Global simulation of tropospheric O3‐NOx‐hydrocarbon chemistry: 3. Origin of tropospheric ozone and effects of nonmethane hydrocarbons

Yuhang Wang,Jennifer A Logan,Daniel J Jacob

doi:10.1029/98jd00156

Abstract

A global three‐dimensional model of tropospheric O3‐NOx‐hydrocarbon chemistry is used to investigate the factors controlling ozone concentrations in the troposphere. Model results indicate a close balance between chemical production and chemical loss of ozone in the tropospheric column at all latitudes (except high latitudes in winter). Using separate tracers for ozone produced in the stratosphere and in different regions of the troposphere, we find that the contribution of transport from the stratosphere to ozone concentrations in the troposphere is about 30% at midlatitudes in winter, 10% in summer, and 5% in the tropics. Production of ozone in the upper, middle, and continental lower troposphere all make significant contributions (10–50%) to ozone concentrations throughout the troposphere. The middle troposphere is a major global source region for ozone even though it is not a region of net production. The springtime maximum of ozone observed at remote sites in the northern extratropics is explained by a phase overlap between ozone transported from the stratosphere which peaks in late winter and ozone produced in the troposphere which peaks in late spring. Our model results do not support previous explanations of the springtime maximum based on wintertime accumulation of ozone or its precursors in the Arctic. The particularly strong springtime maximum at Mauna Loa Observatory (Hawaii) is attributed to long‐range transport of Asian pollution over the North Pacific in spring. A sensitivity simulation without nonmethane hydrocarbons (NMHCs) indicates small decreases of ozone concentrations (<15%) in the remote troposphere and a 20% increase in the global mean OH concentration. Without NMHCs as a source of peroxyacetylnitrate, concentrations of NOx decrease by 30% in the remote lower troposphere but increase by 70% in the continental lower troposphere and by 40% in the upper troposphere. Biogenic isoprene accounts for about half of the NMHC effects in the model.

Highlights

This paper is the third of a series applying a global threedimensional model to simulate tropospheric O3-NOx-hydrocarbon chemistry and to analyze the factors controlling tropospheric ozone
Liu et al [1987] calculated the in situ production of tropospheric ozone in the northern hemisphere by scaling hemispheric estimates of NOx emissions with the ozone production efficiency; they concluded that this in situ source of ozone was much larger than transport from the stratosphere
The budget of tropospheric ozone is largely defined by a balance between in situ chemical production and loss within the troposphere; transport from the stratosphere accounts for only 9% of the global source of ozone in the troposphere, and deposition accounts for only 18% of the global sink

Summary

Introduction

This paper is the third of a series applying a global threedimensional model to simulate tropospheric O3-NOx-hydrocarbon chemistry and to analyze the factors controlling tropospheric ozone. We approach this issue by tracing separately in our global three-dimensional model ozone molecules produced in different regions of the troposphere and in the stratosphere. They increase the ozone production efficiency per unit NOx in the continental boundary layer [Lin et al, 1988] They form organic nitrates such as PAN which provide reservoirs for the long-range transport of anthropogenic NOx to the remote atmosphere [Singh and Hanst, 1981; Jacob et al, 1992; Fan et al, 1994; Moxim et al, 1996]. Wang et al [this issue(b)] presented an extensive evaluation of model results using long-term surface observations of ozone, CO, and C2H6; climatological ozonesonde data; and aircraft measurements of NO, PAN, HNO3, C2H6, acetone, and H2O2 in various regions of the troposphere. The model lifetime of CH3CCl3 against oxidation by OH is 5.1 years below 200 mbar, in close agreement with the estimate of 4.9 ± 0.3 years derived by Prinn et al [1995] from long-term observations of CH3CCl3

Source Regions for Tropospheric Ozone

Springtime Ozone Maximum in the Northern Extratropics

Effects of NMHCs on Tropospheric Chemistry

Findings

Conclusions