Perturbation to global tropospheric oxidizing capacity due to latitudinal redistribution of surface sources of NOx, CH4 and CO

Economic and social projections indicate that during next several decades there will be major geographical redistribution of surface emissions of O3 precursors, such as NOx, CH4 and CO. A net decrease in their emissions from northern hemispheric mid‐latitudes will be accompanied by substantial increases from the tropics. We have investigated a hypothetical scenario of currently underway transition of such emission patterns using a global two‐dimensional photochemical model. With overall O3 precursor releases held constant, a simultaneous transfer of their emissions by 25% from the latitude belt 75°N–35°N to 5°S–35°N increases tropospheric oxidizing capacity such that the methane global lifetime and concentrations fall by more than 3%. Seasonally dependent changes in surface O3 concentrations are also calculated. In influencing OH concentration, redistribution of surface NOx emissions is 2–3 orders of magnitude more efficient per unit mass than CO emissions. Shifts in methane sources have insignificant effects on global photochemistry, but lead to a decrease in its interhemispheric gradient.

In the future, major redistribution of human-related radical concentration. Much of the effort of photochemical emissions of NOx (defined as NO2+NO), hydrocarbons and modelers has been devoted to study the effects of uniformly CO are all likely. A net decrease is predicted from the North increased or decreased global/regional emissions of air accompanied by an increase in emissions from the South pollutants on the chemical balance of the troposphere. In this [Worm Energy Council, 1993;EPA;1995;OECD, 1995;EMEP, 1997]. This redistribution may be accompanied by a

Simulations and Model Description
For the present study, we consider 25% decreases in surface emissions of NOx, CH4 and CO from the developed countries (considered to be located between 35øN-75øN) and 3931 simultaneous increases in their emissions from the NH developing countries (considered to be located between 35øN-5øS) by the same absolute amounts for which they were reduced from the developed countries. This scenario allows geographical redistribution in surface emissions of NOx, CH4 and CO but leaves their global source strengths unchanged. To study the non-linearity in the chemical system induced by this scenario, multiple variations were introduced in the surface emissions of NOx, CH4 and CO. To examine the relative partial effects of adjustment in individual sources of NOx, CH4 and CO on the tropospheric oxidizing capacity, individual model simulations altered emissions for each of these species separately. In the future, global surface emissions of NOx, CH4 and CO may in fact increase. In the present study, this possibility is set aside. Moreover, we realize that the future changes in geographical emission patterns may or may not be the same for all three air pollutants.
All calculations were made using a modified version of the CO respectively were assumed to have shifted from (75øN -35øN) to (35øN-5øS). The model was typically allowed to simulate for at least four decades so as to attain steady state concentrations of CH4 throughout the domain. In this paper, our analysis will concentrate on relative chemical changes brought by the redistribution of surface emissions of NOx, CH4 and CO. For steady state distributions of reactive species corresponding to the reference simulation, see Gupta [1996]. Simulations corresponding to the 25% shift of emissions of all three species combined and NOx, CH4 and CO individually are termed as ALL, NOX, CH4 and CO respectively.

Results and Discussion
Emissions of the air pollutants such as NOx, hydrocarbons and CO from the developing countries, which are mostly confined to tropical latitudes, very effectively perturb the global atmospheric chemical balance for two primary reasons. First, tropical regions experience year-round intensive photochemical activity. Also, at low latitudes, sub-scale convective events draw polluted air out of the planetary boundary layer and redistribute it in the photochemically efficient free troposphere. Along with non-linearities in the production of tropospheric ozone [Liu et al., 1987] and increasing tropical emis-sions, these two factors significantly affect the distributions of chemically reactive trace gases and oxidizing capacity of the global troposphere. Gupta [1996] has described some photochemical effects of increased tropical emissions of NOx, hydrocarbons and CO on the tropospheric chemical balance.

Our calculations show that redistribution of emissions of
NOx, CH4 and CO has significant effects on the tropospheric oxidizing capacity and distributions of chemically active trace species. For simulations ALL and NOX, Figure 1 shows the calculated percent changes in OH concentration for July. For case ALL, a combined decrease in sinks and sources of OH radicals from mid-latitude regions to the tropics led to an overall increase (ranging from 1.4-4.2% at various latitudes and altitudes) in OH concentrations throughout the midlatitude NH troposphere. This net increase in OH concentration at mid-latitudes is caused mainly by the decrease in emissions of CH4 and CO and is counteracted somewhat by the decrease in NOx surface emissions. For case NOX, reduction in NOx emissions alone led to a decrease in OH concentration by 2.6% and 1.6% in the lower and middle NH troposphere respectively. This is due primarily to decrease in 03 concentration (see Figure 2). For tropical regions of the NH, an increase in OH concentration in the range of 3-8% is calculated.

For ALL, despite an increase in emissions of CO and CH4, an increase in OH is obtained because of enhanced NOx emissions which are relatively more efficient in producing 03, and hence OH radicals. A comparison of the increase in OH concentration at NH tropics shows that increased emissions of NOx contributed the most to the calculated increase in OH concentration, with a small opposing effect from increased emissions of CO and CH4.
On the annual average, changes in global OH concentration weighted by CH4 mixing ratio and rate of reaction for CH4+OH [Prather and Spivakovsky, 1990], calculated for ALL, NOX and CO cases are +3.6%, +3.9% and -0.2% re-  (90øN-30øN) and  (30øN-equator).  (75øN-35øN), (35øN-5øS), (90øN-30øN), (30øN-0ø), (0ø-30øS) and (30øS-90øS) respectively.

3). A variable change in surface 03 mixing ratio is obtained
for mid-and high-latitude NH regions. For case NOX, the surface 03 mixing ratio decreases by 0.4-3.4% for the months of June-November. At other times of the year, an increase of 0.5% to 2.1% is calculated for surface 03 mixing ratios despite decreased surface NOx emissions. These seasonally dependent changes in surface 03 mixing ratio are partially governed by the extent of 03 losses by NO forming NO2.
The changes in OH and 03 distributions are expected to alter the steady state lifetime, mixing ratio and interhemispheric gradient of CH 4. For cases ALL and NOX, the decreases in CH 4 time constants are 3.3% and 3.7% respectively. Annually averaged surface mixing ratios of CH4 for the region (75øN-35øN) changed by -4.7%,-3.4%, +0.05% and -1.1% for ALL, NOX,, CO and CH4 respectively (Table I).