission ratios
The objective of this section is to show the effect of the seasonality of NMHC urban composition on the values of emission ratios. For that purpose, we analyzed emission ratios in both Paris and Strasbourg for the most recent measurement periods: summer 2009 and winter 2010. Note that the composition of NMHC urban emissions is still consistent between Strasbourg and Paris in 2009 and 2010 and shows same seasonal changes as the ones discussed in section 3 and 4. Given the consistency of the
Conclusions
In France, a long-term monitoring program, launched by ADEME and the French Ministry of the Environment and operated by the local French Air Quality Monitoring Network, has been performing measurements of 31 NMHC since 2001. NMHC were collected in four French cities: Paris, Grenoble, Marseille and Strasbourg. We analyzed the spatial and seasonal variability of the composition of anthropogenic NMHC emissions in the four cities and we determined the most recent emission ratios relative to
Acknowledgments
The Department of Chemistry and Environment of Mines Douai participates in the Central Laboratory of Air Quality Survey (LCSQA), which is funded by the French Ministry in charge of the environment. The authors would like to gratefully thank the French networks AIRPARIF, ATMO-PACA, ATMO-RH and ASPA for providing NMHC and meteorological data from monitoring stations. The authors are grateful to Joost de Gouw (NOAA/CIRES, Boulder, CO, USA) for the helpful discussions at the beginning of the work.
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Now at: Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), IPSL, CNRS, UMR 8190, France.