Air pollutant units and conversion factors

Air pollutants are measured in various units. Find out about the units and how to convert them so you can compare our data with data from other sources.

Concentration values are the primary measurements of parameters obtained from air quality monitoring stations. Concentrations are reported as:
  • particles (as PM10, PM2.5) – micrograms per cubic metre (µg/m3)
  • NEPH or visibility – while commonly written as 'bsp' NEPH is reported in units of 10-4 m-1. This means that a NEPH value of 1.5 should be read as 1.5x10-4 m-1.On multiplying by 100 you can convert this number to the widely used unit for visibility known as inverse megametres (Mm-1).
  • gases – parts per million (ppm) for CO and parts per hundred million (pphm) for others. Gaseous concentration values are reported referenced to a temperature of 0°C and an absolute pressure of 101.325 kilopascals. This conversion table will allow you to compare data in the DPE air quality monitoring reports with those from other sources.

Pollutant

Concentration units we report in…

to convert from our reported units to…

multiply by
 Ammonia pphm µg/m3 (0°C)
µg/m3 (25°C)		 7.6
7.0

Ozone

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

21.4
19.6

Nitric oxide (NO)

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

13.4
12.3

Nitrogen dioxide (NO2)

pphm

µg/m3 (0°C)
µg/m3 (25°C)

20.5
18.8

Sulfur dioxide (SO2)

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

28.6
26.2

Lead

µg/m3 (0°C)

µg/m3 (25°C)

0.92

Carbon monoxide

ppm

mg/m3 (0°C) 
mg/m3 (25°C)

1.25
1.15
NEPH or visibility 10-4 m-1 Mm-1 or inverse megametres 100