Measurements of formaldehyde at the U.S.–Mexico border during the Cal-Mex 2010 air quality study

Ambient formaldehyde (HCHO), along with other volatile organic compounds (VOCs), was measured using proton-transfer reaction mass spectrometry (PTR-MS) at a ground site along the U.S.–Mexico border during the Cal-Mex 2010 air quality study. During the observation period, the HCHO mixing ratio varied between 1.0 ppbv and 13.7 ppbv. On average, a daily maximum of 6.3 ± 2.6 ppbv occurred around 10 AM and a minimum of 2.8 ± 1.3 ppbv was observed around midnight. The early onset of the HCHO daily maximum (∼ 3 h before the solar noon) indicated the presence of primary HCHO sources and a fast photolysis loss of HCHO, consistent with a measured low ratio of HCHO to acetaldehyde of (2.5 ± 0.8). Using the simulated photolysis rates, we estimated the contribution of OH radical production from HCHO photolysis relative to that from O3 photolysis, with a ratio from 0.8 to 18 and the highest values around traffic rush hours. Hence, our results indicate that HCHO plays a dominant role in regulating the OH radical budget in the area. ► We measured HCHO using a fast-response PTR-MS during the Cal-Mex 2010 study. ► Up to 13.7 ppbv HCHO was measured along the Tijuana–San Diego boarder. ► In this study, HCHO was found to be more important than O3 in contributing to OH radicals and the local photochemistry.