Network monitoring of speciated vs. total non-methane hydrocarbon measurements

Abstract The total non-methane hydrocarbon (TNMHC) level in the atmosphere is defined as the level of total hydrocarbons minus the level of methane. TNMHC observations are made in selected air quality stations (AQS) of Environmental Protection Agency (EPA) across Taiwan. The AQS network is also complemented by a network of photochemical assessment monitoring stations (PAMS) to provide hourly observations of 56 speciated non-methane hydrocarbons (NMHCs). In this study, the relationship between the AQS and PAMS TNMHC values was cross-examined for the period of 2007–2011 at four sites that conducted both types of measurements. Although the two observations differ in their methods of collection, the variations in the two datasets showed high synchronicity. However, because some of the NMHCs were missed in the summation of 56 species, the PAMS TNMHC values were consistently lower than those of the AQS TNMHC by an average of 30%. Although total hydrocarbon (THC) instruments can provide instantaneous TNMHC mixing ratios (by subtracting methane) to approximate the levels of volatile organic compounds (VOCs), they lack speciated information, which is invaluable to establishing the link between the observation site and specific source influences. In contrast, PAMS observations with speciation information can be used to complement this deficiency. Examples of this complementarity were illustrated by comparing four common sites, with two mainly affected by urban/traffic emissions and two primarily affected by industrial emissions. The chemical composition of the air samples varied significantly between the four sites. Ethylene and propylene were found to be prominent at the two industrial sites, linking these samples strongly to the petrochemical industry nearby. This compositional signature was even more pronounced for the sample peak values. These findings suggest that network monitoring of TNMHCs may not be sufficient, and the lack of compositional information can be complemented by the more elaborate monitoring means such as PAMS deployed only at critical sites to provide both area coverage and speciation knowledge.