The invention provides methods and apparatuses for removing aerosols and, in some cases, vapor phase contaminants, such as mercury, from a gas stream. One method is directed to the removal of additional aerosols from a gas stream from which aerosols have previously been removed by using a screen in combination with a primary aerosol collection device. Another method is directed to the removal of both aerosols and vapor phase contaminants by using a sorbent in combination with a screen. Another method is directed to the removal of vapor phase contaminants by using a catalyst to convert the contaminant to a form that is more easily removed from the gas stream and optionally injecting a chemical to increase the rate of conversion. The invention also provides various apparatuses for use in performing the various methods of the invention.