Abstract. As part of the Hemispheric Transport of Air Pollution (HTAP; http://www.htap.org/) project, we analyze results from 16 global and hemispheric chemical transport models and compare these to Clean Air Status and Trends Network (CASTNet) observations in the United States (US) for 2001. Using the policy-relevant maximum daily 8-h ozone (MDA8 O3) statistic, the multi-model ensemble represents the observations well (mean r2=0.57, ensemble bias=+4.1 ppbv for all regions and all seasons) despite a wide range in the individual model results. Correlations are strongest in the NorthEastern US during spring and fall (r2=0.68); and weakest in the Midwestern US in summer (r2=0.46). However, large positive mean biases exist during summer for all Eastern US regions, ranging from 10–20 ppbv, and a smaller negative bias is present in the Western US during spring (~3 ppbv). In most all other regions and seasons, the biases of the model ensemble simulations are ≤5 ppbv. Sensitivity simulations in which anthropogenic O3-precursor emissions (NOx+NMVOC+CO+aerosols) were decreased by 20% in each of four source regions: East Asia (EA), South Asia (SA), Europe (EU) and North America (NA) show that the greatest response of MDA8 O3 to the summed foreign emissions reductions occurs during spring in the West (0.9 ppbv reduction due to 20% reductions from EA+SA+EU). East Asia is the largest contributor to MDA8 O3 at all ranges of the O3 distribution for most regions (typically ~0.45 ppbv). The exception is in the NorthEastern US where European emissions reductions had the greatest impact on MDA8 O3, particularly in the middle of the MDA8 O3 distribution (response of ~0.35 ppbv between 35–55 ppbv). In all regions and seasons, however, O3-precursor emissions reductions of 20% in the NA source region decrease MDA8 O3 the most – by a factor of 2 to nearly 10 relative to foreign emissions reductions. The O3 response to anthropogenic NA emissions is greatest in the Eastern US during summer at the high end of the O3 distribution (5–6 ppbv for 20% reductions). While the impact of foreign emissions on surface O3 in the US is not negligible – and is of increasing concern given the growth in emissions upwind of the US – domestic emissions reductions remain a far more effective means of decreasing MDA8 O3 values, particularly those above 75 ppb (the current US standard).
We conduct a diagnostic analysis of ozone chemistry simulated by four different configurations of a Global Climate-Chemistry Model (GCCM), the Community Earth System Model (CESM) with detailed tropospheric chemistry. The purpose of this study is to evaluate the ability of GCCMs to simulate future ozone chemistry by evaluating their ability to simulate present-day chemistry. To address this we chose four configurations of the CESM that differ in their meteorology (analyzed versus simulated meteorological fields), number of vertical levels, and the coupling of the ice and ocean models. We apply mixed model statistics to evaluate these different configurations against CASTNET ozone observations within different regions of the US by using various performance metrics relevant to evaluating future ozone changes. These include: mean biases and interannual variability, the ozone response to emission changes, the ozone response to temperature changes and ozone extreme values. Using these metrics, we find that although the configuration using analyzed meteorology best simulates temperatures it does not outperform a configuration with simulated meteorology in other metrics. All configurations are unable to capture observed ozone decreases and the ozone north-south gradient over the eastern US during 1995–2005. We find that the configuration with simulated meteorology with 56 vertical levels is markedly better in capturing observed ozone-temperature relationships and extreme values than a configuration that is identical except that it contains 26 vertical levels. We recommend caution in the use of GCCMs in simulating surface chemistry as differences in a variety of model parameters have a significant impact on the resulting chemical and climate variables. Isoprene emissions depend strongly on surface temperature and the resulting ozone chemistry is dependent on isoprene emissions but also on cloud cover, photolysis, the number of vertical levels, and the choice of meteorology. These dependencies must be accounted for in the interpretation of GCCM results.
The two phases of the quasi-biennial oscillation in ozone are simulated using winds generated by a three-dimensional mechanistic stratospheric model input into an off-line ozone transport model. Ozone chemistry is parameterized in the off-line model. The mechanistic model is run with either easterly or westerly zonal winds in the lower equatorial stratosphere, so as to model the equatorial wind structure during the two phases of the equatorial quasi-biennial oscillation (QBO). When forcing is applied at the lower model boundary in the winter hemisphere, the mechanistic model simulates differences in the global circulation between the easterly and westerly phases of the QBO. The resulting modeled total ozone is larger in the polar regions during the easterly phase of the QBO than during the westerly phase, in agreement with observations. Using the residual-mean formalism the authors find that the difference in the modeled budget of ozone between the two phases of the QBO is due to a modulation of the extratropical planetary wave structure, and consequently the ozone transport, by the equatorial zonal-mean winds. Differences in the residual-mean velocities between the two phases of the QBO explain most of the differences in the ozone transport.
We present an approach to constrain simulated atmospheric black carbon (BC) using carbon monoxide (CO) observations. The approach uses: (1) the Community Atmosphere Model with Chemistry to simulate the evolution of BC and CO within an ensemble of model simulations; (2) satellite CO retrievals from the MOPITT/Terra instrument to assimilate observed CO into these simulations; (3) the derived sensitivity of BC to CO within these simulations to correct the simulated BC distributions. We demonstrate the performance of this approach through model experiments with and without the BC corrections during the period coinciding with the Intercontinental Chemical Transport Experiment (INTEX‐B). Our results show significant improvements (∼50%) in median BC profiles using constraints from MOPITT, based on comparisons with INTEX‐B measurements. We find that assimilating MOPITT CO provides considerable impact on simulated BC concentrations, especially over source regions. This approach offers an opportunity to augment our current ability to predict BC distributions.
A simplified box model extracted from tropospheric photochemistry is investigated as the influx of NO ( F NO ) is increased. A subcritical Hopf bifurcation is encountered as F NO is increased, beyond which the steady state is unstable, and the system evolves to an oscillatory state resulting from alternate dominance of two radical chain processes. The first is an O 3 ‐consuming process of net stoichiometry, CO + O 3 → {CO 2 } + {O 2 }, occurring at high [CO] and [O 3 ] and very low [NO x ], but leading to increased [NO x ] via F NO as CO and O 3 are depleted. As [NO x ] thus grows, passes through a maximum, and then declines, the second, an O 3 ‐producing process of net stoichiometry, CO + 2{O 2 } + hν → {CO 2 } + O 3 , is dominant. It remains so as O 3 and CO accumulate (CO via its influx, F CO ) until [NO x ] again reaches very low values at high [O 3 ] and [CO] as a result of the removal of NO 2 by HO photochemically generated from the increasing [O 3 ]. This alternation occurs because each process affects [NO x ] and [HO x ] so as to lead to dominance of the other. A period‐doubling transition to chaotic oscillation occurs as F NO is increased further. The embedding dimension of the chaos is estimated to be four, and the original six‐variable model can be reduced to a four‐variable (CO, O 3 , [NO + NO 2 ] and [HO + HO 2 ]) system that behaves nearly identically to the full six‐variable model. While the oscillatory and chaotic periods seem too long (at least weeks) to be observed in real atmospheres, the model displays the nonlinear nature and dynamic instability of tropospheric photochemistry and offers insight into the behavior of and transitions between higher and lower [NO x ] states, which may be observable. The importance of the ratios [CO]/[NO 2 ] and [O 3 ]/[NO] to net O 3 change is illustrated. The appearance of this instability suggests that predictions based upon the temporal evolution of this or even more complex models of tropospheric chemistry sometimes may be very sensitive to the exact initial conditions of [CO]/[NO 2 ] and [O 3 ]/[NO] prevailing.
To study stratosphere–troposphere exchange, an approach based on the nonconservation of potential vorticity (PV) is developed; this approach arises naturally if one defines the tropopause in terms of PV. The evolution of a tropopause fold simulated by a mesoscale model is studied, as well as the evolution of PV at the tropopause level. The PV framework also permits the identification of the physical processes responsible for the cross-tropopause exchange as either diffusive or diabatic. In this model simulation, the diabatic processes are found to be the most important in the exchange. In particular, it is found that the negative heating gradient in the region of the warm sector of the surface cyclone is responsible for most of the diabatic exchange across the tropopause. The mass exchange during the tropopause folding event is estimated to be around 4.9 × 1014 kg in four days over the domain considered (1600 × 2000 km). This number is shown to correspond to the net difference between exchange from stratosphere to troposphere (23.5 × 1014 kg) and exchange from troposphere to stratosphere (18.6 × 1014 kg). Using the results from the exchange of a passive tracer, the exchange of ozone is estimated to be of the order of 1.1 × 108 kg. Finally, the origin of the air exchanged is found to be from the lower stratosphere and the upper troposphere, for the period of four days studied.
An episodic simulation is conducted to characterize ozone (O 3 ) formation and to investigate the dependence of O 3 formation on precursors in the Houston‐Galveston (HG) area using a regional chemical transport model (CTM). The simulated net photochemical O 3 production rates, P (O 3 ), in the Houston area are higher than those in most other U.S. urban cities, reaching 20–40 ppb hr −1 for the daytime ground NO x levels of 5–30 ppb. The NO x turnaround value (i.e., the NO x concentration at which P (O 3 ) reaches a maximum) is also larger than those observed in most other U.S. cities. The large abundance and high reactivity of anthropogenic volatile organic compounds (AVOCs) and the coexistence of abundant AVOCs and NO x in this area are responsible for the high O 3 production rates and the NO x turnaround value. The simulated O 3 production efficiency is typically 3–8 O 3 molecules per NO x molecule oxidized during the midday hours. The simulation reveals a RO 2 peak up to 70 ppt at night, and the reactions of alkene‐NO 3 and alkene‐O 3 are responsible for more than 80% of the nighttime RO 2 in the residual layer, contributing to over 70% and about 10%, respectively. Isoprene accounts for about 40% of the nighttime RO 2 peak concentration. The nighttime RO 2 level is limited by the availability of alkenes. Hydrolysis of N 2 O 5 on sulfate aerosols leads to an increase of HNO 3 by as much as 30–60% but to a decrease of NO x by 20–50% during the night in the lower troposphere. Heterogeneous conversion of NO 2 to HONO on the surfaces of soot aerosol accelerates the O 3 production by about 1 hour in the morning and leads to a noticeable increase of 7 ppb on average in the daytime O 3 level. The sensitivity study suggests that the near‐surface chemistry over most of the Houston metropolitan area is in or close to the NO x ‐VOC transition regime on the basis of the current emission inventory. Doubling AVOC emissions leads to the NO x sensitive chemistry. Biogenic VOCs contribute about 5% on the average to the total near‐surface O 3 in the Houston area.
