Assessment of parameters describing representativeness of air quality in-situ measurement sites

Abstract

The atmospheric layer closest to the ground is strongly influenced by variable surface fluxes (emissions, surface deposition) and can therefore be very heterogeneous. In order to perform air quality measurements that are representative of a larger domain or a certain degree of pollution, observatories are placed away from population centres or within areas of specific population density. Sites are often categorised based on subjective criteria that are not uniformly applied by the atmospheric community within different administrative domains yielding an inconsistent global air quality picture. A novel approach for the assessment of parameters reflecting site representativeness is presented here, taking emissions, deposition and transport towards 34 sites covering Western and Central Europe into account. These parameters are directly inter-comparable among the sites and can be used to select sites that are, on average, more or less suitable for data assimilation and comparison with satellite and model data. Advection towards these sites was simulated by backward Lagrangian Particle Dispersion Modelling (LPDM) to determine the sites' average catchment areas for the year 2005 and advection times of 12, 24 and 48 h. Only variations caused by emissions and transport during these periods were considered assuming that these dominate the short-term variability of most but especially short lived trace gases. The derived parameters describing representativeness were compared between sites and a novel, uniform and observation-independent categorisation of the sites based on a clustering approach was established. Six groups of European background sites were identified ranging from <i>generally remote</i> to more polluted <i>agglomeration</i> sites. These six categories explained 50 to 80% of the inter-site variability of median mixing ratios and their standard deviation for NO<sub>2</sub> and O<sub>3</sub>, while differences between group means of the longer-lived trace gas CO were insignificant. The derived annual catchment areas strongly depended on the applied LPDM and input wind fields, the catchment settings and the year of analysis. Nevertheless, the parameters describing representativeness showed considerably less variability than the catchment geometry, supporting the applicability of the derived station categorisation

General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales

“…An integration of measurements over such an extended network leads to simplifications in particular related to station representativeness. Overall, the most suited categorization of sites related to particle variability appears to be a mix between the categorization by Henne et al (2010) and the more classical geographical classification . The concept of catchment area (the area in which the surface fluxes are creating detectable and significant signals at the site) seems to apply well to a number of sites located in the plains of Central and Western Europe (KPO, OBK, CBW, JRC, MPZ for example).…”

“…These stations have been obviously placed at those sites to provide a more regional view of aerosol properties. There, the local catchment area, as defined by Henne et al (2010), has in fact, much less influence on the variability of the aerosol parameters. The driving factors explaining observed changes are related to both long-range transport (air mass origin) and station altitude.…”

“…This evolution can only be followed during airborne campaigns, which are limited in time and space. The regional context can be addressed by indicators similar to those listed by Henne et al (2010).…”

Atmospheric ice nucleators active ≥ −12 °C can be quantified on PM<sub>10</sub> filters

“…JFJ is mostly situated in the free troposphere and considered a remote measurement site. However, it intermittently receives air masses influenced by the European atmospheric boundary layer, brought up to the site by either frontal passages, foehn situations or thermally-induced flow systems (Zellweger et al, 2003;Henne et al, 2010).…”

Assessing the regional surface influence through Backward Lagrangian Dispersion Models for aircraft CO<sub>2</sub> vertical profiles observations in NE Spain

“…Surface fluxes in the RPSI area are expected to create a detectable change in the measured CO 2 concentration at the receptor site. The RPSI concept is similar to the "catchment area" exposed in Henne et al (2010). Taking the maximum and the minimum surface fluxes values (diurnal and 3-hourly values) from the European assimilation system CarbonTracker for 2006 (Peters et al, , 2009, the time required to detect a change in the measured concentration of 1ppmv (4σ the noise of the instrument used in this study) is ∼500 s. The grid cells with R t greater than 500 s are expected to be those which potentially contribute the most to the variability at the measurements sites.…”