|The study presents data on optical properties of biomass burning aerosols and on the effects dark- and photochemical aging on optical properties. The novelties of the study are the usage of sub-Saharan biomass fuels and the inclusion of the dark aging conditions in the experiments. The fact that dark aging affects optical properties in different way than photochemical aging is an important finding. Unfortunately, in the manuscript the experiments are quite poorly described both with respect to combustion conditions and chamber experiments. The combustion experiments are performed in a tube furnace using 0.5 g fuel pieces, which offers a good control over the combustion conditions. On the other hand, such a setup differs significantly from any real-world combustion appliance, both with respect to the combustion system geometry and the size of the fuel batch, which likely affects aerosol formation substantially. Therefore, a careful description the combustion conditions would be essential to make the results useful for the scientific community. Presently only MCE, which is a very simple metric depending on the exhaust CO concentration, is presented. The term “combustion temperature” is used throughout the manuscript without defining it (see detailed comments below). I would recommend to report at least the following data from the combustion experiments to make the data of this article more useful.|
- Air-to-fuel ratio
- time-dependent CO, CO2 and hydrocarbon concentrations during combustion of the fuel batch
- Adiabatic combustion temperature
Moreover, the description of the experimental conditions in the chamber seem to be very limited. Clearly presented information on the concentrations of O3, non-methane VOC and NOx in the chamber (e.g. in a table) would be essential to understand the aging experiments. For UV-induced aging experiments, determination of the OH-radical exposure would be a benefit to relate the findings with other chamber experiments. It also seems that all experiments were not successful (e.g. concentrations were too low for some instruments to obtain data) and the authors conclude that further studies are needed.
The paper includes lot of discussion on the effects of particulate chemical composition on its optical properties. The discussed UPLC/DAD-ESI-HR-QTOFMS analyses of chemical constituents in the aerosols are very interesting and potentially very important for the scientific community. However, it is quite strange that no chemical data is presented in this paper and there are just citations to the “Part2-paper” where obviously some of the discussed findings on chemical species are presented. In the current form it is difficult for the reader to find the data since no figures or tables are referred to. In my opinion, if the chemical data is to be published in another paper, it would be more logical to publish first the chemical data. In any case, the discussed chemical data must be clearly visible for the readers. Overall, the paper addresses an important topic and presents potentially scientifically valuable results, but has some significant shortages in description of the experimental conditions and aerosol chemical compositions.
More specific comments are below:
Line 102: The authors state that they “have full control of temperature, airflow, and material combusted.” If that is the case, more details on the combustion process should be available, than the single value of MCE, which has been provided in this paper. Moreover, the term “combustion temperature” is used throughout the manuscript without defining it (Pro tip: the tube furnace setpoint temperature is not necessarily the same as the combustion temperature). In general, during batch combustion of biomass there are many parameters affecting the composition of biomass burning PM, including, but not limited to:
- air-to-fuel ratio
- combustion rate
- fuel moisture content
- fuel particle size
- combustion chamber materials and geometry (these will influence the e.g. radiative heating of the fuel sample from the combustion chamber walls)
- residence time of the flue gases in the combustion appliance
- exhaust gas dilution temperature and dilution ratio
It would be important to include such information for example in the supplementary material.
Line 119: “0.5 g of fuel was burned normally, which produced about 600 to 800 μg m-3 of mass loading in the chamber”. As the chamber volume is 9 m3, the particulate mass derived from the 0.5 g batch is up to 7200 ug, which equals to particulate emission factor of 14400 mg/kg fuel. This is a quite high emission factor for any combustion system. The authors should discuss the relevance of this result in the paper.
Line 135: “For this work, only zero air was used at a flow rate of 10 sL min–1.”
What was the air-to-fuel ratio? Why air was used, although there was a possibility to use lower oxygen contents?
Line 139: “the tube furnace provides a uniform temperature throughout the sample.”
What is the basis for this argumentation? How much is the heat released by the burning fuel particle in comparison to the heat fluxes from the tube furnace walls? The adiabatic combustion temperatures could be calculated based on the fuel composition, air-to-fuel ratios and external heat from the tube furnace walls.
Line 193: “Aerosol growth in the chamber was expected to be due to coagulation, diffusional losses of particles,…” This is already mentioned on the line 184. -> the factors related to particle growth are unnecessary repeated.
Line 206: Please considering putting the section “2.1.3 Chamber cleaning” into the supporting material, as it consists only of technical details on the implementation of the experiments.
Line 222: “Optical properties were measured using the procedure described below soon after the chamber was well mixed.” This is not a very informative sentence.
Line 224: “For the photochemical aging, a new burn was made, and the particles were kept in the chamber for 12 hours with the UV lights on.” What does it mean that a new burn was made? How is this experiment different from the dark aging, apart from the fact that the UV-lights were on, remains unclear.
Line 229-: I suggest to start the paragraph by explaining how the experiment “in a polluted environment” is different to an experiment “clean environment”. In the present form the reader patiently has to go through lot of technical details related to measurement accuracy etc. before even knowing how these experiments were performed. It also remains unclear whether the VOC:s were injected into the chamber before or after feeding the BB-smoke sample? How did adding of VOC:s affect the NMVOC:NOx -ratio in the chamber? Are the NMVOC:NOx -ratios representative to the air of the South African sites in question?
Line 397-399: It is not possible to define the chemical composition of particles, based on the MCE. MCE is a very simple metric depending on the exhaust CO concentration. Generally there is no good correlation between CO and OC/BC in biomass combustion. In my opinion the authors should present measured data on BC and OC concentrations to state such findings. The color of the filter is not a very scientific method for chemical characterization of aerosol particles.
Line 430-433: “Chemical analysis revealed that for smoldering-dominated combustion, Eucalyptus and Acacia had a variety of compounds in common, such as lignin pyrolysis products, distillation products, and cellulose breakdown products. Several lignin pyrolysis products and distillation products are more prevalent in Eucalyptus than in Acacia, while pyrolysis products of cellulose and at least one nitroaromatic species were more prevalent in Acacia.”
Please refer to a table or figure where this information can be seen in a numerical form.
Line 467: “BB aerosol was aged without UV lights on and kept overnight for 24 hours.”
This is not much information about the conditions during the experiment. What was the ozone concentration during the dark aging period. Was O3 added into the chamber? Later (Line 481) it is argued that oxidation of organic aerosol was initiated by O3. Is there no measured information on O3 ? That would be a notable shortage to describe the experimental conditions.
Line 570-573: Data on the concentrations of chemical species in the chamber should be presented in this paper to discuss the findings.
Line 598-599: “Our connecting tubing was short enough (0.5 m) to neglect such a loss.” What is the basis for this conclusion? The length of the tube cannot be used as a proof of low losses.