Gail Tonnesen

Differences in NOy speciation predicted by three photochemical mechanisms

Abstract We describe the production and speciation of NOy in current chemical mechanisms using model simulations with three different chemical mechanisms run under three different chemical scenarios representative of a low-emission rural, a high-emission rural, and a heavily polluted urban scenario. In this study we examine predictions from the Carbon Bond 4 mechanism, the Regional Acid Deposition Model 2 mechanism, and an explicit mechanism used in global modeling. Several reactions are identified which influence the concentrations of important NOy species. The most important reaction under rural conditions is the oxidation of isoprene, which influences the production of PAN and organic nitrate. Differences among mechanisms cause large differences in the NOy concentrations, which propagate throughout the entire mechanism, affecting the chemistry of all species. The largest differences in NOy species among the mechanisms occur in the rural simulations, while predictions are the most similar under urban conditions, for which photochemical mechanisms have been studied and evaluated. The differences in NOy speciation cause uncertainties in the use of these mechanisms for predicting oxidant chemistry over long distances and multi-day episodes under non-urban conditions. Predictions from all mechanisms should be regarded with caution under rural conditions.

Effects of uncertainty in the reaction of the hydroxyl radical with nitrogen dioxide on model-simulated ozone control strategies

Abstract We evaluated the effect of a 20% reduction in the rate constant of the reaction of the hydroxyl radical with nitrogen dioxide to produce nitric acid (OH+NO 2 →HNO 3 ) on model predictions of ozone mixing ratios ([O 3 ]) and the effectiveness of reductions in emissions of volatile organic compounds (VOC) and nitrogen oxides (NO x ) for reducing [O 3 ]. By comparing a model simulation with the new rate constant to a base case scenario, we found that the [O 3 ] increase was between 2 and 6% for typical rural conditions and between 6 and 16% for typical urban conditions. The increases in [O 3 ] were less than proportional to the reduction in the OH+NO 2 rate constant because of negative feedbacks in the photochemical mechanism. Next, we used two different approaches to evaluate how the new OH+NO 2 rate constant changed the effectiveness of reductions in emissions of VOC and NO x : first, we evaluated the effect on [O 3 ] sensitivity to small changes in emissions of VOC (d[O 3 ]/d E VOC ) and NO x (d[O 3 ]/d E NO x ); and secondly, we used the empirical kinetic modeling approach to evaluate the effect on the level of emissions reduction necessary to reduce [O 3 ] to a specified level. Both methods showed that reducing the OH+NO 2 rate constant caused control strategies for VOC to become less effective relative to NO x control strategies. We found, however, that d[O 3 ]/d E VOC and d[O 3 ]/d E NO x did not quantitatively predict the magnitude of the change in the control strategy because the [O 3 ] response was nonlinear with respect to the size of the emissions reduction. We conclude that model sensitivity analyses calculated using small emissions changes do not accurately characterize the effect of uncertainty in model inputs (in this case, the OH+NO 2 rate constant) on O 3 attainment strategies. Instead, the effects of changes in model inputs should be studied using large changes in precursor emissions to approximate realistic attainment scenarios.

Evaluating a Space‐Based Indicator of Surface Ozone‐NOx‐VOC Sensitivity Over Midlatitude Source Regions and Application to Decadal Trends

Determining effective strategies for mitigating surface ozone (O3) pollution requires knowledge of the relative ambient concentrations of its precursors, NO x , and VOCs. The space-based tropospheric column ratio of formaldehyde to NO2 (FNR) has been used as an indicator to identify NO x -limited versus NO x -saturated O3 formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NO x -limited and NO x -saturated conditions may shift in space and time, (2) the ratio of the vertically integrated column may not represent the near-surface environment, and (3) satellite products contain errors. We use the GEOS-Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern midlatitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near-surface O3 production regime. Extending this surface-based predictor to a column-based FNR requires accounting for differences in the HCHO and NO2 vertical profiles. We compare four combinations of two OMI HCHO and NO2 retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite-derived FNR vary with the choice of NO2 product, while the mean offset depends on the choice of HCHO product. Space-based FNR indicates that the spring transition to NO x -limited regimes has shifted at least a month earlier over major cities (e.g., New York, London, and Seoul) between 2005 and 2015. This increase in NO x sensitivity implies that NO x emission controls will improve O3 air quality more now than it would have a decade ago.

Coupling between Chemical and Meteorological Processes under Persistent Cold-Air Pool Conditions: Evolution of Wintertime PM2.5 Pollution Events and N2O5 Observations in Utah’s Salt Lake Valley

The Salt Lake Valley experiences severe fine particulate matter pollution episodes in winter during persistent cold-air pools (PCAPs). We employ measurements throughout an entire winter from different elevations to examine the chemical and dynamical processes driving these episodes. Whereas primary pollutants such as NOx and CO were enhanced twofold during PCAPs, O3 concentrations were approximately threefold lower. Atmospheric composition varies strongly with altitude within a PCAP at night with lower NOx and higher oxidants (O3) and oxidized reactive nitrogen (N2O5) aloft. We present observations of N2O5 during PCAPs that provide evidence for its role in cold-pool nitrate formation. Our observations suggest that nighttime and early morning chemistry in the upper levels of a PCAP plays an important role in aerosol nitrate formation. Subsequent daytime mixing enhances surface PM2.5 by dispersing the aerosol throughout the PCAP. As pollutants accumulate and deplete oxidants, nitrate chemistry becomes less active during the later stages of the pollution episodes. This leads to distinct stages of PM2.5 pollution episodes, starting with a period of PM2.5 buildup and followed by a period with plateauing concentrations. We discuss the implications of these findings for mitigation strategies.

New insights into the column CH2O/NO2 ratio as an indicator of near‐surface ozone sensitivity

Author(s): Schroeder, JR; Crawford, JH; Fried, A; Walega, J; Weinheimer, A; Wisthaler, A; Muller, M; Mikoviny, T; Chen, G; Shook, M; Blake, DR; Tonnesen, GS | Abstract: ©2017. American Geophysical Union. All Rights Reserved. Satellite-based measurements of the column CH2O/NO2 ratio have previously been used to estimate near-surface ozone (O3) sensitivity (i.e., NOx or VOC limited), and the forthcoming launch of air quality-focused geostationary satellites provides a catalyst for reevaluating the ability of satellite-measured CH2O/NO2 to be used in this manner. In this study, we use a 0-D photochemical box model to evaluate O3 sensitivity and find that the relative rate of radical termination from radical-radical interactions to radical-NOx interactions (referred to as LROx/LNOx) provides a good indicator of maximum O3 production along NOx ridgelines. Using airborne measurements from NASAs Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relative to Air Quality (DISCOVER-AQ) deployments in Colorado, Maryland, and Houston, we show that in situ measurements of CH2O/NO2 can be used to indicate O3 sensitivity, but there is an important “transition/ambiguous” range whereby CH2O/NO2 fails to categorize O3 sensitivity, and the range and span of this transition/ambiguous range varies regionally. Then, we apply these findings to aircraft-derived column density measurements from DISCOVER-AQ and find that inhomogeneities in vertical mixing in the lower troposphere further degrades the ability of column CH2O/NO2 to indicate near-surface O3 sensitivity (i.e., the transition/ambiguous range is much larger than indicated by in situ data alone), and we hypothesize that the global transition/ambiguous range is sufficiently large to make the column CH2O/NO2 ratio unuseful for classifying near-surface O3 sensitivity. Lastly, we present a case study from DISCOVER-AQ-Houston that suggests that O3 sensitivity on exceedance days may be substantially different than on nonexceedance days (which may be observable from space) and explore the diurnal evolution of O3 sensitivity, O3 production, and the column CH2O/NO2 ratio. The results of these studies suggest that although satellite measurements of CH2O/NO2 alone may not be sufficient for accurately classifying near-surface O3 sensitivity, new techniques offered by geostationary platforms may nonetheless provide methods for using space-based measurements to develop O3 mitigation strategies.

Air pollutant source characterization using the revised regional Haze tracking metric and a photochemical grid model and implications for regional Haze planning

ABSTRACT The 2017 revisions to the Regional Haze Rule clarify that visibility progress at Class I national parks and wilderness areas should be tracked on days with the highest anthropogenic contributions to haze (impairment). We compare the natural and anthropogenic contributions to haze in the western United States in 2011 estimated using the Environmental Protection Agency (EPA) recommended method and using model projections from the Comprehensive Air Quality Model with Extensions (CAMx) and the Particulate Source Apportionment Tool (PSAT). We do so because these two methods will be used by states to demonstrate visibility progress by 2028. If the two methods assume different natural and anthropogenic contributions, the projected benefits of reducing U.S. anthropogenic emissions will differ. The EPA method assumes that episodic elevated carbonaceous aerosols greater than an annual 95th percentile threshold are natural events. For western U.S. IMPROVE monitoring sites reviewed in this paper, CAMx-PSAT confirms these episodes are impacted by carbon from wildfire or prescribed fire events. The EPA method assumes that most of the ammonium sulfate is anthropogenic in origin. At most western sites CAMx-PSAT apportions more of the ammonium sulfate on the most impaired days to global boundary conditions and anthropogenic Canadian, Mexican, and offshore shipping emissions than to U.S. anthropogenic sources. For ammonium nitrate and coarse mass, CAMx-PSAT apportions greater contributions to U.S. anthropogenic sources than the EPA method assigns to total anthropogenic contributions. We conclude that for western IMPROVE sites, the EPA method is effective in selecting days that are likely to be impacted by anthropogenic emissions and that CAMx-PSAT is an effective approach to estimate U.S. source contributions. Improved inventories, particularly international and natural emissions, and further evaluation of global and regional model performance and PSAT attribution methods are recommended to increase confidence in modeled source characterization. Implications: The western states intend to use the CAMx model to project visibility progress by 2028. Modeled visibility response to changes in U.S. anthropogenic emissions may be less than estimated using the EPA assumptions based on total U.S. and international anthropogenic contributions to visibility impairment. Additional model improvements are needed to better account for contributions to haze from natural and international emissions in current and future modeling years. These improvements will allow more direct comparison of model and EPA estimates of natural and anthropogenic contributions to haze and future visibility progress.

Modeling NH4NO3 Over the San Joaquin Valley During the 2013 DISCOVER‐AQ Campaign

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matter NH4NO3 during episodes of meteorological stagnation in winter. A rich data set of observations related to NH4NO3 formation was acquired during multiple periods of elevated NH4NO3 during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign in SJV in January and February 2013. Here NH4NO3 is simulated during the SJV DISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic model evaluation is performed using the DISCOVER-AQ data set, and integrated reaction rate analysis is used to quantify HNO3 production rates. Simulated NO3- generally agrees well with routine monitoring of 24-hr average NO3-, but comparisons with hourly average NO3- measurements in Fresno revealed differences at higher time resolution. Predictions of gas-particle partitioning of total nitrate (HNO3 + NO3-) and NHx (NH3 + NH4+) generally agree well with measurements in Fresno, although partitioning of total nitrate to HNO3 is sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning results indicate that NH4NO3 formation is limited by HNO3 availability in both the model and ambient. NH3 mixing ratios are underestimated, particularly in areas with large agricultural activity, and additional work on the spatial allocation of NH3 emissions is warranted. During a period of elevated NH4NO3, the model predicted that the OH + NO2 pathway contributed 46% to total HNO3production in SJV and the N2O5 heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO2 pathway for HNO3 production is predicted to increase as NOx emissions decrease.

Evaluation of the Community Multiscale Air Quality Model for Simulating Winter Ozone Formation in the Uinta Basin

The Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) models were used to simulate a 10 day high-ozone episode observed during the 2013 Uinta Basin Winter Ozone Study (UBWOS). The baseline model had a large negative bias when compared to ozone (O3) and volatile organic compound (VOC) measurements across the basin. Contrary to other wintertime Uinta Basin studies, predicted nitrogen oxides (NO x ) were typically low compared to measurements. Increases to oil and gas VOC emissions resulted in O3 predictions closer to observations, and nighttime O3 improved when reducing the deposition velocity for all chemical species. Vertical structures of these pollutants were similar to observations on multiple days. However, the predicted surface layer VOC mixing ratios were generally found to be underestimated during the day and overestimated at night. While temperature profiles compared well to observations, WRF was found to have a warm temperature bias and too low nighttime mixing heights. Analyses of more realistic snow heat capacity in WRF to account for the warm bias and vertical mixing resulted in improved temperature profiles, although the improved temperature profiles seldom resulted in improved O3 profiles. While additional work is needed to investigate meteorological impacts, results suggest that the uncertainty in the oil and gas emissions contributes more to the underestimation of O3. Further, model adjustments based on a single site may not be suitable across all sites within the basin.