We use daily maximum 8 h average surface O3 concentrations (MDA8) for July 1995–2013, meteorological variables from the National Center for Environmental Prediction/National Center for Atmospheric Research Reanalysis, the North American Regional Reanalysis, and output from regional chemistry‐climate simulations to assess relationships between O3 and weather in the western U.S. We also explore relationships among July O3, satellite‐derived NO2, and meteorology. A primary objective of this study is to identify an effective method for correcting the effects of meteorology on July MDA8. We find significant correlations between July MDA8 O3 and meteorological variables for sites in or near Denver, Colorado, and Salt Lake City, Utah. The highest correlations were for 500 hPa heights, surface temperatures, and 700 hPa temperatures and zonal winds. We conclude that increased 500 hPa heights lead to high July O3 in much of the western U.S., particularly in areas of elevated terrain near urban sources of NO2 and other O3 precursors. In addition to bringing warmer temperatures and fewer clouds, upper level ridges decrease winds and allow cyclic terrain‐driven circulations to reduce transport away from sources. Because of strong, nearly linear responses of July MDA8 to 500 hPa heights, it is not reasonable to use uncorrected trends in peak O3 for assessments of the effectiveness of emissions controls for much of the western U.S. Robust linear regressions for July MDA8 and tropospheric NO2 with 500 hPa heights can be used to assess and correct trends in July MDA8 in the Intermountain West.

Transport is a key parameter in air quality research and plays a dominant role in the Colorado Northern Front Range Metropolitan Area (NFRMA), where terrain‐induced flows and recirculation patterns can lead to vigorous mixing of different emission sources. To assess different transport processes and their connection to air quality in the NFRMA during the FRAPPÉ and DISCOVER‐AQ campaigns in summer 2014, we use the Weather Research and Forecasting Model with inert tracers. Overall, the model represents well the measured winds, and the inert tracers are in good agreement with observations of comparable trace gas concentrations. The model tracers support the analysis of surface wind and ozone measurements and allow for the analysis of transport patterns and interactions of emissions. A main focus of this study is on characterizing pollution transport from the NFRMA to the mountains by mountain‐valley flows and the potential for recirculating pollution back into the NFRMA. One such event on 12 August 2014 was well captured by the aircraft and is studied in more detail. The model represents the flow conditions and demonstrates that during upslope events, frequently, there is a separation of air masses that are heavily influenced by oil and gas emissions to the north and dominated by urban emissions to the south. This case study provides evidence that NFRMA pollution not only can impact the nearby foothills and mountain areas to the east of the Continental Divide but that pollution can “spillover” into the valleys to the west of the Continental Divide.

Spectral aerosol optical depth, τa, observed at Mauna Loa, Hawaii, for the past 11 years is analyzed for background variations and the effects of two major volcanic eruptions: El Chichón in 1982 and Mount Pinatubo in 1991. A previously known annual variation and near‐background levels are present in the record. The data are of high accuracy, being primarily obtained from an automatic precision sunphotometer and reduced using the Langley‐plot slope method. The τa values over Mauna Loa were greater immediately after the eruption of El Chichón than after Mount Pinatubo due to more direct transport from El Chichón. However, Pinatubo had a greater temporally integrated impact because of greater erupted sulfur mass. A mean solar irradiance decrease of 6.5 (±2.5) W m−2 per 0.1 τa (500 nm) averaged over 24 hours is observed for both volcanic eruptions. Slight differences are suggested between the eruptions, but the differences are not statistically significant. Small differences between the two eruptions in the aerosol size distributions derived from τa observations are also indicated and are consistent with the suggested difference in total solar irradiance aerosol sensitivity. The near‐background τa values compare well with in situ surface‐based aerosol light‐scattering measurements extrapolated through the upper troposphere.

We describe the resources used, the deployment strategy, and the outcomes of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) experiment, which took place in the summer of 2014 in the Front Range of Colorado. We provide a history of air quality of the region and the outcomes of previously conducted experiments, describe the atmospheric conditions encountered during the campaign, and summarize the scientific findings that the campaign produced, together with the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER‐AQ) intensive, simultaneously carried out by the National Aeronautics and Space Administration. The goal of FRAPPÉ was to measure emission tracers and photochemical tracers from the ground and by aircraft to be able to quantify the contributions of various emission sectors to the photochemical production of ozone in the Colorado Front Range. We found major contributions from the fossil fuel extraction sector as well as the transportation sector, with minor contributions from agriculture, energy generation, and industry. The meteorological conditions were also found to be critical in creating situations conducive to high ozone in the area.