Surface ozone exceedances in Melbourne, Australia are shown to be under NOx control, as demonstrated using formaldehyde:NO2 and glyoxal:formaldehyde ratios

Two and a half years of multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of nitrogen dioxide (NO2), formaldehyde (HCHO) and glyoxal (CHOCHO) are presented alongside in-situ ozone (O3) measurements in Melbourne, Australia. Seasonal and diurnal cycles, vertical profiles and relationships with key meteorological variables are provided. NO2 and CHOCHO were found at highest concentration for low wind speeds implying that their sources were predominantly localised and anthropogenic. HCHO showed an exponential relationship with temperature and a strong wind direction dependence from the northern and eastern sectors, and therefore most likely originated from oxidation of biogenic volatile organic compounds (VOCs) from surrounding forested and rural areas. The glyoxal:formaldehyde ratio (Rgf), reported for the first time in Australia, was consistently high compared to values elsewhere in the world with a mean of 0.105 ± 0.0503 and tended to increase with increasing anthropogenic influence. The HCHO:NO2 ratio (Rfn) was used to characterise tropospheric ozone formation conditions. A strong relationship was found between high temperature, low Rgf, high Rfn and high ozone surface concentrations. Therefore, we propose that both Rgf and Rfn may be useful indicators of tropospheric ozone production regimes and concentrations. The Rfn showed that the vast majority of high ozone production episodes occurred under NOx-limited conditions, suggesting that surface ozone pollution events in Melbourne could be curtailed using NOx emission controls. [Display omitted] •Vertical profiles of NO2, HCHO and CHOCHO are retrieved in Melbourne, Australia.•These results address a lack of data on ozone smog precursors in Australia.•Tropospheric HCHO:NO2 and CHOCHO:HCHO ratios give insight into ozone production mechanisms.•Anthropogenic glyoxal and biogenic HCHO drive a high CHOCHO:HCHO ratio (~10%).•Ozone pollution episodes could be mitigated by nitrogen oxide emission controls.