Abstract

Abstract. For the purposes of developing optimal emissions control strategies, efficient approaches are needed to identify the major sources or groups of sources that contribute to elevated ozone (O3) concentrations. Source-based apportionment techniques implemented in photochemical grid models track sources through the physical and chemical processes important to the formation and transport of air pollutants. Photochemical model source apportionment has been used to track source impacts of specific sources, groups of sources (sectors), sources in specific geographic areas, and stratospheric and lateral boundary inflow on O3. The implementation and application of a source apportionment technique for O3 and its precursors, nitrogen oxides (NOx) and volatile organic compounds (VOCs), for the Community Multiscale Air Quality (CMAQ) model are described here. The Integrated Source Apportionment Method (ISAM) O3 approach is a hybrid of source apportionment and source sensitivity in that O3 production is attributed to precursor sources based on O3 formation regime (e.g., for a NOx-sensitive regime, O3 is apportioned to participating NOx emissions). This implementation is illustrated by tracking multiple emissions source sectors and lateral boundary inflow. NOx, VOC, and O3 attribution to tracked sectors in the application are consistent with spatial and temporal patterns of precursor emissions. The O3 ISAM implementation is further evaluated through comparisons of apportioned ambient concentrations and deposition amounts with those derived from brute force zero-out scenarios, with correlation coefficients ranging between 0.58 and 0.99 depending on specific combination of target species and tracked precursor emissions. Low correlation coefficients occur for chemical regimes that have strong nonlinearity in O3 sensitivity, which demonstrates different functionalities between source apportionment and zero-out approaches, where appropriate use depends on whether source attribution or source sensitivity is desired.

Highlights

  • Regulatory programs have been in place in the United States for more than 50 years to reduce ambient exposure to ozone (O3) which has harmful effects on human health and vegetation (Bell et al, 2004; U.S Environmental Protection Agency (EPA), 2009; National Research Council, 1991)

  • Implementation of O3 tracking capability in Community Multiscale Air Quality (CMAQ)-Integrated Source Apportionment Method (ISAM) for the Carbon Bond 2005 (CB05) gas-phase chemical mechanism adopts the tworegime approach, with nitrogen and volatile organic compounds (VOCs) species explicitly tracked through all chemical transport model science processes to facilitate analysis of their chemical and physical transformations

  • Brute force zero-out CMAQ model simulations serve as a reference to compare the ISAM results applied for a California application in the summer of 2007

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Summary

Introduction

Regulatory programs have been in place in the United States for more than 50 years to reduce ambient exposure to ozone (O3) which has harmful effects on human health and vegetation (Bell et al, 2004; U.S Environmental Protection Agency (EPA), 2009; National Research Council, 1991). Many areas continue to exceed the national ambient air quality standard (NAAQS) for ozone, and uncertainty remains in both the local and distant sources that contribute to exceedances of the NAAQS. In areas that violate the NAAQS, the states and tribes must develop plans to attain the NAAQS by reducing emissions of O3 precursors, including volatile organic compounds (VOCs) and nitrogen oxides (NOx). For air quality managers who are tasked with developing the most expeditious and cost effective emissions control strategies, it is useful to have methods to identify the relative importance of sources that contribute to high O3 concentrations, and to predict how O3 will respond to reductions in VOC and NOx precursor emissions. The Integrated Source Apportionment Method (ISAM) for PM2.5 was previously implemented in the Community Multiscale Air Quality (CMAQ) model (Kwok et al, 2013). This implementation is compared directly with source sensitivity apportionment approaches to provide confidence in the implementation

Review of ozone source apportionment methods
Implementation overview
Ozone regime indicators
Application and evaluation
Sector contributions
Summary and discussion
Findings
Code availability
Full Text
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