Meteorological Modulation of PM2.5-Negative Air Ion Coupling Across Urban Forests of Beijing

Urban forests along the urban–suburban microclimate gradient of Beijing show coupled variation of fine particulate matter (PM2.5) and negative air ions (NAIs), yet how meteorological conditions modulate the strength of the PM2.5–NAI association remains poorly quantified. Using continuous monitoring from January to November 2024 at four Beijing urban forest parks, with the day as the statistical unit, this study combined factor-bin stratified Spearman correlation, clustering of joint meteorological states, and mixed-effects moderation models on 756 complete park-days. The association diverged among parks: negative at the urban park (ρ = −0.23, p < 0.001), positive at the foothill park (ρ = 0.19, p < 0.001), and near zero at the mountain reserve. Relative humidity was the dominant modulator: the wetland park coupled positively across the 40–80% RH range (ρ = 0.38–0.51) and both suburban parks coupled most strongly under humid conditions above 80% RH (ρ = 0.60–0.61, p < 0.001), whereas the urban park coupled most negatively under cold-dry conditions (ρ = −0.73). Four joint meteorological regimes emerged, each with a distinct park-specific coupling signature. NAI × meteorology interactions significantly improved model fit (p = 0.005), with moderation direction varying among parks. These daily-scale associations indicate that meteorological context, not concentration levels alone, conditions the strength of PM2.5–NAI coupling in urban forests, providing a quantitative basis for the sustainable management of urban green infrastructure.