| Summary: | The total solar eclipse on August 21, 2017, provided a rare opportunity to observe and test our understanding of atmospheric dynamics and photochemical dependency on solar irradiance. Here, we utilize observations from the continuous monitoring of both inert and photoreactive trace gases near Boulder, Colorado, for contrasting the unique dynamic and photochemical forcings on the eclipse day. The monitoring station saw a 93% solar obstruction during the peak of the eclipse. Eclipse day data are contrasted with the full month's record from this site. The loss of irradiance caused cooling of the surface air by ~3oC, and weakened convective and tur-bulent mixing. This resulted in a buildup of non-reactive gases (methane, volatile organic com-pounds) as well as nitrogen oxides (NO, NO2) in the surface layer. In contrast, ozone (O3) de-clined by ~15 ppb during the first part of the eclipse compared to median August diurnal mixing ratios. Similar O3 signatures were observed at a series of network stations along the Northern Colorado Front Range. With the loss of irradiance, the initial ratio of NO/(NO+NO2) of ~0.2 dropped steadily, bottoming out at <0.01, but rebounded to ~50% above average levels towards the end of the eclipse. Above average O3 enhancements were seen in the afternoon hours fol-lowing the eclipse. The contrasting behavior of reactive and non-reactive gases, and compari-son with other published eclipse data, allow characterizing these responses as urban/polluted behavior.
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