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Lagrangian particle dispersion models (LPDMs) in backward mode arewidely used to quantify the impact of transboundary pollution ondownwind sites. Most LPDM applications count particles witha technique that introduces a so-called footprint layer (FL) withconstant height, in which passing air tracer particles are assumedto be affected by surface emissions. The mixing layer dynamics arerepresented by the underlying meteorological model. This particlecounting technique implicitly assumes that the atmosphere iswell mixed in the FL. We have performed backward trajectorysimulations with the FLEXPART model starting at Cyprus to calculatethe sensitivity to emissions of upwind pollution sources. Theemission sensitivity is used to quantify source contributions at thereceptor and support the interpretation of ground measurementscarried out during the CYPHEX campaign in July 2014. Here we analysethe effects of different constant and dynamic FL heightassumptions. The results show that calculations with FL heights of100 and 300 m yield similar but still discernible results. Comparison of calculations with FL heights constant at 300 m and dynamically following the planetary boundary layer (PBL) height exhibits systematic differences, with daytime and night-time sensitivity differences compensating for each other. The differences at daytime when a well-mixed PBL can be assumed indicate that residual inaccuracies in the representation of the mixing layer dynamics inthe trajectories may introduce errors in the impact assessment ondownwind sites. Emissions from vegetation fires are mixed up bypyrogenic convection which is not represented in FLEXPART. Neglectingthis convection may lead to severe over- or underestimations of thedownwind smoke concentrations. Introducing an extreme fire sourcefrom a different year in our study period and using fire-observation-based plume heights as reference, we find an overestimation of more than 60  % by the constant FL height assumptions used for surface emissions. Assuming a FL that follows the PBL may reproduce the peak of the smoke plume passing throughbut erroneously elevates the background for shallow stable PBLheights. It might thus be a reasonable assumption for open biomassburning emissions wherever observation-based injection heights arenot available

Assumptions about footprint layer heights influence the quantification of emission sources: a case study for Cyprus