In this subchapter, the following words and phrases shall have the following meanings:

(1) "Act" means the Montana Clean Air Act.

(2) "Administrator," as used in 40 CFR Part 58, means the department.

(3) "Ambient air quality standards" means a permissible level of an air contaminant in the ambient air as defined by the maximum frequency with which a specified level may be exceeded or by a maximum level of an air contaminant in or on body or plant tissues.

(4) "Annual average" means an arithmetic average of any four consecutive valid calendar quarterly averages, where calendar quarterly averages are determined as specified in (3)(a) and (b); except that for hourly data at least 6,570 valid hourly averages must be contained in the four consecutive calendar quarters.

(a) For hourly data, the calendar quarterly average is the arithmetic average of all valid hourly averages collected during the quarter, except that the minimum number of valid hourly averages necessary to determine a valid quarterly average is 65% of the hourly averages contained in the quarter.

(b) For 24-hour data, the calendar quarterly average is the arithmetic average of all valid interval averages, except that the minimum number of valid interval averages necessary to determine a valid quarterly average is 80% of the interval averages contained in the quarter.

(5) "Approved equivalent method" means any method of measuring concentrations of air contaminants regulated in this subchapter which has been approved as an equivalent method by the U.S. EPA pursuant to 40 CFR Part 53, or which has been approved by the department. Methods approved by the department are kept on file and are available for inspection and copying.

(6) "Carbon monoxide" means the gas having the molecular composition of one carbon atom and one oxygen atom.

(7) "Eight-hour average" means the arithmetic average of all valid recorded values during any consecutive eight hours but not less than six valid hourly averages.

(8) "Fluoride" means fluorine combined with one or more other substances.

(9) "Forage" means any plant part which is grazed or browsed.

(10) "Grams per square meter" (gm/m^{2}) means a concentration numerically equal to the mass of an air contaminant (in grams) deposited on one square meter of surface.

(11) "Grazing season average" means, for each sample plot, an arithmetic average of all monthly averages for which sampling occurred in accordance with ARM 17.8.230. The minimum number of monthly averages shall be at least three for any sample plot.

(12) "Hourly average" means an arithmetic average of all valid values recorded between the first minute and 60th minute of the hour (e.g. 1:00 to 2:00), but not less than 2/3 of the data obtainable from the monitoring device during the hour, or an integral sample of more than 40 minutes.

(13) "Hydrogen fluoride" means the gas having the molecular composition of one fluorine atom and one hydrogen atom.

(14) "Hydrogen sulfide" means the gas having molecular composition of one sulfur atom and two hydrogen atoms.

(15) "Interval average" means the arithmetic average of all valid 24-hour averages collected during a specific scheduled sampling interval, except that the minimum number of valid 24-hour averages necessary to determine a valid interval average is one. If a scheduled sampling interval extends into two calendar quarters or two 90-day averaging periods the interval average shall be assigned to the calendar quarter or 90-day averaging period containing the start date of the interval.

(16) "Lead" means elemental lead or lead in combination with any other substance.

(17) "Micrograms per cubic meter" (µg/m^{3}) means a concentration numerically equal to the mass of an air contaminant present (in micrograms) in a one cubic meter of air, corrected to standard conditions.

(18) "Micrograms per gram" (µg/g) means a concentration numerically equal to the mass of an air contaminant (in micrograms) in one gram of dry material.

(19) "Monthly average" means the arithmetic average for a sample plot, taken for all applicable months in accordance with ARM 17.8.230 of all sample plot values of fluoride in or on forage samples collected. The minimum number of sample plot values must be two. The two sample plot values must be separated by at least a 12-day interval. Any number of sample plot values in excess of two for any month must be sampled at least X days from each other, where X is the integer value described by the following equation:

X = (30/number of sample plot values) - 2

Regardless of the number of sample plot values used to calculate a monthly average, at least one sample plot value must lie within 12 days of the end of the month.

(20) "Ninety-day average" means the arithmetic average of all valid interval averages recorded during any 90 consecutive days except that the minimum number of valid interval averages necessary to determine a valid 90-day average is 80% of the interval averages contained in the 90 days.

(21) "Nitrogen dioxide" means the gas having the molecular composition of one nitrogen atom and two oxygen atoms.

(22) "Ozone" means the gas having the molecular composition of three oxygen atoms.

(23) "Particle scattering coefficient" means the fractional change in the light intensity per meter of sight path due to particulate matter.

(24) "Parts per billion" (ppb) means a concentration of an air contaminant numerically equal to the volume of a gaseous air contaminant present in one billion volumes of air at the same conditions of temperature and pressure.

(25) "Parts per million" (ppm) means a concentration of an air contaminant numerically equal to the volume of a gaseous air contaminant present in one million volumes of air at the same conditions of temperature and pressure.

(26) "Regional administrator," as used in 40 CFR Part 58, means the department.

(27) "Sample plot value" means the results of any chemical analysis performed on a composite of forage clippings taken from a given sample plot on a specific sampling day.

(28) "Scheduled sampling interval" means the time period commencing with the start of one scheduled sampling day and ending at the start of the next scheduled sampling day, where "scheduled" means a predetermined routine sampling frequency. If the sampling schedule is changed during any calendar quarter or 90-day averaging period the scheduled sampling interval shall be the largest possible time period based on any of the sampling schedules.

(29) "Standard conditions" means a temperature of 25º Celsius and a pressure of 760 millimeters of mercury.

(30) "Sulfur dioxide" means the gas having the molecular composition of one sulfur atom and two oxygen atoms.

(31) "Thirty-day average" means an arithmetic average of all recorded values during any consecutive 30 days, but not less than 20 valid 24-hour average recorded values or an integral sample of more than 20 days.

(32) "Twenty-four hour average" means an arithmetic average of each valid recorded value during any consecutive 24 hours, but not less than 18 valid hourly averages or an integral sample of more than 18 hours.

(33) "Valid recorded value" means data recorded, collected, transmitted, and analyzed as required by ARM 17.8.212.

(34) "Year" means any 12 consecutive months.