An internal mix air atomizing spray nozzle assembly having a nozzle body that defines a central liquid flow passage, an air guide for directing pressurized air within the nozzle, and a downstream air cap having an upstanding impingement post and a plurality of discharge orifice defining passages about the impingement post. The air cap passages each are oriented at compound angle with respect to a central axis for discharging atomized liquid flow streams in a conical spray pattern swirling in a predetermined rotative direction about the central axis, and the impingement post has a diffuser cap that defines a flat central liquid impingement surface and an outer annular array of non radial diffuser veins for directing the atomized liquid and pressurized air in a swirling fashion about the impingement post in the same relative direction as the swirling spray pattern from the air cap passages.