droplets evaporate and leave fine particulate matter formed by crystallization of ed  solids. 30  VOC emissions typically result from the leakage from process heat exchangers that service  hydrocarbon process streams as well as from chemical treatment with VOC containing material  added to the circulating water. VOC emissions are expected fro m cooling towers used in  refineries and chemical plants, where the circulating water is used to cool down the process  stream. Thus, VOC emissions from airport cooling towers are uncommon.  Typically PM emissions from airport cooling towers  are PM$_{10 }$ and are the result of the total  dissolved solids (TDS) in the circulating water which are carried out with the water that entrained in the air being discharged from the  cooling tower. PM$_{10 }$ emissions from cooling  towers are calculated using the methodology presented in Chapter 13.4,  Wet Cooling Towers 31  ,  of EPA's AP-42. The AP-42 method assumes that all TDS emitted in “drift” particles (liquid  water entrained in the air stream and carried out of the tower through the induced draft fan stack)  are PM$_{10}$; however, this method is overly conservative for wet cooling towers with medium to  high TDS levels.  The general methodology for calculating  PM$_{10 }$ from wet cooling towers is  expressed by  Equation A5-10  ( Particulate Matter Emissions for Cooling Towers ).  E$_{PM }$= CWFR x TDS/10$^{6 }$x DR/100 x 60 x 8.34  Where:  E$_{PM }$= total particulate matter emissions from a single cooling tower over a given time  period, expressed in pounds per hour.  CWFR = circulating water flow rate, expressed in gallons per minute.  TDS = total dissolved solids concentration, expressed in parts per million (ppm)*.  DR = drift rate, expressed as a percentage of the cooling tower water CWFR.  60 = conversion factor from minutes to hours.  8.34 = pounds of water per gallon of water.  *ppm is defined as one part of the pollutant to one million parts of solution. In other words, one  ppm is equal to 1/10$^{6}$. The equation includes the 1/10$^{6 }$factor (e.g. 12,000 ppm / 10$^{6 }$= 0.012).  Depending on the media, ppm can be on a volume or mass basis.  31  , http://www.epa.gov/ttnchie1/ap42/ch13/final/c13s04.pdf.  30  EPA, AP-42, Chapter 13.4,  Wet Cooling Towers ,  http://www.epa.gov/ttnchie1/ap42/ch13/final/c13s04.pdf.  Equation A5-10. Particulate Matter Emissions for Cooling Towers  35$_{ }$ A5.3.2.1  Methodology  APPENDIX A5$_{ }$