Estimating local versus regional contributions to tropospheric ozone: An example case study for Las Vegas Mark Green and Dave DuBois Desert Research Institute

The Problem With new 8-hour ozone standard of 75 ppb and tendency of polluted air to affect downwind areas, local versus transported pollutant contributions to ozone is important With new 8-hour ozone standard of 75 ppb and tendency of polluted air to affect downwind areas, local versus transported pollutant contributions to ozone is important Monitoring concentrated in urban areas and large geographic areas of the southwestern US may exceed the new standards but sufficient data does not exist to define the areas exceeding standards Monitoring concentrated in urban areas and large geographic areas of the southwestern US may exceed the new standards but sufficient data does not exist to define the areas exceeding standards How do we even know where to monitor with limited resources to define the areas of high concentrations? How do we even know where to monitor with limited resources to define the areas of high concentrations? Use existing monitoring data, EI’s, and Chemical Transport Models to help design expanded monitoring network needed Use existing monitoring data, EI’s, and Chemical Transport Models to help design expanded monitoring network needed

This presentation We present some results of analysis of local versus regional contributions to ground-level ozone in Las Vegas done in support of the Clark County Regional Ozone and Precursors Study (CCROPS) We present some results of analysis of local versus regional contributions to ground-level ozone in Las Vegas done in support of the Clark County Regional Ozone and Precursors Study (CCROPS) We demonstrate some methodologies that might be useful elsewhere We demonstrate some methodologies that might be useful elsewhere Disclaimer/Excuse- we were paid by Clark County, Nevada only to collect upper air data (SODARS, a radar wind profiler, and radiosondes). They paid someone else for analysis of ozone patterns, but we did some analysis anyway because we were interested. So we were not as thorough as we would have been had we gotten paid to do the data analysis. Disclaimer/Excuse- we were paid by Clark County, Nevada only to collect upper air data (SODARS, a radar wind profiler, and radiosondes). They paid someone else for analysis of ozone patterns, but we did some analysis anyway because we were interested. So we were not as thorough as we would have been had we gotten paid to do the data analysis.

2005 Danger if you are downwind of California! (or Vegas or Phoenix) Annual number of exceedances 4 th highest O3

Typical summer California wind flow pattern- combination of sea-breeze and mountain-valley wind circulations

Frequency of Tehachapi Pass tracer above background (July 12- Aug ) High frequency of flow from O 3 rich San Joaquin Valley to the SE, then NE

Frequency of El Centro tracer above background (July 12 – Aug ) Convergence zone transports emissions from Mexicali through LA Basin and San Joaquin Valley to the north and east

8 days (all in June and July) with one or more long-term Clark County sites greater than 8 hour standard of 85 ppb O 3 (May and June had high frequency of troughs passing through) Nearly every day South Coast maximum > San Joaquin Valley maximum > Clark County maximum

High regional ozone 500 mb chart

Low regional ozone 500 mb chart

Summer 2005 daily 8-hour maximums- Clark County, Jean, California Mohave Desert California Mojave Desert maximum typically considerably higher than Clark County maximum- provides high background

Summer 2005 daily 8-hour maximums- Jean and Clark County maximums Clark County maximum and Jean track well- bigger offset in July to early August than May-June; more local impact?

Summer 2005 daily 8-hour maximums- San Joaquin Valley, Mojave Desert, Palm Springs Climatology of California ozone patterns- August SJV max> Mohave max>Palm Springs, max shifts northward?

Used cluster analysis of wind field patterns to form groups of days with similar surface winds For each cluster of days, used cluster analysis to group hours with similar wind patterns Generated resultant winds and average ozone concentrations for each site for each daily cluster for each group of hours Used difference in ozone concentration from upwind to downwind of Las Vegas to estimate local versus regional transport contributions Generated and plotted HYSPLIT backtrajectories for each cluster Monitoring sites used in cluster analysis

Percentage of days in each cluster, by month; total number of days per cluster Dates Strong SW flow Light northerly flow SJV+LV increment Local terrain forced flowothers May June July Aug Total days per cluster Climatology of transport patterns helps build conceptual model- Local terrain forced flow common all summer; strong SW early summer, SJV+LV increment July peak

Nighttime downslope flow for terrain forced cluster

By 7-10 am heating of east facing Spring Mtns causes upslope flows

Late morning to afternoon, valley flow develops, transporting urban precursors to the northwest and O 3 is high there. About 9 ppb local enhancement

Synoptic flows weak, HYSPLIT backtrajectories from all over, not real helpful

San Joaquin Valley + Las Vegas has light SW transport overnight, elevated O 3 at high elevation sites

SJV + LV high O 3 most of monitoring area, high background at Jean + local enhancement of about 9 ppb

Backtrajectories for SJV + LV show flow from SF Bay area through San Joaquin Valley, over Tehachapi pass and then into Las Vegas in convergence zone

Strong SW flow shows high background O3

Afternoon shows highest ozone at background site – 1 exceedance day at Jean

Backtrajectories for strong SW flow show about equal frequency from SJV and South Coast

Cluster Regional Winds Midday surface winds Trajectory pathways Ozone levels Las Vegas ozone increment Strong SWSW SJV, South Coast Highest at background sites – 1 exceedance None apparent Light NNNNHighest at Boulder City – No exceedances 11 ppb SJV + LVWSWSESJVHighest NW LV Valley – 3 exceedances 8 ppb Local Terrain Forced VariedSEVariedHighest in NW LV Valley – 4 exceedances (2 fire days) 9ppb Pattern summary table

m AGL m AGL m AGL 9-11 am 3-6 pm7-10 pm Much vertical change in wind direction in morning, becomes SW all levels afternoon

Summary Consideration of regional and local (and global?) transport important in understanding causes of high ozone Consideration of regional and local (and global?) transport important in understanding causes of high ozone Spatial pattern analysis, Cluster analysis and backtrajectory analysis among the methods helpful in developing conceptual models Spatial pattern analysis, Cluster analysis and backtrajectory analysis among the methods helpful in developing conceptual models Far too few rural monitoring sites of O 3 and precursors in the western US to understand extent and cause of high ozone in the WRAP region Far too few rural monitoring sites of O 3 and precursors in the western US to understand extent and cause of high ozone in the WRAP region Need coordinated monitoring, modeling, and data analysis effort over a large region Need coordinated monitoring, modeling, and data analysis effort over a large region Use modeling, EI’s, and analysis of existing data to design monitoring networks Use modeling, EI’s, and analysis of existing data to design monitoring networks