Concentration monitoring of volatile organic compounds and ozone in Xi'an based on PTR-TOF-MS and differential absorption lidar

Abstract

Ozone has become a severe ambient pollutant. As the critical precursors of the formation of ozone, volatile organic compounds (VOCs) can adversely affect the atmospheric environment. In order to obtain reliable data on the sources and pollution characteristics of VOCs and ozone in the city for precise emission reduction, the concentrations of VOCs and ozone in Xi'an city were monitored by navigation observation using both a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and differential absorption lidar. Three different functional areas were selected, including a residential area (RA), a commercial district (CD), and an industrial park (IP). The PTR-TOF-MS measurement results found total concentrations of VOCs (TVOCs) with mean values of 136 ppb, 175 ppb, and 267 ppb at RA, CD, and IP sites, respectively. The dominant category of VOCs was oxygenated VOCs (OVOCs) at all three sites, accounting for 86%, 81%, and 75% of TVOCs at the RA, CD, and IP sites, respectively. Among OVOCs and aromatic hydrocarbons (AHs), acetone and toluene were the predominant chemical species, accounting for 51% and 58% at the IP site, respectively. Propylene was the most abundant aliphatic hydrocarbon (ALH) at the RA and CD sites, accounting for 42% and 48% of the TVOCs, respectively. The average ozone formation potentials (OFP) of the measured VOCs were 546 μg/m3, 574 μg/m3, and 772 μg/m3 at the RA, CD, and IP sites, respectively; OVOCs were the dominant contributors to OFP at all three sites. Ozone concentrations were measured by differential absorption lidar, with mean values of 82 μg/m3, 92 μg/m3, and 117 μg/m3 at the RA, CD, and IP sites, respectively. Ozone concentration was correlated to VOCs concentration. Local meteorological conditions including wind speed and air pressure influenced VOCs and ozone concentrations. The assessment of the measurement results of local environmental protection departments indicated that forming relevant VOCs and ozone control policies and priority strategies should be considered in order to reduce the adverse effects of these pollutants.