Estimation of the source strength of soil NO_x in the Nashville, Tennessee, urban area

Soil biogenic NOx emissions are an important part of the total global NOx budget. The objective of this study was to investigate the role of the soil NOx source in an urban-metropolitan area. We measured the emissions from soils within the urban-metropolitan land use area of Nashville, Tennessee, and surrounding Davidson County, to estimate the importance of this source in the overall NOx budget of Davidson County. Nashville is currently in nonattainment for ozone (O-3), and a State Implementation Plan (SIP) has been promulgated to help achieve compliance of the O-3 standard. Using geographic information systems imagery and aerial photographs, we determined the extent of urban-metropolitan soils within the Nashville area. Results indicated that 409 km(2), or approximately 34% of the county, was classified as residential or urban, with the predominant vegetation being grass (68%). The residential sector was the largest with 323 km(2), the urban area comprising 87 km(2), and a total area of 8 km(2) of golf courses. Soil NOx emission measurements were made in approximate proportion to the major land use types using static chamber techniques. Results indicated that urban-metropolitan soils were not an important source of soil NOw(x). Approximately 10 Mg NOx, or 0.003% of the total annual NOx budget of Davidson County, is estimated to be emitted by urban soils annually. Additionally, the remaining soil sources in the county, forests and agricultural land, contributed 179 Mg, or 0.66% of the total annual NOx budget. While soil NOx emissions were concluded to be a negligible component of the Davidson County budget, their contribution to the NOx budget of the surrounding four counties in the STP area was significant. Extrapolation of soil NOx emissions for three of the four counties within the SIP area indicated that soil biogenic NOx emissions contributed from 7.0 to 9.8% of the daily average NOx budget during the months of June through August. Because of the soil temperature dependence of soil NOx emissions, it was estimated that during the hottest July days the soil biogenic component could contribute over 17% of the total NOx in some SIP counties.