Lifetime Of NOx Research Articles

Seasonal budgets of reactive nitrogen species and ozone over the United States, and export fluxes to the global atmosphere

A three‐dimensional, continental‐scale photochemical model is used to investigate seasonal budgets of O3 and NOy species (including NOx and its oxidation products) in the boundary layer over the United States and to estimate the export of these species from the U.S. boundary layer to the global atmosphere. Model results are evaluated with year‐round observations for O3, CO, and NOy species at nonurban sites. A seasonal transition from NOx to hydrocarbon‐limited conditions for O3 production over the eastern United States is found to take place in the fall, with the reverse transition taking place in the spring. The mean NOx/NOy molar ratio in the U.S. boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, in accord with observations, and reflecting largely the seasonal variation in the chemical lifetime of NOx. Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, is estimated to account for 30% of the chemical sink of NOx in the U.S. boundary layer in summer. Model results indicate that peroxyacylnitrates (PANs) are most abundant in the U.S. boundary layer in spring (25% of total NOy.), reflecting a combination of active photochemistry and low temperatures. About 20% of the NOx emitted from fossil fuel combustion in the United States in the model is exported out of the U.S. boundary layer as NOx or PANs (15% in summer, 25% in winter). This export responds less than proportionally to changes in NOx emissions in summer, but more than proportionally in winter. The annual mean export of NOx and PANs from the U.S. boundary layer is estimated to be 1.4 Tg N yr−1, representing an important source of NOx on the scale of the northern hemisphere troposphere. The eventual O3 production in the global troposphere due to the exported NOx and PANs is estimated to be twice as large, on an annual basis, as the direct export of O3 pollution from the U.S. boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O3 in the northern hemisphere troposphere on an annual basis.

Sources and chemistry of NOx in the upper troposphere over the United States

The origin of NOx in the upper troposphere over the central United States is examined using aircraft observations obtained during the SUCCESS campaign in April–May of 1996. Correlations between NOy (sum of NOx and its oxidation products) and CO at 8–12 km altitude indicate that NOx originates primarily from convective transport of polluted boundary layer air. Lightning and aircraft emissions appear to be only minor sources of NOx. Chemical steady state model calculations constrained by local observations of NO underestimate the measured NOx/NOy concentration ratio at 8–12 km altitude by a factor of two on average. The magnitude of the underestimate is correlated with concentrations of condensation nuclei, which we take as a proxy for the age of air in the upper troposphere. We conclude that the NOx/NOy ratio is maintained above chemical steady state by frequent convective injections of fresh NOx from the polluted boundary layer and by the long lifetime of NOx in the upper troposphere (5–10 days). In contrast to previous studies, we find no evidence for fast heterogeneous recycling from HNO3 to NOx in the upper troposphere.

Factors influencing the upper free tropospheric distribution of reactive nitrogen over the South Atlantic during the TRACE A experiment

This paper evaluates the potential sources of the enhanced levels of NO that were observed throughout the upper troposphere over the equatorial and tropical South Atlantic. During September/October 1992 NOx (NO + NO2) mixing ratios in the 8‐ to 13‐km region averaged 150 parts per trillion by volume (pptv) and were greatly affected by what appeared as spatially large “plumes” (100 to 1000 km) with NO enhancements of over 800 parts per trillion by volume. Other trace gases were also enhanced within these plumes (e.g., CO, CO2, CH4, CH3Cl, C2H2, C2H6, C3H8, and PAN). However, for these tracers of surface emissions, inconsistent patterns of enhancement were found with respect to one another and to NO. We analyzed these plumes for indications of coherent relationships between the enhanced levels of NO and the enhanced levels of biogenic and combustion‐related tracers. This analysis indicated that the tracer relationships were primarily produced by their common injection via deep convection into the upper troposphere. A corollary analysis using a combustion tracer reference frame in combination with meteorological analysis indicates a longer than expected lifetime of NOx in the upper troposphere. This analysis also suggests that an efficient mechanism may exist in the upper troposphere for recycling HNO3 back into NOx with a rate comparable to that predicted for the HNO3 formation. During the Transport and Atmospheric Chemistry Near the Equator Atlantic study period this in‐situ formation of NOx is estimated to provide the equivalent of approximately 0.7 TgN/yr of NOx within the South Atlantic basin's upper troposphere. This magnitude of local in situ source is estimated to be comparable to the combined inputs from lightning and biomass burning, which are both injected via deep convection. Our analysis also suggests that lightning can contribute as much as half of the external input of NOx into this region of the upper troposphere with biomass burning possibly representing the remainder.

Assessment of ozone photochemistry in the western North Pacific as inferred from PEM‐West A observations during the fall 1991

This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASA's western Pacific Exploratory Mission (PEM‐West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z < 6 km and positive for z > 6–8 km. For nearly all altitudes and latitudes, observed nonmethane hydrocarbon (NMHC) levels were shown to be of minor importance as ozone precursor species. Air parcel types producing the largest positive values of P(O3) included fresh continental boundary layer (BL) air and high‐altitude (z > 7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free‐tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column‐integrated O3 photochemical production and destruction. By contrast, for the tropical regime column‐integrated O3 showed photochemical destruction exceeding production by nearly 80%. Both transport of O3 rich midlatitude air into the tropics as well as very high‐altitude (10–17 km) photochemical O3 production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high‐altitude NOx to the western Pacific. Given that the high‐altitude NOx lifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical O3 formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large part of the Pacific.

Ozone production in the rural troposphere and the implications for regional and global ozone distributions

The relationship between O3 and NOx (NO + NO2) which was measured during summer and winter periods at Niwot Ridge, Colorado, has been analyzed and compared to model calculations. Both model calculations and observations show that the daily O3 production per unit of NOx is greater for lower NOx. Model calculations without nonmethane hydrocarbons (NMHC) tend to underestimate the O3 production rate at NOx higher than 1.5 parts per billion by volume and show the opposite dependence on NOx. The model calculations with NMHC are consistent with the observed data in this regime and demonstrate the importance of NMHC chemistry in the O3 production. In addition, at eight other rural stations with concurrent O3 and NOx measurements in the central and eastern United States the daily O3 increase in summer also agrees with the O3 and NOx relationship predicted by the model. The consistency of the observed and model‐calculated daily summer O3 increase implies that the average O3 production in rural areas can be predicted if NOx is known. The dependence of O3 production rate on NOx deduced in this study provides the basis for a crude estimate of the total O3 production. For the United States an average summer column O3 production of about 1×1012Cm−2S−1 from anthropogenically emitted NOx and NMHC is estimated. This photochemical production is roughly 20 times the average cross‐tropopause O3 flux. Production of O3 from NOx that is emitted from natural sources in the United States is estimated to range from 1.9×1011 to 12×1011 cm−2 s−1, which is somewhat smaller than ozone production from anthropogenic NOx sources. Extrapolation to the entire northern hemisphere shows that in the summer, 3 times as much O3 is generated from natural precursors as those of anthropogenic origin. The winter daily O3 production rate was found to be about 10% of the summer value at the same NOx level. However, because of longer NOx lifetime in the winter, the integrated O3 production over the lifetime of NOx may be comparable to the summer value. Moreover, because the natural NOx sources are substantially smaller in the winter, the wintertime O3 budget in the northern hemisphere should be dominated by ozone production from anthropogenic ozone precursors. The photochemical lifetime of O3 in the winter in the mid‐latitude is approximately 200 days. We propose that this long lifetime allows anthropogenically produced O3 to accumulate and contribute substantially to the observed spring maximum that is usually attributed to stratospheric intrusion. Furthermore, the anthropogenic O3 may be transported not only zonally but also to lower latitudes. Thus the long‐term interannual increase in O3, observed in the winter and spring seasons at Mauna Loa, may be due to the same anthropogenic influences as the similar winter trend observed at Hohenpeissenberg, Germany.