Elsevier

Atmospheric Environment

Volume 81, December 2013, Pages 421-432
Atmospheric Environment

WRF-Chem simulation of NOx and O3 in the L.A. basin during CalNex-2010

https://doi.org/10.1016/j.atmosenv.2013.08.064Get rights and content

Highlights

  • NOx emissions and O3 chemistry in the Los Angeles Basin are evaluated.

  • NOx is overestimated in the baseline scenario (24% reduction relative to NEI’05).

  • A 45% reduction of NOx emissions relative to NEI’05 significantly improves the comparison of model to observations.

  • Modeled Weekend–Weekday differences are much smaller, indicating weekend NOx emissions are still overestimated.

  • Improved understanding of volatile organic compound emissions and photochemical processing on weekdays are needed.

Abstract

NOx emissions and O3 chemistry in the Los Angeles (L.A.) Basin during the CalNex-2010 field campaign (May–June 2010) have been evaluated by analyzing O3 and NOy (NO, NO2, HNO3, PAN) observations using a regional air quality model (WRF-Chem). Model simulations were conducted at 4-km spatial resolution over the basin using the Carbon-Bond Mechanism version Z (CBM-Z) and NOx emissions reduced by 24% relative to 2005 National Emissions Inventory (NEI’05), according to recent state emission statistics (BASE_NOx scenario). In addition, a 22–26% NOx emission reduction from weekday to weekend was applied. WRF-Chem reproduced the observed diurnal cycle and day-to-day variations in surface O3, Ox, HNO3 and HCHO (correlation r2 = 0.57 − 0.63; pairs of data n > 400; confidence value p < 0.01) at the CalNex supersite at Caltech but consistently overestimated surface NO and NO2. A 45% reduction of NOx emissions relative to NEI’05 (LOW_NOx scenario), as suggested by the OMI-NO2 column trend in California over the same period, improved the agreement of modeled NO2, NOx, and O3 with observations on weekdays. Three-dimensional distributions of daytime O3 and NOy were compared with five daytime NOAA WP-3D flights (three on weekdays and two on weekends) to study the Weekend-to-Weekday (WE-to-WD) effects by using the LOW_NOx scenario. Aircraft data showed a 17.3 ppb O3 increase and a 54% NOy reduction in the boundary layer on weekends relative to weekdays, while modeled WE-to-WD differences were much smaller, with a 2.9 ppb O3 increase and 16% NOy reduction only. Model results on weekends underestimated O3 by 23% and overestimated NOy and HNO3 by 40% and 27%, respectively, which may indicate that weekend NOx emissions (45% reduction relative to NEI’05 with a 22–26% reduction on weekends compared to weekdays) were still overestimated in the model. Comparisons of PAN to HNO3 ratios also indicated that the enhanced photochemistry on weekends was not well represented in the model. Although modeled weekday O3 was close to the observations in the boundary layer, modeled PAN and HNO3 were overestimated by 30% and 22%, respectively, and modeled NOy was underestimated by 24% on weekdays. Interpreted as emission ratios, the slopes of volatile organic compound (VOC) species versus CO concentrations indicated that speciated VOC emissions in the model were not accurately represented, impacting the photochemistry in the model. These findings argue for the need to improve our understanding of VOC emissions and their photochemical processing in the model.

Introduction

The Los Angeles (L.A.) basin has been ranked as the most polluted megacity in the U.S. with respect to O3 (The American Lung Association, 2012). High O3 in the L.A. basin results from the extremely high emission of O3 precursors, nitrogen oxides NOx = NO + NO2 and volatile organic compounds (VOC). Over the past two decades (1985–2005), the total emissions of VOC and NOx in the South Coast Air Basin (SCAB) have been reduced significantly (California Air Resources Board, CARB, 2012c; data available at http://www.arb.ca.gov/app/emsinv/emssumcat.php). Although decades of emission reductions have been effective at mitigating O3 levels, many monitoring locations in the SCAB are still not in attainment of the National Ambient Air Quality Standard (NAAQS) (www.aqmd.gov).

Regional air quality models are useful tools to study this air pollution problem. Previous modeling studies have revealed that the photochemical pollution in the basin is exacerbated by the fact that the high emission intensity area is surrounded by mountains, which tends to trap the air pollution as it is blown eastward by the prevailing westerly winds (e.g. Lu and Turco, 1995, Lu and Turco, 1996). However, the application of regional models to the L.A. basin is challenging due to the combined effects of complex terrain-meteorology and high emissions in the basin. The challenges and the evaluations of the terrain-meteorology simulations using the WRF-Chem model for the L.A. basin have been described in an accompanying paper, Chen et al. (2013). The primary goal of the current paper is to evaluate the model capability to simulate the photochemistry in the basin.

Another motivation for revisiting O3 pollution in the basin is the need to understand the effect of NOx emissions, which changes in terms of the total amounts, major sources and temporal variation. Many studies have observed the recent decrease of NOx emissions in the U.S. and most of them show evidence that the decrease is due to the implementation of NOx control technology in power plants (e.g. Kim et al., 2006, Kim et al., 2009). Fuel-consumption-based calculations and ambient measurements have also been used to estimate exhaust NOx emissions from mobile sources (e.g. Parrish, 2006, Harley et al., 2005). A 9–10% per year reduction trend was found during the 2007–2009 period (McDonald et al., 2012) in the L.A. basin, where mobile sources account for ∼80% of total NOx emissions. In the study of Russell et al. (2010), OMI satellite data and ground surface observational data were used to constrain NOx emissions in the California region. The observed decrease of NO2 vertical columns in Los Angeles and the surrounding cities was 9% per year for 2005–2008. Brioude et al. (2011) presented top-down estimates of anthropogenic NOx surface fluxes and found that NOx emissions were 32 ± 10% lower in the L.A. county relative to NEI’05 for 2010. However, statistics from the California Air Resources Board (CARB) show a much slower reduction. CARB reports a decrease in total NOx from 2005 to 2010 of around 24% (CARB, 2012c; data available at http://www.arb.ca.gov/app/emsinv/emssumcat.php). In addition to the uncertainties of total NOx emission reductions, observations also showed a large Weekend-to-Weekday (WE-to-WD) NOx emission reduction, which leads to higher O3 on weekends (Pollack et al., 2012, Russell et al., 2010).

In this work, we apply the WRF-Chem model to simulate O3 and NOy species for selected days in May–June 2010 in the L.A. Basin during the CalNex-2010 intensive field experiment, focusing on three weekday daytime flights of the NOAA WP-3D research aircraft on May 4, 14 and 19, and two weekend daytime flights on May 16 and June 20. We conducted simulations at 4-km spatial resolution over the L.A. Basin using the Carbon-Bond Mechanism version Z (CBM-Z). In an accompanying paper (Chen et al., 2013), we described the model configurations and evaluations of meteorological and carbon monoxide (CO) predictions. This study evaluates chemical predictions resulting from two adjusted NOx emission scenarios. The monitoring database includes CARB stations, surface observations on the campus of the California Institute for Technology (Caltech), and the aircraft data. Chemical species evaluated in our study include ozone (O3), NOx (= NO + NO2), formaldehyde (HCHO), nitric acid (HNO3) and peroxyacetyl nitrate (PAN).

In this manuscript, we validate the model chemical predictions through comparisons of the modeled surface O3, NOy, NO, NO2, Ox, HCHO and HNO3 mixing ratios with those measured at the Caltech site. The NOx emission inventory is evaluated by comparing the simulated O3 and NOx to measurements at 20 CARB surface sites. More definitive conclusions regarding the total NOx emissions, model capabilities and Weekend-to-Weekday (WE-to-WD) effects are obtained by comparing the model against daytime aircraft observations on weekdays and weekends. The WRF-Chem model deficiencies are discussed in the context of these comparisons.

Section snippets

WRF-Chem description

The Weather Research and Forecasting (WRF) model coupled with online chemistry (WRF-Chem) version 3.1.1 was used in this study (http://ruc.noaa.gov/wrf/WG11/; Grell et al., 2005, Fast et al., 2006). Three nested domains with 36-, 12- and 4- km horizontal resolution cover the regions of the western United States, California and the L.A. basin, respectively (Fig. 1 in Chen et al., 2013). The topography in the 4-km domain is shown in Fig. 1a. There are 30 vertical layers extending from the surface

Comparisons at surface sites

We evaluated WRF-Chem simulations against surface observations at the Caltech super site and CARB sites. The measurements at the Caltech super site were compared to simulations with the baseline emission scenario (BASE_NOx). CARB observations were compared with model results using two emission scenarios BASE_NOx and LOW_NOx. Modeled and observed hourly concentrations, model bias, standard deviations (S.D.), root mean square error (RMSE) and correlations r2 are listed in Tables 2 and 3. To

Summary and conclusions

A regional dynamical/chemical model (WRF-Chem) was used to simulate and analyze O3, NOx emissions, and NOx oxidation productions in the L.A. basin during the CalNex-2010 campaign in May and June 2010. A series of 4 km WRF-Chem model simulations using the NEI’05 derived emissions inventories (two scenarios with NOx emissions reduced by 24% and 45% relative to NEI’05) were compared with in-situ aircraft measurements and surface observations to evaluate the NOx emission inventory, model

Acknowledgments

The authors would like to thank Caltech for providing the site and CARB for the infrastructure at the site. The author is grateful to Stu McKeen and Si-wan Kim for providing the emission inventory and helping set up the model. Thanks are also given to Ken Aikin at NOAA for providing the WP-3D meteorological data. This study was funded by the NOAA Global Atmospheric Composition and Climate Program.

References (55)

  • G. Yarwood et al.

    Modeling weekday to weekend changes in emissions and ozone in the Los Angeles basin for 1997 and 2010

    Atmos. Environ.

    (2008)
  • American Lung Association

    State of the Air Report

    (2012)
  • C.L. Blanchard et al.

    Differences between weekday and weekend air pollutant levels in southern California

    J. Air Waste Manage.

    (2003)
  • J. Brioude et al.

    Top-down estimate of anthropogenic emission inventories and their interannual variability in Houston using a mesoscale inverse modeling technique

    J. Geophys. Res-atmos.

    (2011)
  • California Air Resource Board

    2009 Almanac Emission Projection Data

    (2012)
  • W.P.L. Carter

    2003. VOC Reactivity Data

    (2013)
  • W.P.L. Carter

    Development of ozone reactivity scales for volatile organic-compounds

    J. Air Waste Manage.

    (1994)
  • D. Chen et al.

    WRF-chem Simulation of Meteorology and CO in the Los Angeles Basin During CalNex-2010

    (2013)
  • L.R. Chinkin et al.

    Weekday versus weekend activity patterns for ozone precursor emissions in California's South Coast Air Basin

    J. Air Waste Manage.

    (2003)
  • J.A. de Gouw et al.

    Validation of proton transfer reaction-mass spectrometry (PTR-MS) measurements of gas-phase organic compounds in the atmosphere during the New England Air Quality Study (NEAQS) in 2002

    J. Geophys. Res-atmos.

    (2003)
  • D.B. Dreher et al.

    A fuel-based inventory for heavy-duty diesel truck emissions

    J. Air Waste Manage.

    (1998)
  • W.P. Dube et al.

    Aircraft instrument for simultaneous, in situ measurement of NO3 and N2O5 via pulsed cavity ring-down spectroscopy

    Rev. Sci. Instrum.

    (2006)
  • J.D. Fast et al.

    Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology-chemistry-aerosol model

    J. Geophys. Res-atmos.

    (2006)
  • E.M. Fujita et al.

    Diurnal and weekday variations in the source contributions of ozone precursors in California's South Coast Air Basin

    J. Air Waste Manage.

    (2003)
  • A. Geyer et al.

    Vertical profiles of NO3, N2O5, O-3, and NOx in the nocturnal boundary layer: 2. Model studies on the altitude dependence of composition and chemistry (vol. 109, art no D16399, 2004)

    J. Geophys. Res-atmos.

    (2004)
  • A.B. Guenther et al.

    Isoprene and monoterpene emission rate variability: model evaluations and sensitivity analyses

    J. Geophys. Res.

    (1993)
  • R.A. Harley et al.

    Changes in motor vehicle emissions on diurnal to decadal time scales and effects on atmospheric composition

    Environ. Sci. Technol.

    (2005)
  • Cited by (33)

    • A comprehensive study on ozone pollution in a megacity in North China Plain during summertime: Observations, source attributions and ozone sensitivity

      2021, Environment International
      Citation Excerpt :

      These performance in city scale simulation were comparably lower than those in the regional simulations especially for NO2, which is possibly caused by the model biases in estimating the planetary boundary layer (PBL) or unpredicted human activities at nighttime (Li et al., 2017b, 2010). However, the statistical results were still fall into the range in the city-scale literatures (Bocquet et al., 2015; Chen et al., 2013), indicating a satisfied modeling precipitation process of this study. As the importance to establish efficient O3 control strategies, the sources of O3 formations from either local emission sources or regional transportations must be defined.

    View all citing articles on Scopus
    1

    Now at: National Center for Atmospheric Research, Boulder, Colorado, USA.

    2

    Now at: Department of Mechanical Engineering, University of Colorado Boulder, Boulder, Colorado, USA.

    View full text