Hydroxyl Radical Chemistry Below A Forest Canopy

Author : Dandan Wei
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (11 download)

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Download or read book Understanding the Oxidation of Biogenic Volatile Organic Compounds in the Amazon Rainforest written by Dandan Wei and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Biogenic volatile organic compounds (BVOCs) in the forested environments are receiving increased attention as more information emerges about their local, regional impacts on air quality and climate. The formation of tropospheric ozone (O3) and the generation of secondary organic aerosols (SOA) are among the many processes influenced by oxidation of VOCs. Although large amounts of work have been done in this field, the chemical processes associated with biogenic VOCs still remain poorly understood in forested environments. An accurate understanding of the VOCs oxidation is necessary for quantitative predictions of the concentrations of particulate matter, oxidation capacity, and consequent environmental and climate impacts. In this work, we focus on: (i) the characterization of the meteorological and chemical background conditions in the Amazon rainforst using field data; (ii) the environmental and biological controls on the temporal variations in the most abundant BVOC species - isoprene; (iii) linking turbulence and chemistry to evaluate the in-canopy mixing and chemical processes using a one-dimensional canopy model. As human encroachment increases in the Amazon rain forest, it is important to determine how anthropogenic emissions of reactive gases affect regional atmospheric chemistry. In the present study, we investigate the extent to which urban air plumes modify the levels of ozone (O3) and nitrogen oxides (NOx) in the downwind rain forest. The median mixing ratios of the background O3, NOx, and NOx oxidation products (NOz) were 20 (15 parts per billion on a volume basis, ppbv), 0.6 ppbv (0.6 ppbv), and 1.0 ppbv (0.5 ppbv) during the dry (wet) season at the study site. Compared to the background environment, air plumes from the city of Manaus had enhanced median mixing ratios for O3 and NOz by 30-50% and 40-90%, respectively. However, the enhancements of NOx in the air plumes were less than 20%, indicating that the majority of NOx was chemically converted to O3 and NOz during transport. Results from a photochemical model showed that an injection of 8 ppbv of NOx into the rain forest can cause up to 260% and 150% increases in O3 and hydroxyl radical (OH) levels compared to the background conditions, indicating the likely extent that NOx can modify the air quality and oxidative capacity in the Amazon rain forest. Slopes of the O3-temperature linear relationships increased with NOx levels from 3.7 to 6.5 ppbv per degree Kelvin during the dry season and 1.7 to 5.5 ppbv per degree Kelvin during the wet season. Average rates of change of the slope with respect to NOz were approximately 1.8 and 2.3 times higher than those with respect to NOx for the dry and wet seasons. One key conclusion of this study is that NOz substantially contributed to the O3 ozone formation response to temperature under enhanced NOx nitrogen oxide conditions in the forested environment.The Amazon rain forest is a major global isoprene source, but little is known about its seasonal ambient concentration patterns. To investigate the environmental and phenological controls over isoprene seasonality, we measured isoprene mixing ratios, concurrent meteorological data, and leaf area indices from April 2014 to January 2015 above a rain forest in the central Amazon, Brazil. Daytime median isoprene mixing ratios varied throughout the year by a factor of two. The isoprene seasonal pattern was not solely driven by sunlight and temperature. Leaf age and quantity also contributed to the seasonal variations of isoprene concentrations, suggesting leaf phenology was a crucial variable needed to correctly estimate isoprene emissions. A zero-dimensional model incorporating the estimated emissions, atmospheric boundary layer dynamics, and air chemistry was used to assess the contributions of each process on the variability of isoprene. Surface deposition was an important sink mechanism and accounted for 78% of the nighttime loss of isoprene. Also, chemical reactions destroyed isoprene and during 6:00 to 18:00 hours local time 56, 77, 69, and 69 % of the emitted isoprene was chemically consumed in June, September, December, and January, respectively. Entrainment fluxes from the residual layer contributed 34 % to the early-morning above-canopy isoprene mixing ratios. Sensitivity analysis showed that hydroxyl radical (HO) recycling and segregation of isoprene-HO played relatively lesser roles (up to 16 %) in regulating ambient isoprene levels. Nitric oxide (NO) levels dominated isoprene chemical reaction pathways associated with consumption and production of HO under low-NO and high volatile organic compound (VOC) conditions. While surface deposition and oxidative processes altered isoprene levels, the relative importance of these factors varied seasonally with leaf phenology playing a more important role.Many scientific questions and uncertainties remain concerning in-canopy processes and forest-atmosphere exchange. Among those, chemical transformations and turbulent vertical mixing that occurs within and above the canopy play an important role in the formation pathways of SOA. We evaluate the eddy diffusivity parameterization in a one-dimensional coupled canopy-chemistry mode (CACHE). In-canopy mixing is weak in the original CACHE model, indicating the original K-theory parameterization may not be adequate to capture the turbulence within the canopy. The CACHE model with the observed eddy diffusivity enhanced turbulence within the canopy and effectively reduced in-canopy concentrations and weakened the vertical gradient for OH, isoprene, alpha-pinene, limonene, and formaldehyde. The performances of the condensed chemical mechanism (RACM2) and nearly-explicit master chemical mechanism (MCM) in simulating the BVOC oxidation are evaluated. Both RACM2 and MCM show similar predictions of O3 isoprene, monoterpenes, and formaldehyde. However, The MCM predicted higher OH and NO concentrations than RACM2, indicating that the OH-NOx chenimstry is sensitive to the choice of chemical mechanisms in the forested environment. We also access the dependence of OH on the ambient levels of NO. The classical dependence of OH on NO were reproduced by both RACM2 and MCM, with a threshold NO value of 1.0 ppbv above the canopy. Accompanying the increase of NO concentration from 0.2 ppbv to 1.0 ppbv, the OH concentrations above the canopy increased by 150% (from 0.8 x 10^6 to 2.0 x 10^6) predicted by CACHE with the MCM, highlighting the important role of NOx in sustaining OH concentrations in the remote Amazon rain forest.