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Formaldehyde (HCHO) is an important air pollutant with direct cancer risk and ozone-forming potential. HCHO sources are complex because HCHO is both directly emitted and produced from oxidation of most gas-phase reactive organic carbon. We update the secondary production of HCHO in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM) in the Community Multiscale Air Quality (CMAQ) model. Production of HCHO from isoprene and monoterpenes is increased, correcting an underestimate in the current version. Simulated June–August surface HCHO during peak photochemical production (11:00–15:00 LT, local time) increased by 0.6 ppb (32 %) over the southeastern USA and by 0.2 ppb (13 %) over the contiguous USA. The increased HCHO compares more favorably with satellite-based observations from the TROPOspheric Monitoring Instrument (TROPOMI) and from aircraft-based observations. Evaluation against hourly surface observations indicates a missing nighttime sink that can be improved by increased nighttime deposition, which reduces June–August nocturnal (20:00–04:00 LT) surface HCHO by 1.1 ppb (36 %) over the southeastern USA and 0.5 ppb (29 %) over the contiguous USA. The ability of CRACMM to capture peak levels of HCHO at midday is improved, particularly at sites in the northeastern USA, while peak levels at sites in the southeastern USA are improved, although still lower than observed. Using established risk assessment methods, lifetime exposure of the population in the contiguous USA (∼ 320 million) to ambient HCHO levels predicted here may result in 6200 lifetime cancer cases, with 40 % from controllable anthropogenic emissions of nitrogen oxides and reactive organic compounds. Chemistry updates will be available in CRACMM version 2 (CRACMM2) in CMAQv5.5. [ABSTRACT FROM AUTHOR]

The transition of onroad vehicles from internal combustion engine vehicles to electric vehicles (EVs) has been promoted in the United States at federal and state levels to reduce emissions of conventional air pollutants and CO 2. One state level effort is a plan from California to mandate that all new passenger vehicle sales are EVs by 2035. Using the Community Multiscale Air Quality (CMAQ) model, we analyze the potential air quality impacts of having varying amounts of EVs in the California fleet for a historical (2016) and a future (2028) year, with emphasis on ozone (O 3) and fine particulate matter (PM 2.5). We find that for both PM 2.5 and O 3 , reductions in concentrations scale approximately linearly with increasing EV fleet turnover. Improvements in air quality are widespread throughout the state, with the largest improvements occurring near Los Angeles and throughout the Central Valley. With full electrification of onroad vehicle fleets, the statewide population-weighted annual average PM 2.5 concentration is reduced by about 0.5 μg/m3 for both 2016 and 2028. Reductions in state population-weighted average fourth highest MDA8 O 3 are 6.6 ppb and 4.3 ppb for 2016 and 2028. The results indicate that there are air quality benefits, and thus health benefits, to be gained from the transition to EVs and that these benefits do not diminish with increasing rates of EV adoption. [Display omitted] • Vehicle electrification results in significant reductions in PM 2.5 and ozone. • Air quality improvements are largest in Los Angeles and the Central Valley. • Reductions in PM 2.5 and ozone scale approximately linearly with electrification. • Reductions in population weighted average PM 2.5 exceed unweighted average. • Brake wear, tire wear, and road dust are still significant sources of emissions. [ABSTRACT FROM AUTHOR]