Section 24-142. Emission of air contaminant; standard smoke chart

(a) No
  person shall cause or permit the emission of air contaminant of:  (1)  A
  density  which appears as dark or darker than number two on the standard
  smoke chart or of an opacity which obscures vision to a degree equal  to
  or greater than smoke of number two density on the standard smoke chart;
  or
    (2)  A  density which appears as dark or darker than number one on the
  standard smoke chart, but less than number two on said chart, or of such
  opacity as to obscure vision to a degree equal to or greater than  smoke
  of  number one density on the standard smoke chart, but less than number
  two on said chart, if such an emission continues  for  longer  than  two
  minutes in the aggregate in any sixty minute period.
    (b)  The density or opacity of an air contaminant shall be measured at
  the point of its emission, except:
    (1) When the point of emission cannot be readily observed, it  may  be
  measured  at  an  observable  point  on  the  plume nearest the point of
  emission; or
    (2) In the case of air contaminant emitted from a  source  outside  of
  New   York,   it   shall   be  measured  after  the  plume  crosses  the
  jurisdictional boundary of New York city.