Role of chemical production and depositional losses on formaldehyde in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM)

Abstract. Formaldehyde (HCHO) is an important air pollutant due to its direct health effects as an air toxic that contributes to elevated cancer risk, its role in ozone formation, and its role as a product from oxidation of most gas phase reactive organic carbon. We make several updates affecting secondary production of HCHO in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM) in the Community Multiscale Air Quality (CMAQ) model. Secondary HCHO from isoprene and monoterpenes is increased, correcting an underestimate in the current version. Simulated 2019 June–August surface HCHO during peak photochemical production (11 am–3 pm) increased by 0.6 ppb (32 %) over the southeastern US and by 0.2 ppb (13 %) over the entire contiguous US. The increased HCHO compares more favorably with satellite-based observations from TROPOMI and observations from an aircraft campaign. Evaluation against hourly surface observations indicates a missing nighttime sink for HCHO which can be ameliorated by adding bidirectional exchange of HCHO and a leaf wetness dependent deposition process which increases nighttime deposition, decreasing 2019 June–August nocturnal (8 pm–4 am) surface HCHO by 1.1 ppb (36 %) over the southeastern US and 0.5 ppb (29 %) over the entire contiguous US. The ability of CRACMM to capture peak levels of HCHO at midday is improved, particularly at sites in the northeastern US, while peak levels at southeastern US sites are improved though still lower than observed. Using established risk assessment methods, lifetime exposure of the contiguous U.S. population (~320 million) to ambient HCHO levels predicted here may result in 6200 lifetime cancer cases, 40 % of which are from controllable anthropogenic emissions of nitrogen oxides and reactive organic compounds. Chemistry updates will be available in CRACMM version 2 (CRACMM2) in CMAQv5.5.