Asymmetric mid-plane gas in ALMA images of HD 100546

In this paper we present new ALMA observations towards the protoplanet-hosting transitional disc of the Herbig Ae/Be star HD 100546. This includes a resolved 1.3 mm continuum, (CO)-C-13, and the first detection of (CO)-O-18 in this disc, which displays azimuthal asymmetry in regions spatially coincident with structures previously identified in HST images related to spiral arms. The lower limit on the mass of the dust disc is calculated to be 9.6 x 10(-4)M(circle dot). A firm lower limit on the total gas mass calculated from optically thin, mid-plane-tracing (CO)-O-18 (2 - 1) emission is 0.018 M-circle dot assuming interstellar medium abundances. These mass estimates provide an estimate of gas-to-dust ratio in the disc of 19; the ratio will increase if (CO)-O-18 is relatively underabundant in the disc compared to CO and H-2. Through deprojection and azimuthal averaging of the image plane we detect 1.3 mm continuum emission out to 290 +/- 10 au, (CO)-C-13 to 390 +/- 10 au, and (CO)-O-18 to 300 +/- 10 au. We measure a radially increasing millimetre spectral index between wavelengths of 867 mu m and 1.3 mm, which shows that grain sizes increase towards the star, with solid particles growing to centimetre scales in the inner disc.