  We present a method to correct the chromatic and airmass dependent extinction
for N-band spectra taken with the TIMMI2 instrument at the ESO / La Silla
observatory. Usually, the target and calibrator star have to be observed at
similar airmass in order to obtain reliable spectrophotometric fluxes. Our
method allows to correct the atmospheric extinction and substantially improves
the spectrophotometric flux calibration, when the standard stars were observed
at a very different airmass than the targets. Hundreds of standard star
measurements in several passbands (N1, N8.9, N10.4, N11.9) were used to derive
mid-IR extinction coefficients. We demonstrate that applying our correction of
the differential extinction to test data results in a spectrophotometric
accuracy up to 2% within the literature flux.
