Time Integrated Radiative Forcing from North American NOX Emissions: Climate Effect over 20- and 100-year Time Scales

The distribution of tropospheric ozone (O₃) globally depends on the emission of precursors (e.g., NOx), chemistry, and transport. In this study, we quantify the response of radiative forcing over 20- and 100-year time scales, to O₃ and methane (CH₄) perturbations caused by a marginal increase (0.1 Tg N) in anthropogenic emissions of NOx in January and July from 21 (10° × 10° grid) geographical locations in North America. Changes in the perturbations have been calculated with the global climate-chemistry transport model STOCHEM. Addition of NOx emissions led to an initial increase in global O₃ burdens up to 0.9 Tg, which decayed after 4 months. Global CH₄ burdens decreased (by increasing OH) by up to –0.7 Tg and decayed gradually after 6 months. Global radiative forcings resulting from the regional emission increases were calculated, accounting for changes in both O₃ (using an offline radiation code) and CH₄ (using a simple conversion of 0.37 mW m⁻² ppb⁻¹, assuming that CH₄ is well mixed in the atmosphere). Our results revealed that O₃-induced time-integrated radiative forcings exhibit both positive (initial) and negative (long-term) phases in the two (20- and 100-year) time horizons. For the positive phase, both the 20- and 100-year time periods peaked at 0.454 mW m⁻² yr; however, for the negative phase, the 20-year peaked at –0.246 mW m⁻² yr and the 100-year peaked at –0.300 mW m⁻² yr. CH₄, on the other hand, showed a single negative phase which peaked at –1.070 mW m⁻² yr for the 20-year time period and –1.302 mW m⁻² yr for the 100-year time period. The total net radiative forcings (assuming a linear additive for relatively small perturbations) of the CH₄ term and the two O₃ terms over a 100-year time period from all 21 locations produce a net climate cooling effect (negative forcings), irrespective of the season of the emission pulses. However, over a 20-year time period in winter, some emission pulses at low latitudes produce a net climate warming effect (positive forcings). Both the O₃ and CH₄ burdens and the associated radiative forcings depend strongly on the geographical location as well as the season of the emission pulses. They are most sensitive to emissions from low latitudes and least sensitive to emissions from mid-latitudes and high latitudes.