Author: Wang, Yanan
Title: Soil nitrous acid emissions : mechanisms, impact factors, and their effects on air quality
Advisors: Wang, Tao (CEE)
Degree: Ph.D.
Year: 2024
Subject: Soils -- Environmental aspects
Fertilizers -- Environmental aspects
Nucleic acids
Air quality
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: 163 pages : color illustrations
Language: English
Abstract: Nitrous acid (HONO) plays a crucial role in the production of the highly reactive hydroxyl radical (OH), which acts as a major oxidant to remove various gases emitted into the atmosphere and produce secondary air pollutants. Previous studies have revealed the significant contribution of HONO emissions from soil to atmospheric HONO. The release of HONO from soil is mainly attributed to nitrification and denitrification processes, in which microbial communities and genes play a vital role. These processes are influenced by various environmental factors, including soil moisture, temperature, nitrogen (N) content, etc.
As a major factor, the effects of N fertilizers on HONO emissions have not been studied. In addition to HONO emissions introduced by fertilizers, emissions from unfertilized soils also accounted for a large part of total soil emissions due to their long duration. However, limited research has been conducted to quantify HONO emissions for different soil types, resulting in a lack of parameterization schemes to assess their impact on air quality. To address these research gaps, parameterization schemes linking HONO emissions from both fertilized and unfertilized soils to soil water content (SWC) and temperature were developed and implemented in a chemical transport model. This improved the simulation of atmospheric HONO levels.
We conducted experiments to measure soil HONO emissions after applying three commonly used fertilizers and found high HONO emissions from soils at high soil moisture, which contrasts to previous lower predictions at high soil moisture. This source is missing from current state-of-the-art air quality models. The effects of fertilization on HONO release lasted for approximately one week. Based on laboratory results, we developed a parameterization scheme and incorporated it into a chemical transport model. The inclusion of post-fertilization soil HONO emissions improved HONO predictions and demonstrated the significant impact of this emission source on air quality. Additionally, we found that the use of urea fertilizer resulted in the highest release of HONO compared to other commonly used fertilizers (ammonium bicarbonate and ammonium nitrate). Therefore, we propose that adjusting fertilizer structures may help to reduce HONO emissions and improve air quality in polluted regions with intense agriculture.
We also measured soil background (unfertilized) HONO and NO emission fluxes of 48 soil samples collected from diverse soil types across China. The results show much higher emissions of HONO than those of NO, which were caused by the higher promotion effects of long-term fertilization on the abundance of HONO-producing genes than NO-producing genes. This enhancement was greater in northern China than in southern China, which accounts for the higher emission fluxes in the former region than in the latter region. Simulations using a chemical transport model with the lab-derived parametrization show greater impacts of soil background HONO emissions on air quality than NO emissions, and larger impacts in the regions with lower anthropogenic emissions. With the projected continuous reduction of anthropogenic emissions, the contribution of soil emissions to reactive nitrogen levels is expected to increase substantially. Our study highlights the necessity for quantification of HONO emissions from soil together with NO emissions to improve the estimates of reactive N emissions and their impact on air quality.
Overall, this thesis measured both fertilized and unfertilized soil HONO emissions. The derived parameterization schemes improved the performance of the HONO simulations and demonstrated the important role of soil HONO emissions in atmospheric oxidative capacity and atmospheric chemistry.
Rights: All rights reserved
Access: open access

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