Biomass burning at Cape Grim: exploring photochemistry using multi-scale modelling

Abstract. We have tested the ability of high resolution chemical transport modelling (CTM) to reproduce biomass burning (BB) plume strikes observed at Cape Grim in Tasmania Australia from the Robbins Island fire. The model has also been used to explore the contribution of near-field BB emissions and background sources to ozone (O3) under conditions of complex meteorology. Using atmospheric observations, we have tested model sensitivity to meteorology, BB emission factors (EF) corresponding to low, medium and high modified combustion efficiency (MCE) and spatial variability. The use of two different meteorological models varied the first (BB1) plume strike time by up to 15 hours, and duration of impact between 12 and 36 hours, while the second plume strike (BB2) was simulated well using both meteorological models. Meteorology also had a large impact on simulated O3, with one model (TAPM-CTM) simulating 4 periods of O3 enhancement, while the other model (CCAM) simulating only one period. Varying the BB EFs which in turn varied the non methanic-organic compound (NMOC) / oxides of nitrogen (NOx) ratio had a strongly non-linear impact on O3 concentration, with either destruction or production of O3 predicted in different simulations. As shown in the previous work (Lawson et al., 2015), minor rainfall events have the potential to significantly alter EF due to changes in combustion processes. Models which assume fixed EF for O3 precursor species in an environment with temporally or spatially variable EF may be unable to simulate the behaviour of important species such as O3. TAPM-CTM is used to explore the contribution of the Robbins Island fire to the observed O3 enhancements during BB1 and BB2. Overall, the model suggests the dominant source of O3 observed at Cape Grim was aged urban air (age = 2 days), with a contribution of O3 formed from local BB emissions. The model indicates that in an area surrounding Cape Grim, between 25–43 % of O3 enhancement during BB1 was formed from BB emissions while the fire led to a net depletion in O3 during BB2. This work shows the importance of assessing model sensitivity to meteorology and EF, and the large impact these variables can have in particular on simulated destruction or production of O3. This work also demonstrates how a model can be used to elucidate the degree of contribution from different sources to atmospheric composition, where this is difficult using observations alone.