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Every sixth day at each site, a sample of ambient air is collected into an evacuated 6-liter stainless steel Summa cannister. In order to fill the canister to the desired pressure and obtain a representative sample over the 24-hour sampling period, a canister sampler is used to regulate the flow of air into the canister at approximately 6.5 cubic centimeters (cc) per minute.
Samples are taken every sixth day so that the samples will represent every day of the week and every range of weather conditions. If samples were taken weekly, they would not cover any variation in emissions throughout the week.