Dust and Volatiles in the Disintegrating Comet C/2019 Y4 (ATLAS)

C/2019 Y4 (ATLAS) is an Oort cloud comet with an orbital period of$\sim$5895$\,{\rm yr}$. Starting in March 2020, its nucleus underwentdisintegration. In order to investigate the gas and dust properties of C/2019Y4 (ATLAS) during its disintegration, we obtained long-slit spectra at3600--8700$\,{\rm\mathring{A}}$ and $BVRI$ multi-band images with the Xinglong2.16-Meter Telescope in April 2020. Our observations revealed that C/2019 Y4(ATLAS) exhibited strong emission bands of CN, C$_2$, C$_3$, and NH$_2$ whichare superimposed on a dust scattering continuum, typical of cometary spectra inthe optical. The production rates of CN, C$_2$, and C$_3$ derived using theHaser model and the corresponding C$_2$/CN and C$_3$/CN ratios suggest thatC/2019 Y4 (ATLAS) is a ``typical'' Oort cloud comet under the A'Hearnclassification, although it appears less dusty as revealed by the $Af\rho$quantities. Its dust-scattering reflectivity is slightly red, with a gradientof $\sim$5% per $10^3\,{\rm\mathring{A}}$. We model the reflectivity gradientin terms of porous dust and find that the red color is accounted for by porousdust.