The impact of solar activity on ambient ultrafine particle concentrations: An analysis based on 19-year measurements in Boston, USA

Highlights: • This is the first study to investigate the impact of solar activity on ambient ultrafine particle concentrations over a period of 19-years. • Solar activity has a strong impact on daily ambient ultrafine particle concentrations. • Temporal variability of ambient ultrafine particle concentrations linked to solar activity may be critical for studies in public health, air pollution, and climate change. Solar radiation plays a major role in atmospheric photochemistry, contributing to the formation and growth of ultrafine particles (PN). PN affect global Earth's radiation balance, climate system, and human health. However, the impact of solar activity on ambient PN remains unclear. In this study, we investigated the associations between daily ambient PN concentrations [particle number (PN)/cm³] and solar radio flux [solar activity index (F_10.7 in sfu)] as a solar activity parameter, shortwave solar radiation (SWR), daylight time (DL), cosmic ray-induced ionization (CRII), and air pollution [PM_2.5, black carbon (BC) and SO₂] over a 19-year period in Boston, MA. We used generalized additive models adjusted for local environmental conditions. We found that F_10.7 was the strongest predictor for daily PN concentrations over all time lags (0–28 days of lags) and seasons. The effects were higher in winter and fall. In winter, an interquartile (IQR) of 60 sfu F_10.7 corresponded to an increase of 5770 PN/cm³ in the day of PN collection. In fall, an IQR of 75.5 sfu F_10.7 was associated with an increase of 5429 PN/cm³. The effects of F_10.7 on PN concentrations were slightly greater when the models were adjusted for air pollution. In summer, ambient PN concentrations were statistically significantly associated with F_10.7, SWR, and BC, with the strongest association found for PN and BC in the day of PN collection. Unlike the effects of F_10.7, SWR and local pollutants on PN concentrations, DL and CRII were negatively associated with ambient PN in the analyses. These findings suggest that solar activity may have a significant impact on daily ambient PN concentrations that affect the Earth's climate system and human health.