The ambient Ozone, refered as O_{3} and also called called groundlevel or tropospheric Ozone impacts everyone on earth regardless of the country, as shown on the image on the right ^{[1]}.
(Attribution: WMO GAW research on reactive gases )
Unlike particulate matter (PM_{2.5}), the groundlevel Ozone is not emitted directly. It is instead produced through a series of chemical reactions that occur in presence of nitrogen oxides, volatile organic compounds, sunlight and high temperatures, as shown on the following visual:
Quantifying the impact of this ground level Ozone on Health is done via the Air Quality Index standard which each countries defines. What is interesting is that half of the world is using a standard based on milligrams measurement, while the rest is using ppb based measurement. But is this really a problem? This is what we will be looking at in this article.

Ozone Measurement Principles
In this system, the Ozone concentration is measured as an amount of light energy per volume of air, from which the concentration in ppbv is deducted. The lower detectable limit for this system is 0.4 ppb (corresponding to an AQI of 0.3, based on the US EPA 8hour Ozone standard). This system does not measure the mass as such, but there is a standard way of doing the convertion from ppmv to mg/m^{3}.
Converting Atmospheric Pollutant Concentrations: from ppmv to mg/m^{3}
or
1 ppb = 10^{3} ppm`. The conversion factor depends on the temperature at which you want the conversion (usually 25 degrees Centigrade in the US), as well as the ambient pressure. At an ambient pressure of 1 atmosphere, the general equation is:

c
= concentration in mg/m^{3}(i.e., milligrams of gaseous pollutant per cubic meter of ambient air) 
MW
= molecular weight of the gaseous pollutant 
ppmv
= parts per million by volume (i.e., volume of gaseous pollutant per million volumes of ambient air) 
t
= ambient temperature in degrees centigrade. 
12.187
= inverse of the Universal Gas Law constant^{[4]}
20 ppmv
of Ozone to `mg/m3 at
25 °C` and 1 atmophere
, the following formula is used: where:
48.00
= `MW(O_{3)` = molecular weight of Ozone O3 }European and US Convertion Standards
Gas  Standard Conditions for Temperature and Pressure ( STP)  
"STP US" Conditions at 25°C (US EPA standard) ^{[5]} 1013 mbar and 298K  "STP European Union" Conditions at 20°C (EU standard) ^{[6]} 1013 mbar and 293K  "Normal" Conditions at 0°C 1013 mbar and 273K  
O_{3}  Ozone  1 ppb = `1,97` µg/m3  1 ppb = `2,00` µg/m3  1 ppb = `2,15` µg/m3 
NO_{2}  Nitrogen Dioxyde  1 ppb = `1,88` µg/m3  1 ppb = `1,91` µg/m3  1 ppb = `2,05` µg/m3 
Note: For those interested in knowing why 20°C is used as the standard reference temperature, you can check Ted Doiron's article on "20 °C—A short history of the standard reference temperature for industrial dimensional measurements".

Impact from ambient temperature
At 20°C and 1 atm, `120 mg/m^3` converts to 120/2.00, i.e.
60.0 ppmv
. So let us assume that this is the actual measurement from the Ozone sensor. The question is then, what if the ambient temperature was peaking to 42°C, as it sometimes happen during summer heat waves, then what would be the correct mass? The convertion formula is: $$c = { ppmv \times 12.187 \times MW \over 273.15 + t } = 111.37 $$
This results in a difference of `8.6 mg/m^3` of measured Ozone. When applying the CAQI standard, the corresponding AQI is
46.4
(instead of 50
using the standard 20°C condition). This is actually an acceptable difference. The generalized adjustement formula depending on the ambient temperature is summurized with the graph on the right. The xaxis is the ambient temperature, and the yaxis the calculated AQI is the actual temperature would be used instead of the reference one (20°C).
Impact from atmospheric pressure
12.187
is actually the inverse of the Universal Gas Law constant R
. So, to understand the impact of atmospheric pressure, the following formula can be used: $$R = {{ P \times V } \over {n \times T}} = {{ P \times 22.4 } \over { 1 \times 273 }} = { P \over 12.1875 } $$
In other words, one just need to divide the convertion factor by the current atmosphere. Assuming that the pressure
p
is expressed in millibars (1 atm
= 1013.25 mb
), the generalized convertion formula becomes: where:

c
= concentration in mg/m^{3}(i.e., milligrams of gaseous pollutant per cubic meter of ambient air) 
MW
= molecular weight of the gaseous pollutant 
ppmv
= parts per million by volume (i.e., volume of gaseous pollutant per million volumes of ambient air) 
t
= ambient temperature in degrees centigrade. 
p
= ambient atmospheric pressure in millibars.
Conclusions
ppm
and `mg/m^3`), this is actually not a problem since there is a standard way to convert readings from `mg/m^3 to
ppm` and viceversa. Moreover, the impact of using the reference STP (Standard Temperature & Pressure) instead of the actual ambient temperature and pressure is minimal, i.e. just units of Index in difference for the Ozone. Credits: Ozone visual recreated using Icon pack by Icons8 and taken from American Chemical Society.