Exceptional Event Demonstration for Ozone Exceedances in Clark County, Nevada – August 18-21, 2020 Final Report Prepared for U.S. EPA Region 9 San Francisco, CA September 2021 This document contains blank pages to accommodate two-sided printing. Exceptional Event Demonstration for Ozone Exceedances in Clark County, Nevada – August 18-21, 2020 Prepared by Prepared for Steve Brown, PhD Clark County Department of Environment Crystal McClure, PhD and Sustainability Cari Gostic Division of Air Quality David Miller, PhD 4701 W. Russell Road, Suite 200 Nathan Pavlovic Las Vegas, NV 89118 Charles Scarborough Ph 702.455.3206 Ningxin Wang, PhD www.clarkcountynv.gov Sonoma Technology 1450 N. McDowell Blvd., Suite 200 Petaluma, CA 94954 Ph 707.665.9900 | F 707.665.9800 sonomatech.com Final Report STI-920053-7477 September 1, 2021 Cover graphic illustrates daily HMS smoke over the United States for one day prior to and during the August 18-21 event. See Section 3.1.2 for more information. ● ● ● Contents Contents Figures ......................................................................................................................................................................................... iv Tables.......................................................................................................................................................................................... viii Executive Summary ..................................................................................................................... ES-1 1. Overview ..................................................................................................................................... 1-1 1.1 Introduction ......................................................................................................................................................... 1-1 1.2 Exceptional Event Rule Summary ................................................................................................................ 1-4 1.3 Demonstration Outline .................................................................................................................................... 1-5 1.4 Conceptual Model ............................................................................................................................................. 1-8 2. Historical and Non-Event Model ......................................................................................... 2-1 2.1 Regional Description ........................................................................................................................................ 2-1 2.2 Overview of Monitoring Network ............................................................................................................... 2-1 2.3 Characteristics of Non-Event Historical O3 Formation ........................................................................ 2-3 3. Clear Causal Relationship Analyses .................................................................................... 3-1 3.1 Tier 1 Analyses .................................................................................................................................................... 3-1 3.1.1 Comparison of Event with Historical Data ............................................................................... 3-1 3.1.2 Ozone, Fire, and Smoke Maps ...................................................................................................... 3-6 3.1.3 HYSPLIT Trajectories ...................................................................................................................... 3-15 3.1.4 Media Coverage and Ground Images ..................................................................................... 3-50 3.2 Tier 2 Analyses ................................................................................................................................................. 3-56 3.2.1 Key Factor #1: Q/d Analysis ........................................................................................................ 3-56 3.2.2 Key Factor #2: Comparison of Event Concentrations with Non-Event Concentrations................................................................................................................................. 3-75 3.2.3 Satellite Retrievals of Pollutant Concentrations .................................................................. 3-81 3.2.4 Supporting Pollutant Trends and Diurnal Patterns ............................................................ 3-87 3.3 Tier 3 Analyses ................................................................................................................................................. 3-95 3.3.1 Total Column and Meteorological Conditions .................................................................... 3-95 3.3.2 Matching Day Analysis ............................................................................................................... 3-110 3.3.3 GAM Statistical Modeling .......................................................................................................... 3-119 3.4 Clear Causal Relationship Conclusions ................................................................................................. 3-140 4. Natural Event Unlikely to Recur........................................................................................... 4-1 5. Not Reasonably Controllable or Preventable .................................................................. 5-1 6. Public Comment ...................................................................................................................... 6-1 7. Conclusions and Recommendations .................................................................................. 7-1 8. References ................................................................................................................................. 8-1 ● ● ● iii ● ● ● Figures Figures 2-1. Regional topography around Clark County, with an inset showing county boundaries and the air quality monitoring sites analyzed in this report. ................................................................................. 2-0 2-2. Clark County topography, with an inset showing air quality monitoring sites that measure ozone in the Clark County area................................................................................................................................ 2-0 2-3. Time series of 2015-2020 ozone concentrations at Joe Neal. .................................................................... 2-4 2-4. Time series of 2015-2020 ozone concentrations at Paul Meyer. ............................................................... 2-5 2-5. Time series of 2015-2020 ozone concentrations at Walter Johnson. ...................................................... 2-6 2-6. Time series of 2015-2020 ozone concentrations at Green Valley. ............................................................ 2-7 2-7. Seasonality of 2015-2020 ozone concentrations from Joe Neal. .............................................................. 2-8 2-8. Seasonality of 2015-2020 ozone concentrations from Paul Meyer. ......................................................... 2-9 2-9. Seasonality of 2015-2020 ozone concentrations from Walter Johnson. ............................................. 2-10 2-10. Seasonality of 2015-2020 ozone concentrations from Green Valley. ................................................. 2-11 2-11. Ozone time series at all monitoring sites ....................................................................................................... 2-12 3-1. Time series of 2020 MDA8 ozone concentrations from Joe Neal. ............................................................. 3-2 3-2. Time series of 2020 MDA8 ozone concentrations from Paul Meyer. ....................................................... 3-3 3-3. Time series of 2020 MDA8 ozone concentrations from Walter Johnson. .............................................. 3-4 3-4. Time series of 2020 MDA8 ozone concentrations from Green Valley. .................................................... 3-5 3-5. Daily ozone AQI one day prior to and during the August 18-21 event. ................................................ 3-7 3-6. Daily PM2.5 AQI one day prior to and during the August 18-21 event. .................................................. 3-8 3-7. Daily HMS smoke over the United States for one day prior to and during the August 18-21 event. ............................................................................................................................................................................... 3-10 3-8. Visible satellite imagery from over California, Nevada, Arizona, and Utah on August 17, 2020. ................................................................................................................................................................................ 3-11 3-9. Visible satellite imagery from over California, Nevada, Arizona, and Utah on August 18, 2020. ................................................................................................................................................................................ 3-12 3-10. Visible satellite imagery from California, Nevada, Arizona, and Utah on August 19, 2020. ....... 3-13 3-11. Visible satellite imagery from California, Nevada, Arizona, and Utah on August 20, 2020. ....... 3-14 3-12. Visible satellite imagery from California, Nevada, Arizona, and Utah on August 21, 2020.