Abstract
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.
Highlights
AbundaninttheU.S.boundarlayyerin spring(25%of totalNO3.)r,eflectinagcombinatioonf activephotochemistrayndlow temperaturesA. bout20% of theNOx emittedfrom fossilfuel combustionin theUnitedStatesin themodelis exportedoutof theU.S. boundarylayer asNOx or PANs (15% in summer2, 5% in winter)
Fossil fuel combustion in the United States is estimated to account for about 10% of the total sourceof 03 in thenorthernhemispheretroposphereon an annualbasis
Fossil fuel combustion accounts for about 50% of the total fuel combustioins oxidizedwithintheboundarylayerof indusemissionof nitrogenoxides(NOx = NO + NO2) to the atmo- trialcontinentsw,hereNOxconcentratioanrsehighandhencethe sphere[IntergovernmentalPanel on Climate Change, (IPCC), 03 productioenfficienciyslow
Summary
UnitedStatesin fall whereboundarylayermixingis excessive dueto insufficienpt recipitation[JacobandPrather, 1990;Jacob. [this issue]to investigateseasonalvariations.For this purpose, we use time series of observations at non-urban sites that are at least 1 year long (Table 2). The model capturesthe springsummermaximumover the United States,althoughthe seasonal peakin the modeloverthe easternUnited Statesis shiftedto the endof the summer(August)reflectingthe seasonapl eakin GCM temperatures[Jacobet al., 1993a]. Ozone sonal cycle is larger in the easternthan in the westernUnited States,bothin the observations[Logan, 1989] andin the model; Figure 2 comparesthe simulatedand observeddistributions this regionaldifferenceis drivenby the high NOx emissionsin. Comorderto isolatetheseinfluenceswe conductedtwo sensitivity parisonof thesesensitivitysimulationsto the standardsimulation simulations(:1) a "no-chemistrys"imulationwhere is solely in Figure 4 showsthat regional productionmakes an important controlledby transporftromtheboundarieasnddeposition(;2) a contributionto 03 concentrationsin surfaceair over the United "zero-NOx"simulationwhereno NOx is emittedfrom the model Statesin all seasonsexcept winter.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have