Contrasting behavior of slow and fast photoreactive gases during the August 21, 2017, solar eclipse

Abstract

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 slow and fast photochemically reacting trace gases near Boulder, Colorado, for evaluating 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 ~3°C, and weakened convective and turbulent mixing. This resulted in a buildup of slow photoreactive gases (methane, short-chain non-methane hydrocarbons), as well as total nitrogen oxides (the sum of nitric oxide (NO) + nitrogen dioxide (NO2)) in the surface layer. In contrast, ozone (O3) declined by ~15 ppb during the first phase 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 approximately two times above August median levels for this time of day towards the end of the eclipse. Above average O3 enhancements were seen in the afternoon hours following the eclipse at this and a series of other nearby surface O3 monitoring sites. The contrasting behavior of these slow and fast photoreactive gases, and comparison with other published eclipse data, allow characterizing these responses as more typical for an urban/polluted environment.