Aircraft observations of aerosols during a haze pollution in the North China Plain: Spatial-temporal distribution, vertical distribution and physicochemical characteristics

Atmospheric aerosols serve as critical drivers of haze pollution formation through bidirectional feedback mechanisms with planetary boundary layer (PBL) structure. Here, ground and aircraft observations from a severe haze pollution event in the North China Plain (NCP) in 2016 were analyzed to study the spatial and temporal distribution, as well as the vertical distribution and evolutionary characteristics of aerosols. Results suggested that PM_2.5 in the haze pollution gradually accumulated through three outbreak phases (OB) and three maintenance phases (MT). A unimodal distribution was observed from ground-based observations of aerosol number concentration (N_a), revealing the different formation and evolution characteristics of haze pollution across different pollution phases. Notably, NO₃⁻ drove the accumulation of pollution while SO₄²⁻ drove the maintenance. The aircraft observations data showed that the influence height of haze pollution varied at different phase, which were 850 m (OB1), 1450 m (OB2), 2200 m (early MT2) and 1500 m (late MT2). The proportion of NO₃⁻ decreased above influence height during OB1 and OB2, whereas that of SO₄²⁻ exhibited the opposite trend. Compared with OB phases, the contributions of NO₃⁻ and SO₄²⁻ significantly increased below influence height during MT2, whereas the contribution of Org decreased. N_a of OB phases initially increased and then decreased with height below influence height, while during the MT phase, it exhibited a significant decreasing trend with height. The spectra distribution of N_a was similar at different altitudes and phases, but the differences increased gradually with altitude. A narrow spectral width was found during late MT2 at 2000 m, the backward trajectory analysis showed that air masses from long-distance transport over Mongolia may be a main cause.