Understanding trends in stratospheric NOy and NO2

Abstract

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.