text: """ calculating the particle area
  """
  
  import numpy as np
  from particula.util.input_handling import in_radius, in_scalar
  
  
  def area(
      radius=None,
      area_factor=1,
  ):
      """ Returns particle's surface area: 4 pi r^2 .
  
          Parameters:
              radius       (float) [m]
              area_factor  (float) [ ]      (default: 1)
  
          Returns:
                           (float) [m^2]
      """
  
      radius = in_radius(radius)
      area_factor = in_scalar(area_factor)
  
      return 4*np.pi*(radius**2)*area_factor
  