## 4 Conclusions  
The effect of operational parameters and biomass physical properties on biochar surface properties was investigated. When the airflow rate increased, the BET surface area and basic functional groups of biochar also increased due to increases in reaction temperature, but no acidic functional groups were observed because of the oxidative nature of the process. The CEC of biochar also increased when the airflow rate increased, driven by a higher degree of oxidation in the top-lit updraft gasifier. The variations in the physical properties of the biomass were found to significantly affect the surface chemistry of the biochar. The increase in moisture content resulted in lower biochar pH and small BET surface area due to reduced oxidative effects in the reactor. Furthermore, the increase in woodchip particle size reduced the BET surface area of biochar and initially increased the pH of biochar, but  
Figure 3: (**A**) pH of biochar from wood chips produced at varying biomass compactness (airflow 20 L/min, moisture content 10%, particle size 7 mm), and (**B**) the correlation between pH and reaction temperature. Different letters represent significant difference level at 10%.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Biomass Compactness (kg)**} & \multicolumn{2}{c}{**Functional Groups**} & \multicolumn{2}{c}{**BET Surface Area**} & \multicolumn{2}{c}{**Atomic Ratios**} \\ \cline{2-5}  & **Carboxylic (mmol g\({}^{-1}\))** & **Basic (mmol g\({}^{-1}\))** & **m\({}^{2}\) g\({}^{-1}\)** & **H/C** & **O/C** \\ \hline
0 & 0 b & 0.115 a & 405.3 & 0.271 ab & 0.100 a \\
1 & 0.004 b & 0.093 a & 266.7 & 0.246 b & 0.063 b \\
2 & 0.011 a\({}^{-}\)b & 0.081 a & 281.9 & 0.248 ab & 0.067 b \\
3 & 0.016 a & 0.073 a & 191.8 & 0.331 a & 0.070 b \\ \hline \hline \end{tabular}  
Different superscripts represent significant differences in the order of a \(>\) b.  
\end{table}
Table 5: Chemical properties of biochar from wood chips produced with varying biomass compactness. BET: Brunauer-Emmett-Teller.  
further increase in particle size above 7 mm caused lower pH. As a result, the carboxylic functional groups reduced when the particle size increased to 7 mm, but increased for larger particles. The basic functional groups had an opposite trend versus particle size. Likewise, the increase in biomass compactness caused a decrease in the pH and smaller BET surface area of biochar because of less available air for combustion relative to the amount of biomass in the gasifier chamber, which caused less aggressive oxidation. Therefore, increase in compactness increased the carboxylic functional groups and reduced the basic functional groups.  
The manuscript was written through contributions of all authors. [PERSON] and [PERSON] conceptualized the experiments and wrote the manuscript. [PERSON], [PERSON] ([PERSON]), [PERSON] ([PERSON]), and [PERSON] performed the experiments of physical and chemical properties, and proofread the manuscript. [PERSON], [PERSON] ([PERSON]), and [PERSON] were awarded funds to carry out this project. All authors have read and agreed to the published version of the manuscript.  
This material was based upon the work supported by the U.S. National Science Foundation, Grant No. 1462732, the USDA National Institute of Food and Agriculture, Hatch project number NC02613, and the USDA National Institute of Food and Agriculture, Grant No. 12690802. The lead author was also partially supported by the SNI (Sistema Nacional de Investigacion, in Spanish) funded by the SENACYT (National Bureau of Science, Technology and Innovation) from the Government of Panama.  
We thank [PERSON] and [PERSON] at North Carolina State University for the access to their labs to perform the surface chemical analyses of the biochar samples. The authors declare no conflict of interest.