Measurement report: Three years of size-resolved eddy-covariance particle number flux measurements in an urban environment

Abstract

Abstract. Size-resolved particle number fluxes in the size range of 10 nm < particle diameter (Dp) < 200 nm were measured over a 3-year period (April 2017–March 2020) using the eddy-covariance technique at an urban site in Berlin, Germany. The observations indicated the site as a net source of particles with a median total particle number flux of FTNC=0.86 × 108 m−2 s−1. The turbulent surface–atmosphere exchange of particles was clearly dominated by ultrafine particles (Dp < 100 nm) with a share of 96 % of total particle number flux (FUFP=0.83 × 108 m−2 s−1). Annual estimates of median FTNC and FUFP slightly decreased by −9.6 % (−8.9 % for FUFP) from the first to the second observation year and a further −5.9 % (−6.1 % for FUFP) from the second to the third year. The annual variation might be due to different reasons such as the variation of flux footprints in the individual years, a slight reduction of traffic intensity in the third year, or a progressive transition of the vehicle fleet towards a higher share of low-emission standards or electric drive. Size-resolved measurements illustrated events of bidirectional fluxes, i.e. simultaneous emission and deposition fluxes within the size spectrum, which occurred more often in spring, late summer, and autumn than in winter. Multi-year observations of size-resolved particle fluxes proved to be important for a deeper understanding of particle exchange processes with the urban surface and the pronounced influence of traffic at this urban site.

Real world ultrafine particle emission factors for road-traffic derived from multi-year urban flux measurements using eddy covariance

“…The study area was defined as a 2 km radius around the site, since this area contains the 80% contour line of the flux source area in every single of the three observation years as determined by flux source area modelling ( cf. 20 ). For the study area, data of 22 automatic traffic counting stations were available from the Senate Department for the Environment, Urban Mobility, Consumer Protection and Climate Action of the Berlin city administration (red dots in Fig.…”

“…For further details on the gap-filling procedure and the measurement setup, the authors refer to ref. 20 and 24.…”

“…Our analysis did not point to any bias in the method which tended to a systematic under- or overestimation of particle concentrations by the gap-filling. 20 For these PNSDs size-integrated number concentrations were calculated in order to process particle number fluxes, i.e. total particle number concentration (TNC, 10 nm < D p < 200 nm), ultrafine particle number concentration (UFP, 10 nm < D p < 100 nm), nucleation mode particles (NUC, 10 nm < D p < 30 nm), Aitken mode particles (AIT, 30 nm < D p < 100 nm), and accumulation mode particles (ACC, 100 nm < D p < 200 nm).…”

“…The resulting particle number fluxes are subject to the following sign convention: emission fluxes have a positive sign and are directed to the atmosphere, whereas deposition fluxes are negative and directed to the surface. For further details on data handling, quality control, or data assurance the reader is referred to Straaten et al 20 .…”

“…To date, only very few studies are available which report findings from multiple years of eddy covariance particle fluxes and provide size-resolved information on particle EFs. 20 We hypothesise that long-term eddy-covariance observations are a very valuable method to derive particle EFs for a mixed urban fleet covering a large variation of boundary conditions such as driving patterns, turbulent transport and meteorological conditions. We aim to quantify a ‘typical’ mixed-fleet road traffic emission factor which is representative for urban Berlin or other large German cities with similar fleet compositions.…”