Understanding trends in stratospheric NOy and NO2

Nitrous oxide (N2O), an important greenhouse gas, has been increasing since 1980 at a rate of about +3% per decade. Recently, a notably greater rate of increase of about +5% per decade since 1980 was reported for measurements of stratospheric nitrogen dioxide (NO2) over Lauder, New Zealand. Since N2O is the dominant source of odd-nitrogen compounds in the stratosphere, including NO2, this presents an obvious conundrum. Analysis here shows that these apparently conflicting trends are generally consistent when viewed in a global-change framework, specifically, when concurrent trends in stratospheric ozone and halogens are included. Using a combination of photochemical and three-dimensional chemistry-transport models, we predict a 1980-2000 trend in the NO2, as measured over Lauder, New Zealand, of +4.3%/decade when these concurrent trends are considered. Of this, only +2.4%/decade is attributed directly to the increase in N2O; the remainder includes +2.5%/decade due to the ozone change and -0.6%/decade to the increased halogens' impact on odd-nitrogen partitioning. The slant column densities of NO2, as measured from the zenith scattered sunlight during twilight, are found to (1) overestimate the trend by +0.4%/decade as compared to the true vertical column densities and (2) display a diurnally varying trend with a maximum during the night and large gradients through sunrise and sunset in good agreement with measurement. Nonetheless, measurements such as these are essential for identifying global change and provide a lesson in understanding it: careful simulation of the time, location, and geometry of measurements must be combined with concurrent trends in related chemical species and climate parameters.