Contrasting Air Pollution Responses to Hourly Varying Anthropogenic NO_x Emissions in the Contiguous United States

Abstract. Monthly mean concentrations of air pollutants such as tropospheric nitrogen dioxide (NO₂) columns retrieved from satellite instruments are frequently used to infer NO_x emissions. An underlying assumption, also implicit in some global models, is that hourly variations in emissions average out in monthly means. To characterize the impacts of hourly emission variations, we use a global model with a refined ~14 km resolution over the contiguous United States (CONUS; MUSICAv0) and a regional CONUS inventory for July 2018. Switching from daily to hourly nitric oxide (NO) emissions (typically higher during the day and lower at night) yields differing spatial responses in surface nitrogen oxides (NO_x ≡ NO+NO₂) and ozone (O₃) concentrations in western versus eastern CONUS and in urban versus rural areas. Neglecting hourly variations in CONUS NO emissions products leads to pixel-level monthly mean errors of -49 % to +86 % (-1 to +8 ppb) for surface NO₂ and -22 % to +11 % (-7 to +5 ppb) for O₃, with tropospheric NO₂ columns showing similar spatial patterns (-12 % to +56 %). Although comparable in magnitude to a uniform 30 % NO emission reduction (-12 % to +9 %, -7 to +3 ppb for O₃), these distinct spatial patterns in the concentration responses reflect the influence of location-specific emission timing and meteorology. We conclude that models used to infer NO_x emissions from monthly mean concentrations may alias hourly emission variations into the inferred magnitude of emitted NO.