LAUSR

Abstract A one-year observation of atmospheric peroxyacetyl nitrate (PAN) in the border of Beijing-Tianjin-Hebei (BTH) was conducted during 2018–2019 to assess the photochemical pollution in this region. PAN daily average was 2.23, 3.72, 5.34 and 2.44 ppb(v) in October, January, April and July. The higher PAN here compared to previous studies on urban Beijing implies the deterioration of photochemical pollution in BTH region. High positive correlation between PAN and CO in October and January points to the important role of tropospheric accumulation in PAN level in fall and winter; while ΔO3/ΔPAN analysis implies that the photochemical pollution was more aged in this border than in urban Beijing, and more aged in October than in April. Based on the calculations of PAN photochemical formation and thermal decomposition, PAN yield rate was determined, 0.20, 0.07, 0.47 and 0.08 ppt·s−1 for daily average, 0.87, 0.32, 0.95 and 0.93 ppt·s−1 for daily maximum, respectively in October, January, April, and July. Local photochemical pollutions still existed in the border of BTH in non-winter seasons. Potential source contribution function analysis (PSCF) found that the grids that most potentially contribute to high PAN appeared around the border site in July, 50–100 km away in direction of southwest in January, both around this site and along the southern trajectory clusters in October and April. Transport of PAN from South Hebei and Tianjin to Beijing could be seen in non-summer seasons. Regression analyses further quantified the contribution of regional transport to PAN in the border, roughly 65% (April and October), 93% (January) and 48% (July) at noon time. Finally, the comparison in PAN during 12:00–16:00 under various winds explores the more severe photochemical pollutions in South Hebei and Tianjin compared to Beijing in non-summer seasons, but the same level of photochemical pollutions in various cities of BTH in summer.