被动采样法表征降水离子浓度和通量的局限性

降水(湿沉降)是大气污染物进入地表环境的主要途径,由于传统的被动采样法易受到颗粒物和气体干沉降的干扰,而采集到的混合沉降样品不能科学表征降水化学组成,也不能准确量化湿沉降通量,因此目前正逐步被主动采样法所取代. 采用主动和被动2种采样法同步采集了降水样品,以考察被动采样法表征降水离子浓度和通量的适用性. 结果表明:被动采样法采集的降水样品中离子浓度明显偏高,其中ρ(NO₃-)、ρ(Mg2+)、ρ(Ca2+)、ρ(K+)、ρ(Cl-)、ρ(SO₄2-)、ρ(Na+)和ρ(NH₄+)分别比主动采样法高65%、58%、43%、41%、35%、26%、10%和9%. 受大气污染物干沉降影响,被动采样法获得的NO₃-、NH₄+和SO₄2-混合沉降通量分别比主动采样法获得的湿沉降通量高79%、18%和35%,差异最显著的是NO₃-. 被动采样法可以收集氮和硫的湿沉降,但不能有效捕获二者以颗粒物和气体形式发生的干沉降,对二者沉降总通量(干沉降通量+湿沉降通量)低估达39%〔23 kg/(hm2·a)〕和40%〔20 kg/(hm2·a)〕. 因此,鉴于大气污染形势日趋严重,污染物的干沉降作用凸显,被动采样法已难以准确测算污染物从大气向地表的沉降总通量,需要全面考虑细颗粒物(粒径≤2.5 μm)和气态物种(如HNO₃)的贡献. 

Limitations of Passive Sampling Technique of Rainfall Chemistry and Wet Deposition Flux Characterization

Precipitation (wet deposition) is one of the major pathways for removal of air pollutants from the atmosphere. The traditional passive sampling method (bulk collector) has been gradually replaced by the active sampling technique (wet-only device), because the former was vulnerable to dry deposition of aerosols and gaseous pollutants, thereby affecting the chemical composition of precipitation and the quantification of wet deposition fluxes. Here, a wet-only device was co-located with a bulk collector in urban Beijing, allowing for the contribution of dry deposition to the bulk collector to be inferred. The results showed that the concentrations of NO₃-, Mg2+, Ca2+, K+, Cl-, SO₄2-, Na+and NH₄+ observed by the bulk sampler were 65%, 58%, 43%, 41%, 35%, 26%, 10% and 9% higher than those by wet-only deposition, respectively. In addition, due to the interference of dry deposition, bulk deposition fluxes of NO₃-, NH₄+ and SO₄2- were 79%, 18% and 35% higher than those by wet-only deposition, respectively. Bulk deposition measured by open rain collectors could capture most nitrogen (N) and sulfur (S) compounds via wet deposition, but were incomplete by dry deposition of gases and particles. The total deposition (wet plus dry) of N and S to ecosystems will therefore be underestimated by the use of bulk collectors in urban areas, with underestimated values of 23 kg/(hm2·a) (39%) and 20 kg/(hm2·a) (40%) respectively. Dry deposition plays an indispensable role in the removal of air pollutants during elevated air pollution. Consequently, traditional bulk deposition method has difficulty in characterizing the total flux of pollutants from the atmosphere to the earth's surface. Consideration of the contributions of fine particles (≤2.5 μm) and gaseous species is urgently required.