Response of formaldehyde to meteorology in Beijing: Primary or secondary contributions.

During high-temperature periods in summer, formaldehyde (HCHO) levels increase due to secondary production. However, recent studies have also shown a rise in the HCHO concentration in winter, but the underlying cause remains unclear. Here, HCHO observations in urban Beijing were conducted, the impact of meteorological differences between warm and cold seasons to HCHO concentrations was investigated. Additionally, the positive matrix factorization model was applied to the source apportionment of HCHO, with a focus on changes during pollution events. The results indicated that, during warm seasons, the secondary production of HCHO was driven by high temperature influenced by the low-pressure front, with the contribution of secondary production + background peaking at 85.9 % in the afternoon, exhibiting a unimodal diurnal variation. Conversely, during cold seasons, the influence of a uniform pressure field, coupled with weak winds, low boundary layers and high humidity, led to HCHO accumulation from primary emissions, resulting in multiday high-concentration pollution. During the most severe pollution periods, anthropogenic primary emissions contributed up to 91.7 %. Therefore, while the contribution of volatile organic compounds to HCHO levels through secondary production has been recognized, the significant impact of primary emissions during cold seasons cannot be overlooked.