Measuring OVOCs and VOCs by PTR-MS in an urban roadside microenvironment of Hong Kong: relative humidity and temperature dependence, and field inter-comparisons

Abstract. Volatile organic compound (VOC) control is an important issue of air quality management in Hong Kong because ozone formation is generally VOC limited. Several oxygenated volatile organic compound (OVOC) and VOC measurement techniques, namely, (1) off-line 2,4-dinitrophenylhydrazine (DNPH) cartridge sampling followed by High Performance Liquid Chromatography (HPLC) analysis, (2) on-line gas chromatography (GC) with flame ionization detection (FID), and (3) off-line canister sampling followed by GC with mass spectrometer detection (MSD), FID, and (electron capture detection) ECD, were applied during this study. For the first time, the proton transfer reaction–mass spectrometry (PTR-MS) technique was also introduced to measured OVOCs and VOC in urban roadside area of Hong Kong. The integrated effect of ambient relative humidity (RH) and temperature (T) on formaldehyde measurements by PTR-MS was explored in this study. A Poly 2-D regression was found to be the best nonlinear surface simulation (r = 0.97) of the experimental reaction rate coefficient ratio, ambient RH and T for formaldehyde measurement. This correction method was found to be better than correcting formaldehyde concentrations directly via the absolute humidity of inlet sample, based on a two-year field sampling campaign at Mong Kok (MK) in Hong Kong. For OVOC species, formaldehyde, acetaldehyde, acetone and MEK showed good agreements between PTR-MS and DNPH-HPLC with with slopes of 1.00, 1.10, 0.76 and 0.88, respectively, and correlation coefficients of 0.79, 0.75, 0.60 and 0.93, respectively. Overall, fair agreements were found between PTR-MS and on-line GC- FID for benzene (slope = 1.23, r = 0.95), toluene (slope = 1.01, r = 0.96) and C2-benzenes (slope = 1.02, r = 0.96) after correcting benzene and C2-benzenes levels which could be affected by fragments formed from ethylbenzene. For the inter-comparisons between PTR-MS and off-line canister measurements by GC-MSD/FID/ECD, benzene showed good agreement with a slope of 1.05 (r = 0.62), though PTR-MS had lower values for toluene and C2-benzenes with slopes of 0.78 (r = 0.96) and 0.67 (r = 0.92), respectively. All in all, the PTR-MS instrument is suitable for OVOC and VOC measurements in urban roadside areas.