Lagrangian particle dispersion models (LPDMs) in backward mode are
widely used to quantify the impact of transboundary pollution on
downwind sites. Most LPDM applications count particles with
a technique that introduces a so-called footprint layer (FL) with
constant height, in which passing air tracer particles are assumed
to be affected by surface emissions. The mixing layer dynamics are
represented by the underlying meteorological model. This particle
counting technique implicitly assumes that the atmosphere is
well mixed in the FL. We have performed backward trajectory
simulations with the FLEXPART model starting at Cyprus to calculate
the sensitivity to emissions of upwind pollution sources. The
emission sensitivity is used to quantify source contributions at the
receptor and support the interpretation of ground measurements
carried out during the CYPHEX campaign in July 2014. Here we analyse
the effects of different constant and dynamic FL height
assumptions. The results show that calculations with FL heights of
Transport processes in the atmosphere co-determine local air
composition as trace substances can be transported over thousands
of kilometres by the wind
Trajectory models are one of the main tools for analysing the transport pathways of air pollution from their source regions to a measurement site. These models track the path of an air tracer
particle forward or backward in time in order to establish
relationships between the pollution source and the receptor site
The methodology in LPDMs is based on the assumption that any
regions passed by the trajectories can potentially affect the
receptor site. Since emissions from surface sources are
distributed over a vertical layer adjacent to the ground, only
trajectories passing this so-called footprint layer (FL)
Most applications of source quantification with LPDMs in backward
mode assume constant FL heights of
The aim of this study is to quantify the impact of varying FL
heights on the results of source quantification. We use backward
simulations with the LPDM model FLEXPART that were carried out
during the CYPHEX ground campaign in July 2014 to study the source
areas and transport routes of air pollution at Cyprus. Results from this
detailed analysis of transport routes were used in the study of
We have performed backward simulations with the LPDM model
FLEXPART 9.2
The positions of the tracer particles are calculated on a 3-D grid
with a horizontal resolution of 0.2
The output of FLEXPART is a 4-D function of emission sensitivity
In addition to the described standard output, FLEXPART provides the
PBL height for all individual tracer particles at their
geographical position at each time step. The PBL heights are
calculated by the model according to
Since we are interested in area sources at ground level, the
emission sensitivity for volume sources in
Eq. (
Subdivision of the difference in emission sensitivity
The first method uses the assumption of a constant FL height. Here, we choose a layer of The second method is new in this study and uses a FL height The third method is specifically developed for the application on fire emissions. It is based on the plume rise model (PRM) injection heights from the Global Fire Assimilation System (GFAS)
To analyse the impact of FL height variations,
we calculate the emission sensitivity for constant and dynamic FL
heights and analyse their differences. The absolute difference is
described by
To analyse the changes in emission sensitivity that are introduced
by varying FL heights, Eq. (
Following this case-by-case analysis, the overall difference in
emission sensitivity, spatially and/or temporally integrated, is
the result of the following four characteristic differences:
The fields of emission sensitivity
Spatial integration finally results in the total mass mixing ratio
that is transported to the receptor. It represents the
In this study, we use the Global Fire Assimilation System GFAS1.2
However, in the period of the campaign, fire intensity was too weak
to influence the
First we analyse the impact of FL height
variations in a case study of the 19 July 2014,
00:00 UTC (03:00 LT). This case study describes the transport of
air masses from southeastern Europe to Cyprus on a continental
transport route during the five previous days. For transport
routes along land masses, we can expect a pronounced diurnal cycle
in PBL heights with peaks during daytime up to 2–3
Frequency distribution of PBL heights occurring during the 5-day backward simulation from 19 July 2014, 00:00 UTC (03:00 LT).
As a starting point, we analyse the emission sensitivity for the
constant
In this example, the spatial distribution exhibits a pattern of
positive and negative values. Thus, the application of the
constant
Temporal changes in the overall difference in emission sensitivity
Therefore, we explore the impact of temporal variations in FL
height respective PBL heights and spatially integrate the emission
sensitivity. Then, it only depends on time and its evolution can
be represented in a simple time series over the simulation period.
For a comparison, this time series is calculated for the emission
sensitivity of the constant
To account for the local and temporal variability in more detail,
the overall difference is subdivided in contributions of the
individual effects following Eq. (
By merging these four separated effects, a margin between
The analysed case study is only representative of a specific
synoptic situation. We extend the analysis on the whole available
set of 216 cases studies to account for a variety of synoptic
situations. Typically in summertime, a dominant pattern of
northerly winds is expected in the eastern Mediterranean that
drives the transport of continental air masses from eastern Europe.
However, four distinct periods of west and southwesterly flow in
the eastern Mediterranean were observed in July 2014 that cut off
the expected continental transport routes and brought maritime air
to Cyprus
Box-and-whisker plot for the overall differences in emission sensitivity and the contributions of the different effects in reference to the
To compare different case studies, the procedure of the previous
section, following Eq. (
At first sight, it is obvious that negative differences dominate
with mean values around
Mean values and calculated intervals in per cent for the interquartile range and the 2nd to 98th
percentile as shown in the box-and-whisker plot in Fig.
Total
The detailed analysis of all case studies shows that the use
of a variable FL height within the PBL lowers the emission
impact. During summer at Cyprus, the
When PBL heights fall below
Overall, a negative difference in emission sensitivity is
observed. However, 21 positive outliers with a maximum of
In this section, we analyse the impact of specific emission sources. This impact depends on the emission sensitivity and, additionally, on the local distribution and source strength of pollutants.
We calculate the
The spatial distribution of
For these extreme fires the emission heights significantly exceed
the
For a more detailed analysis we spatially integrate the 3-D field
of
At the peak of
Overall, the
In this study, we analysed the impact of different footprint layer
(FL) height assumptions on the quantification of source
contributions with the Lagrangian particle dispersion model (LPDM)
FLEXPART. The FL height defines the vertical layer adjacent to
the ground in which surface emissions are present and assumed to
affect passing air tracer particles. Consequently, the assumed FL
height determines a dilution of any emitted pollutants and the
number of trajectories exposed to the emissions. Both effects
counteract each other: a growth in FL height leads to a stronger
dilution that is coupled to a stronger impact. In a well-mixed
layer with a homogeneous trajectory distribution and thus
a constant tracer particle density, the counteracting effects are
balanced. The dilution is in addition to the vertical mixing
represented in the underlying meteorological model. It thus has
the potential to compensate for any underestimation of vertical
mixing in the trajectories. Then, FL height variations are
buffered and the results of source quantification are independent
of the FL height.
For the comparison, we used in total 216 FLEXPART simulations that
were carried out during the CYPHEX campaign at Cyprus in July
2014. The case studies provide different meteorological conditions
and transport patterns. For convective conditions over day, the
PBL heights significantly exceed the
For an application example, we arrange emission data of a strong
fire event in Greece in August 2007 with our case study which
simulates long residence times in that fire region. In
From this study it can be concluded that the assumption of the FL
height in LPDMs is of vital importance for the relationship
between emission sources and the atmospheric composition at the
receptor site. While small variations in FL height up to
All
data are archived at the Max Planck Institute for Chemistry in
Mainz and are available on the public ftp server in subdirectory
Hueser/Flexpart4CYPHEX: server:
IH conducted the FLEXPART simulations, data analysis and wrote the manuscript.
HH co-organised the CYPHEX campaign and supervised IH. AH extracted data from ECMWF and co-wrote the manuscript. JK provided
The authors declare that they have no conflict of interest.
This study was conducted for the CYPHEX campaign and was funded by the Max Planck Society (MPG). The work on fire emissions was funded by the European Union through the CAMS_44 contract with ECMWF. Furthermore, we thank ECMWF for permitting access to the operational archive. The article processing charges for this open-access publication were covered by the Max Planck Society. Edited by: Silvia Kloster Reviewed by: two anonymous referees