Introducing the novel concept of cumulative concentration roses for studying the transport of ultrafine particles from an airport to adjacent residential areas

Abstract. Airports are often surrounded by urban residential areas, which is both motivation and challenge for studying their potential impact on local air quality. Airports are a relevant source for ultrafine particles (UFP), which can pose a risk to human health due to their small size (particle diameter D_p ≤ 100 nm). However, in urban environments UFP originate from a multitude of biogenic and anthropogenic sources. Here, we investigate UFP in close proximity to an airport to disentangle its impact on local air quality from other urban sources.

We present observations and analysis of airborne UFP concentrations and size distributions determined at two sites in close proximity to Munich Airport. Therefore, two novel measurement stations were established north and south to the airport, but were neither situated on the axis of prevailing wind directions nor impacted by fly overs. This set-up allowed us to explore a mainly advection driven distribution of UFP into the most populated adjacent residential areas. The observation period covered a full year from August 2021 to July 2022. We analysed the dataset in three steps: (1) First, we derived UFP concentration roses using the wind data as reported in 10 m height at the airport to represent the local wind field. An increase in particle number concentrations and a shift of the modal maximum towards smaller mobility diameters became evident for wind directions including those approaching from the airport. During the airport’s operation hours at daytime median particle number concentrations were 2.2 and 1.6 fold compared to nighttime at the northern and southern station. However, our data had a high variability and the direction-based analysis was uncertain due to other potential UFP sources in the surroundings and the assumption of a homogeneous, local wind field. (2) Next, we derived concentration roses employing the airflow observations from the two measuring stations at 5.3 m height. While the annual concentration rose in principle yielded the same conclusions as the first analysis step, a significant seasonal and diurnal variability of UFP and wind became evident. The influencing factors were likely other urban local UFP sources, an increased surface roughness due to green vegetation, and the atmospheric boundary layer development. (3) In order to assess the possible advection of UFP from the direction of Munich Airport relative to all other directions over the course of the year, we calculated cumulative concentration roses with both local and site-scale wind data. Under the assumption of a homogeneous local wind field, the fraction of all UFP sampled in airflows approaching from the airport’s direction was 21 % (N322) and 40 % (S229). Considering a local background, the range of UFP advection from Munich Airport to the adjacent residential areas was up to 10 % in the North and 14 % in the South. It has to be noted, that these values highlight the relative magnitude of maximum impact of the airport on local air quality, as they do not separate from other UFP sources between airport and measuring sites. Additionally, they integrate over a time period, for which the airport did not reach its full capacity compared to pre-COVID-19 times.