Heterogeneous chemistry of N₂O₅ and CINO₂ in China : implications for parameterizations, particulate nitrate formation, and impact on atmospheric oxidative capacity

Abstract

Dinitrogen pentoxide (N₂O₅) is a crucial nighttime reservoir of NOX in the atmosphere. Reactive uptake of N₂O₅ on atmospheric aerosols produces particulate nitrate (NO₃-) and nitryl chloride (ClNO₂), which have remarkable impacts on haze formations and atmospheric oxidation capacity, respectively. Previous studies established the global significance of the multiphase chemistry of N₂O₅ and ClNO₂ in air quality issues. However, a systematic understanding of the related mechanistic and kinetic aspects regarding N₂O₅ and ClNO₂ is limited especially in highly urbanized polluted regions, like some parts of China. This thesis presents a series of original ambient observations of N₂O₅ and ClNO₂ in the developed regions of north, east, and south China. The observation datasets demonstrate the abundance and variation of these important species in the real atmosphere. Combining with related observations such as NOx, O₃, VOCs, and aerosol chemical compositions, field observations in this thesis reveal key kinetic parameters of N₂O₅ heterogeneous chemistry and the atmospheric roles of N₂O₅ and ClNO₂. Major findings originated from this thesis are introduced as follows and presented in detail in subsequent chapters. A field campaign conducted in urban Beijing in 2017 showed significant N₂O₅ heterogeneous uptake in dry weather and sandstorm events. For the first time, a remarkable N₂O₅ hydrolysis rate (~0.01 s-1) with a high N₂O₅ uptake coefficient (γ(N₂O₅), 0.044) was found in a sand-dust event. The γ(N₂O₅) values (0.032 ± 0.010) were also determined for the intercepted urban air masses, in which γ(N₂O₅) exhibited significant correlations (R2=0.46) with Va/Sa, the ratio of particle volume to surface area. This result indicated a volume-limited mechanism in N2O5 hydrolysis. However, no significant relationship was found between the γ(N₂O₅) and chemical compositions of aerosols (e.g., Cl-, NO₃-, and H₂O). This result differed from previous studies where γ(N₂O₅) was enhanced by aerosol water and Cl-and reduced by NO ₃-. Current parameterizations of γ(N₂O₅) were found inconsistent with the observed γ(N₂O₅) here. According to the dependence of the observed γ(N₂O₅) on the Va/Sa ratio, a new parameterization of γ(N₂O₅) that matched with the observed γ(N₂O₅) was proposed in this study to describe γ(N₂O₅) in dry conditions. A series of field campaigns of ClNO₂ and N₂O₅ were also performed during 2017­2018 over the North China Plain (NCP). Instead of campaign-by-campaign analysis, systematic comparisons of wintertime field observations were performed regarding a rural site (Wangdu), a mountain site (Mt. Tai), and an urban location (Beijing) over the NCP. Nocturnal levels of ClNO₂ were lower at urban and rural sites but more elevated at the mountain site, reaching a maximum level of 1 ppbv. These winter ClNO₂ levels were generally lower than summer ones at the same locations due to net effects of lower NO₃ production rate (P(NO₃)), higher ratios of N₂O₅/NO₃, and smaller N₂O₅ uptake coefficients (γ(N₂O₅)) observed in winter. Significant daytime peaks of ClNO₂ were observed during all winter campaigns, featured by its high mixing ratios (up to 1.3 ppbv) during noon hours (10:00~12:00 LT). Vertical transport of ClNO₂ from the upper part of nocturnal mixing layers and a prolonged photochemical lifetime of ClNO₂ in winter may explain the daytime abundance of ClNO₂. The daytime-averaged Cl production rates (P(Cl)) when ClNO₂ showed daytime peaks were 0.17, 0.11, and 0.12 ppbv h-1 at the rural, urban, and mountain sites, respectively, which were 3~4 times higher than the campaign-averaged conditions. The Cl liberated during the daytime-ClNO₂ episodes increased HOx levels by 20~130 % and daily O3 productions by 5~27 % based on box model analysis. These results highlight the significance of daytime ClNO₂ during winter and imply the potential role of upper part of nocturnal mixing layers in atmospheric oxidations. A field observation performed at a rural site in Nanjing in 2018 focused on the production yield of Cl₂ and ClNO₂ (φ(Cl₂) and φ(ClNO₂)) from N₂O₅ uptake. Elevated mixing ratios of ClNO₂ (maximum 3.7 ppbv) were recorded during most nights. The φ(ClNO₂) value was high (0.56 ± 0.20) and positively correlated with the ratio of [Cl-]/[H₂O]. By incorporating the suppressing effect of organic aerosols, the performance of the revised φ(ClNO₂) parameterization was improved especially for the low to medium yield range (0~0.75). The abundance of Cl₂ and ClNO₂ significantly correlated during most nights. Further evidence showed that Cl₂ was probably a co-product of ClNO₂ produced by reactive uptake of N₂O₅ on acidic chloride-bearing particles. This new result contrasted with the previous view that Cl₂ was produced by direct ClNO₂ uptake. The φ(Cl₂) originated from N₂O₅ uptake was firstly defined in this thesis, which exhibited apparent correlations with the estimated [H+] and [Cl-]. Based on such relationships, a novel parameterization of φ(Cl₂) was proposed, which can be adopted in chemical transport models to represent nocturnal formations of Cl₂ and its impact on daytime photochemical oxidations. This thesis also analyzed the observation results of N₂O₅, ClNO₂, and nighttime Cl₂ at a remote coastal station in Hong Kong SAR, China. Features of the intercepted marine, continental, and mixed air masses were introduced. γ(N₂O₅) values were derived using the observation data of N₂O₅ and related species. Similar to the study in the 2017 April Beijing campaign, the parameterized γ(N₂O₅) was found inconsistent with the field-derived γ(N₂O₅) during this observation. This study thus developed another parameterization of γ(N₂O₅) for application in the coastal environments. The new parameterized γ(N₂O₅) considered Va/Sa, [H₂O], [Cl-], and [NO₃-] as size-dependent parameters and, for the first time, incorporated the reacto-diffusive length in formulating parameterizations. The derived parameterization of γ(N₂O₅) well represented the observed γ(N₂O₅), showing its capability in representing the reactive uptake of N₂O₅ in air quality models. Overall, this thesis has investigated the characteristics and impacts of the N₂O₅ heterogeneous chemistry in various environments of China, proposed new mechanisms of N₂O₅ uptake and the production of ClNO₂ and Cl₂, and revised the parameterizations of key kinetic parameters regarding this chemistry. These results provide insights into key atmospheric processes (gas-phase and heterogeneous chemistry) and improve the understanding of the fundamental science of nitrogen oxides and halogens. Implications include better representations of these critical atmospheric processes in chemical transport models to predict the air quality and scientific evidence for policymakers to mitigate haze and photochemical pollution.