A study of atmospheric oxidation chemistry in Australasia using MAX-DOAS measurements

Abstract

Oxidation reactions provide the fundamental mechanism for chemical cycling of atmospheric constituents. Understanding the key chemical and meteorological factors determining atmospheric oxidation chemistry has important implications for air pollution and climate change. With strong but isolated urban pollution sources and endemic plants known to emit high levels of volatile organic compounds (VOCs), Australasia is a fascinating place to study atmospheric oxidation chemistry in a range of remote, coastal and urban environments. To date however, Australia is vastly under represented in the observational atmospheric chemistry literature.

In this thesis, the passive solar multi-axis differential optical absorption spectroscopic (MAX-DOAS) technique is used to study key molecules in the atmospheric oxidation cycle including nitrogen dioxide (NO2), formaldehyde (HCHO), nitrous acid (HONO), glyoxal (CHOCHO) and iodine monoxide (IO). In particular, the role of these molecules in forming hydroxyl radicals (OH) and ozone (O3), two key daytime tropospheric oxidants, is studied using urban measurements at Broadmeadows, Victoria and at Garden Island in Western Australia. In the first comprehensive study demonstrating the MAX-DOAS technique in Australasia, the technique is verified using a range of analysis sensitivity studies, inter-instrument comparison and validation against in-situ and remote sensing methods. This includes the first long term MAX-DOAS-satellite comparison in the Southern Hemisphere, where MAX-DOAS measurements were in excellent agreement with Tropospheric Monitoring Instrument (TROPOMI) results in Melbourne, Australia and at Lauder in New Zealand.

The HONO observed at Broadmeadows was consistently at daytime concentrations exceeding what is expected given the known mechanisms. The maximum daytime HONO levels correlated with soil moisture levels indicating that soil-based emissions may play a role in the missing HONO source. The exponential dependence on temperature observed for HCHO at Broadmeadows suggests that the primary formaldehyde source there is oxidation of biogenic VOCs. In contrast, glyoxal appears to be more dependent on biomass burning or anthropogenic emissions. Chemical trajectory modelling studies at Garden Island suggest that isoprene oxidation is expected to be the dominant HCHO source, while anthropogenic emissions are expected to be the dominant CHOCHO source.

At Broadmeadows, HONO photolysis was found to be the greatest boundary-layer OH source in all months, contributing on average between 45-50 % of OH production. HCHO and ozone (O3) were often present at sufficient levels in summer to be commensurate with HONO as surface OH producers. The vertical profiling capability of the MAX-DOAS technique showed that while HONO typically dominated the OH production close to the surface, ozone and formaldehyde photolysis were the dominant mechanisms at higher altitudes. At Garden Island, a background marine boundary layer level of IO was detected which did not appear to have a locally significant O3-destruction orOH-formation role, but nevertheless provide evidence of ubiquitous iodine chemistry in the marine boundary layer. Using the ratio of HCHO to NO2, significant VOC emissions on hot summer days (as indicated by high observed HCHO levels), were found to shift the ozone production regime from VOC-limited to NOx-limited. The ozone production regime was found to be mostly NOx-limited at Garden Island. This has important implications for air pollution, for while NO2 was not found in exceedance of guideline pollution levels, O3 smog could still be reduced on hot days by curbing nitrogen oxide emissions.