Person:

Kondo, Yasuyuki

The late winter/early spring (February/March, 1994) measurements of Pacific Exploratory Mission-West (PEM-W) B have been analyzed to show latitudinal distributions (45°N to 10°S) of the mixing ratios of reactive nitrogen species (NO, peroxyacetylnitrate (PAN), HNO3, and NOy), ozone, and chemical tracers (CO, NMHCs, acetone, and C2Cl4) with a focus on the upper troposphere. Mixing ratios of all species are relatively low in the warm tropical and subtropical air south of the polar Jetstream (≈28°N) but increase sharply with latitude in the cold polar air north of the Jetstream. Noteworthy is the continuous increase in reservoir species (PAN and HNO3) and the simultaneous decrease in NOx toward the northern midlatitudes. The Harvard global three-dimensional model of tropospheric chemistry has been used to compare these observations with predictions. In the upper troposphere the magnitude and distribution of measured NOy and PAN as a function of latitude is well represented by this model, while NOx (measured NO + model calculated NO2) is underpredicted, especially in the tropics. Unlike several previous studies, where model-predicted HNO3 exceeded observations by as much as a factor of 10, the present data/model comparison is improved to within a factor of 2. The predicted upper tropospheric HNO3 is generally below or near measured values, and there is little need to invoke particle reactions as a means of removing or recycling HNO3. Comparison between measured NOy and the sum of its three main constituents (PAN + NOx + HNO3) on average show a small mean shortfall (<15%). This shortfall could be attributed to the presence of known but unmeasured species (e.g., peroxynitric acid and alkyl nitrates) as well as to instrument errors.

Interactions between atmospheric hydrogen oxides and aircraft nitrogen oxides determine the impact of aircraft exhaust on atmospheric chemistry. To study these interactions, the Subsonic Assessment: Ozone and Nitrogen Oxide Experiment (SONEX) assembled the most complete measurement complement to date for studying HOx (OH and HO2) chemistry in the free troposphere. Observed and modeled HOx agree on average to within experimental uncertainties (±40%). However, significant discrepancies occur as a function of NO and at solar zenith angles >70°. Some discrepancies appear to be removed by model adjustments to HOx-NOx chemistry, particularly by reducing HO2NO2 (PNA) and by including heterogeneous reactions on aerosols and cirrus clouds.

During October/November 1997, simultaneous observations of NO, HO2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40–60°N), to derive ozone production rates, P(O3), and to examine the relationship between P(O3) and the concentrations of NOx (= NO + NO2) and HOx (= OH + peroxy) radicals. A positive correlation is found between P(O3) and NOx over the entire data set, which reflects the association of elevated HOx with elevated NOx injected by deep convection and lightning. By filtering out this association we find that for NOx>70 pptv, P(O3) is nearly independent of NOx, showing the approach of NOx-saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer.

The factors controlling the concentrations of HO radicals (= OH + peroxy) in the upper troposphere (8–12 km) are examined using concurrent aircraft observations of OH, HO, HO, CHOOH, and CHO made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of O, HO, NO, peroxyacetyl nitrate (PAN), HNO, CH, CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HO and of the larger chemical family HO (= HO + 2 HO + 2 CHOOH + HNO + HNO). Observations of OH and HO are successfully simulated to within 40% by a diel steady state model constrained with observed HO and CHOOH. The model captures 85% of the observed HO variance, which is driven mainly by the concentrations of NO (= NO + NO) and by the strength of the HO primary sources. Exceptions to the good agreement between modeled and observed HO are at sunrise and sunset, where the model is too low by factors of 2–5, and inside cirrus clouds, where the model is too high by factors of 1.2–2. Heterogeneous conversion of NO to HONO on aerosols (γ = 10) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO on ice crystals (γ = 0.025) could explain the discrepancy in cirrus. Primary sources of HO from O()+HO and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO were OH+HO (NO50 pptv). Observed HO concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO to HO on aerosols (γ = 0.2). Observed CHOOH concentrations are underestimated by a factor of 2 on average. Observed CHO concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CHOH and with cirrus clouds. Heterogeneous oxidation of CHOH to CHO on aerosols or ice crystals might provide an explanation (γ ∼ 0.01 would be needed).