Concentration of nitrogen oxides (ppm: mg/L)†
Sampling Period* 1st Denuder 2nd Denuder 1st Denuder–2nd Denuder
First half 10.9 2.0 8.9
Latter half 9.7 1.9 7.9
Estimate of HONO concentration (ppm:mg/L)     (8.4)
*The total sampling periods was under 2% in all exposure period. †The collection efficiency of sodium carbonate annular denuder is different according to type of nitrogen oxides. HONO is perfectly trapped by the coated walls, while nonacidic gases such as NO2 or NO passed through the coated walls. The first denuders trap all HONO and approximately 2–3% of NO2 or NO. The second denuders trap only approximately 2–3% of NO2 or NO
Table 1: Concentrations of exposure HONO.