As of January 4 of this year, it now costs $250 to sit and smoke a cigarette on Boston Common. Make that $350 for a joint, including the $100 fine for marijuana possession. Smoking or vaporizing anything in any city park now carries a $250 fine. Supposedly this is a necessary measure to protect innocent parkgoers from the dangers of secondhand smoke in the open air, not to mention the dangers of secondhand vapor in the open air. The measure has already been criticized in a letter to the Globe that described it as a tax on the homeless, who can’t choose to smoke in a private space. It remains to be seen how evenhanded–or how possible–enforcement will be.