Observations of reactive nitrogen and ozone at a rural site in Michigan

TitleObservations of reactive nitrogen and ozone at a rural site in Michigan
Publication TypeThesis
Year of Publication2001
AuthorsThornberry TDean
DegreeDoctor of Philosophy
Number of Pages110 pp.
UniversityUniversity of Michigan
CityAnn Arbor, MI
Thesis TypeDoctor of Philosophy
KeywordsVOLATILE ORGANIC COMPOUNDS
Abstract

The primary goals of the research presented in this dissertation were 1) to determine the partitioning of reactive nitrogen and investigate the factors affecting the partitioning at a rural site with significant local biogenic VOC emissions subject to highly variable levels of reactive nitrogen depending on the direction of transport, and 2) to investigate the factors affecting the local photochemistry related to ozone production. Data from the PROPHET Summer 1998 intensive were used for the examination reactive nitrogen partitioning and data from the Summer 2000 intensive were used for evaluation of the local ozone photochemistry. During the PROPHET 1998 Summer intensive, measurements of total NOy, NOx (NO + NO2), PAN, PPN, MPAN, C3-C5 alkyl nitrates, total isoprene nitrates, HNO3, HONO, and aerosol NO3- and NO2- provided the opportunity for an investigation of several issues related to reactive nitrogen oxide chemistry at this site. Observations of NOy, NOx, PANs, and alkyl nitrates with respect to synoptic scale transport allowed analysis of source region contribution to observed nitrogen oxide levels and comparison of relative contributions of individual constituents to the overall reactive nitrogen budget under different levels of total reactive nitrogen loading. Measurements of MPAN and the first quantitation of ambient isoprene nitrates permitted an investigation of the contribution of local biogenic VOC chemistry to the processing and distribution of reactive nitrogen under varying levels of anthropogenic input. While it has been previously discussed that isoprene nitrates might comprise a substantial fraction of the missing NOy in high isoprene environments, and that isoprene nitrate formation might represent a signficant loss process for NOx [O'Brien et al., 1995, 1997; Liang et al., 1998] no measurements of ambient isoprene nitrates were available to allow the testing of these hypotheses. During the period when all measurements are present, a comparison between total NOy and the sum of the individually measured NOy components allowed us to test for potentially missing NOy species and examine the role of acidic gases and aerosols at the site. Additional measurements of HONO made during the Summer 2000 intensive allowed an improved, time-resolved examination of the contribution of HONO to the reactive nitrogen budget at the PROPHET site. Measurements of photolysis frequencies made during the Summer 2000 intensive, that were not made during Summer 1998, were used in conjunction with measurements of NO, NO2-, O3, OH, HO2, and isoprene to examine the photochemical ozone production and ozone production efficiency at the PROPHET site under various levels of reactive nitrogen precursors. Chapter 2 provides a description of the PROPHET site, research facilities, and the experimental methods of the measurements related to the analyses in this dissertation. The instrumental methods and measurements of NO, NO2, and NOy are discussed in detail and summaries of the measurements of other species and parameters are presented. Chapter 3 describes the results and analysis of the reactive nitrogen measurements and ozone photochemistry during the PROPHET Summer 1998 and Summer 2000 intensives. The Summer 1998 data is used primarily for examining the behavior of reactive nitrogen because of the more complete set of reactive nitrogen species measurements made during the Summer 1998 intensive, while the Summer 2000 data is used for an analysis of the in situ ozone photochemistry because of the availability of measured photolysis frequencies. Chapter 4 summarizes the results and conclusions from the analyses of Chapter 3 and presents a number of outstanding questions and suggestions for furture work.