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A comparison of aromatic hydrocarbon measurements is reported forthe CENICA supersite in the district of Iztapalapa during the MexicoCity Metropolitan Area field experiment in April 2003 (MCMA 2003).Data from three different measurement methods were compared: aProton Transfer Reaction Mass Spectrometer (PTR-MS), long pathmeasurements using a UV Differential Optical Absorption Spectrometer(DOAS), and Gas Chromatography-Flame Ionization analysis (GC-FID)of canister samples. The principle focus was on the comparisonbetween PTR-MS and DOAS data. Lab tests established that the PTR-MSand DOAS calibrations were consistent for a suite of aromaticcompounds including benzene, toluene, p-xylene, ethylbenzene,1,2,4-trimethylbenzene, phenol and styrene. The point samplingmeasurements by the PTR-MS and GC-FID showed good correlations(r=0.6), and were in reasonable agreement for toluene,C2-alkylbenzenes and C3-alkylbenzenes. The PTR-MS benzene data wereconsistently high, indicating interference from ethylbenzenefragmentation for the 145 Td drift field intensity used in theexperiment. Correlations between the open-path data measured at 16-mheight over a 860-m path length (retroreflector in 430 m distance),and the point measurements collected at 37-m sampling height werebest for benzene (r=0.61), and reasonably good for toluene,C2-alkylbenzenes, naphthalene, styrene, cresols and phenol(r>0.5). There was good agreement between DOAS and PTR-MSmeasurements of benzene after correction for the PTR-MS ethylbenzeneinterference. Mixing ratios measured by DOAS were on average afactor of 1.7 times greater than the PTR-MS data for toluene,C2-alkylbenzenes, naphthalene and styrene. The level ofagreement for the toluene data displayed a modest dependence on winddirection, establishing that spatial gradients – horizontal,vertical, or both – in toluene mixing ratios were significant, andup to a factor of 2 despite the fact that all measurements wereconducted above roof level. Our analysis highlights a potentialproblem in defining a VOC sampling strategy that is meaningful forthe comparison with photochemical transport models: meaningfulmeasurements require a spatial fetch that is comparable to the gridcell size of models, which is typically a few 10 km2.Long-path DOAS measurements inherently average over a larger spatialscale than point measurements. The spatial representativeness can befurther increased if observations are conducted outside the surfaceroughness sublayer, which might require measurements at altitudes ashigh as 10 s of metres above roof level

Comparison of aromatic hydrocarbon measurements made by PTR-MS, DOAS and GC-FID during the MCMA 2003 Field Experiment