  We report on high-resolution and spatially-resolved spectra of Io in the 4.0
{\mu}m region, recorded with the VLT/CRIRES instrument in 2008 and 2010, which
provide the first detection of the {\nu}1 + {\nu}3 band of SO2 in Io's
atmosphere. Data are analyzed to constrain the latitudinal, longitudinal, and
diurnal distribution of Io's SO2 atmosphere as well as its characteristic
temperature. equatorial SO2 column densities clearly show longitudinal
asymmetry, but with a maximum of around 1.5e17 cm-2 at central meridian
longitude L = 200-220 and a minimum of around 3e16 cm-2 at L = 285-300, the
longitudinal pattern somewhat differs from earlier inferences from Ly {\alpha}
and thermal IR measurements. Within the accuracy of the measurements, no
evolution of the atmospheric density from mid-2008 to mid-2010 can be
distinguished. The decrease of the SO2 column density towards high latitude is
apparent, and the typical latitudinal extent of the atmosphere found to be (+-)
40{\deg} at half-maximum. The data show moderate diurnal variations of the
equatorial atmosphere, which is evidence for a partially sublimation-supported
atmospheric component. Compared to local noon, factor of 2 lower densities are
observed around 40{\deg} before and 80{\deg} after noon. Best-fit gas
temperatures range from 150 to 220 K, with a weighted mean value of 170 (+-) 20
K, which should represent the column-weighted mean kinetic temperature of Io's
atmosphere. Finally, although the data include clear thermal emission due to
Pillan (in outburst in July 2008) and Loki, no detectable enhancements in the
SO2 atmosphere above these volcanic regions are found, with an upper limit of
4e16 cm-2 at Pillan and 1e17 cm-2 at Loki.
