Spatial and temporal variability in the ratio of trace gases emitted from biomass burning

Fires are a major source of trace gases and aerosols to the atmosphere. The amount of biomass burned is becoming better known, most importantly due to improved burned area datasets. The partitioning of biomass burned into emitted trace gases and aerosols, however, has received relatively little attention. To convert estimates of biomass burned to trace gas and aerosol emissions, most studies have used emission ratios (or emission factors [EFs]) based on the arithmetic mean of field measurement outcomes, stratified by biome. However, EFs vary substantially in time and space, even within a single biome, and in addition it is unknown whether the measurement locations provide a representative sample for the various biomes. Here we used the available body of EF literature in combination with satellite-derived information on vegetation characteristics and climatic conditions to better understand the spatio-temporal variability in EFs. While focusing on CO, CH4, and CO2, our findings are also applicable to other trace gases and aerosols. We explored relations between EFs and different satellite datasets thought to drive part of the variability in EFs (tree cover density, vegetation greenness, temperature, precipitation, and the length of the dry season). Although reasonable correlations were found for specific case studies, correlations based on the full suite of available measurements were less satisfying (r-max=0.62). This may be partly due to uncertainties in the driver datasets, differences in measurement techniques, assumptions on the ratio between flaming and smoldering combustion, and incomplete information on the location and timing of measurement. We derived new mean EFs, using the relative importance of each measurement location. These weighted averages were within 18% of the arithmetic mean. We argue that from a global modeling perspective, future measurement campaigns could be more beneficial if measurements are made over the full fire season, or alternatively if relations between ambient conditions and EFs receive more attention.