Changes in summertime ozone in Colorado during 2000-2015

In 2016, the Denver Metro Area (DMA)/Northern Colorado Front Range (NCFR) was reclassified from a Marginal to a Moderate O-3 Non-Attainment Area due to the prevalence of high summer ozone (O-3) occurrences. Hourly surface O-3 data collected during 2000-2015 from a total of 80 monitoring sites in the State of Colorado were investigated for geographical features in O-3 behavior and O-3 changes over time. We particularly focus on summer O-3 (June, July, August), which is the time when most exceedances of the O-3 National Ambient Air Quality Standard (NAAQS) have been recorded. Variables investigated include the statistical (5th, 50th (median), and 95th percentile) distribution of O-3 mixing ratios, diurnal amplitudes, and their trends. Trend analyses were conducted for 20 site records that had at least ten years of data. The majority of Colorado ozone monitoring sites show an increase of the 5th (16 total; 11 of these are statistically significant (p-value <= 0.05) trends) and 50th (15 total; 4 statistically significant trends) percentile values. Changes for the 95th percentile values were smaller and less consistent. One site showed a statistically significant declining trend, and one site an increasing trend; the majority of other sites had slightly negative, albeit not statistically significant declining O-3. Ozone changes at the two highest elevations sites (>2500 m asl) are all negative, contrasting increasing O-3 at U.S. West Coast sites. NCFR urban sites do not show the rate of decreasing higher percentile O-3 as seen for the majority of urban areas across the U.S. during the past 1-2 decades. The amplitudes of diurnal O-3 cycles were studied as a proxy for nitrogen oxides (NOx) emissions and the diurnal O-3 production chemistry. The majority of sites show a decrease in the median summer O-3 diurnal amplitude (19 total/10 statistically significant). This is mostly driven by the increase in nighttime O-3 minima, which is most likely a sign for a declining rate of nighttime O-3 loss from titration with nitric oxide (NO), indicating a change in O-3 behavior from declining NOx emissions. Since median and upper percentile surface O-3 values in the DMA have not declined at the rates seen in other western U.S. regions, thus far the reduction in NOx has had a more pronounced effect on the lower percentile O-3 distribution than on high O-3 occurrences that primarily determine air quality. An assessment of the influence of oil and gas emissions on Colorado, and in particular DMA O-3, is hampered by the sparsity of monitoring within oil and gas basins. Continuous, long-term, high quality, and co-located O-3, NOx, and VOC monitoring are recommended for elucidating the geographical heterogeneity of O-3 precursors, their changing emissions, and for evaluation of the effectiveness of O-3 air quality regulations.