Using the NO2/NOx Ratio to Understand the Spatial Heterogeneity of Secondary Pollutant Formation Capacity in Urban Atmospheres

Abstract:

Urban atmospheres are chemically reactive environments in which anthropogenic emissions can react with natural and anthropogenic components, on time scales of seconds to days, to increase the oxidative potential of the urban atmosphere and to create secondary air pollutants. The oxides of nitrogen (NO_x = NO + NO₂) are generated by combustion activities, including power plants and vehicles. Tailpipe emissions of NO_x contain 10-20% NO₂, and 90-80% NO. NO can be oxidized to NO₂ by O₃, HO₂ and RO₂species. The latter oxidants can be produced through the photo-oxidation of VOCs. It is likely then that oxidative capacity within a city is spatially heterogeneous since the distribution of anthropogenic (NOx, VOCs) and biogenic (VOCs) emissions varies spatially within the city.

The spatial heterogeneity in local oxidative capacity of the urban atmosphere has seldom been measured or modeled at the fine spatial scale of ~250m. In summer 2013 we measured NO and NO₂ using passive samplers over two weeks at 144 sites in the Portland Metro area. We used the ratio of NO₂ to NO_x as an indicator of the local atmospheric oxidative potential. We found the measured percentage of NO₂ in NO_x ranged from a minimum of 20% to a maximum of 76%, with an average value of 54% (std dev = 12%). The measured NO₂/NO_x ratio was statistically significantly correlated with both freeways and tree canopy within 250m (r = -0.25 and 0.31 respectively), showing a decreasing fraction of NO₂ with increasing length of freeway and an increasing fraction of NO₂in areas with greater tree cover, as expected.

We will use the NO₂ measurements to allocate county-level emissions to the spatial scale of ~250m for the Portland Metro area. Using WRF-Chem, we will then model the urban chemistry at this fine spatial scale, and compare the modeled NO to measured NO. Comparing the modeled and measured NO values will serve a dual purpose: it will help validate the fine spatial-scale WRF-Chem model for Portland, and it will provide an insight into the effectiveness of using the NO₂/NO_x ratio as an indicator of the potential for urban secondary pollutant formation.