The bio-chars used in this study contained both carbonized and non-carbonized domains, which potentially can express varied reactivities with sorbates and thus represent present different sorption mechanisms.Analysis of the sorption data suggests that monuron, diuron and linuron are likely binding to the bio-chars via multiple sorption mechanisms. The non-linearity of absorption isotherms varied between bio-chars. The non-linearity of sorption isotherms for monuron, diuron and linuron observed on the bio-chars is a characteristic of sorption processes arising from site-specific interactions occurring on the carbonized phase of the bio-char.The carbonized fraction of bio-chars is sometimes referred to as a “glassy” domain, whereas the non-carbonized soil organic matter is a rubbery domain.Generally, the sorption of organic compounds such as herbicides on carbonized phase of bio-char can be characterized by nonlinear adsorption ; however, sorption on the non-carbonized phase is better described by a partitioning mechanism that follows a linear isotherm.A lower non-linearity was observed in the low temperature bio-char sorption results and higher non-linearity was observed in the high temperature bio-char . These results indicate that a “glassy” domain sorption mechanism is involved in sorption of phenyluea herbicides to bio-char produced under high temperatures. The mechanism of low temperature produced bio-char sorption is similar to that involved in sorption to soil organic matter. The incomplete carbonization of low temperature bio-char results in bio-char with larger amounts of noncarbonized carbon than high temperature bio-char.

The microbial availability of carbon associated with the rubbery domain of low temperature bio-char is relatively higher than that associated with the carbonized phase of higher temperature bio-char.Hence,plastic garden pots the sorption capacity of phenylurea herbicides to high rubbery domain bio-char may be reduced over time due to degradation of the rubbery domain as bio-char ages after field application. Abundance of rubbery and glassy domains can also be inferred from the bio-char H/C ratios. Bio-chars with high H/C ratios, such as EB, contain larger amounts of the original organic residues. A decrease in H/C ratio indicates more complete carbonization and higher saturation in the bio-char. The 1/n value for diuron and linuron sorption data increased with the atomic H/C ratio of bio-chars, which indicates that the higher the aromaticity of sorbent, the higher the non-linearity of the sorption isotherms. It is noted that this positive correlation was observed in the higher Kow herbicides , but not in the lower Kow herbicide . This indicates that glassy domain of bio-chars plays an important role in high lipophilic herbicide sorption. The high sorption capacity of bio-chars for the phenylurea herbicides reported in this study is consistent with previously published data.Bio-char amendment to agricultural soil significantly enhanced sorption of linuron and diuron and reduced leaching of 12 kinds of phenylurea herbicides from soil to groundwater.The large capacity for bio-charsto adsorb herbicides also substantially reduced leaching of linuron, alachlor, and metalaxyl in a sandy soil. Sorption capacity of herbicides to bio-char amended soil can be lower than theoretical sorption capacity based on bio-char and soil sorption capacity measured by batch sorption experiments. Organo-mineral interactions between soil and bio-char can compete binding sites on bio-char surface with herbicides, which can diminish bio-char herbicide sorption capacity.During the ageing of bio-char, the organo-mineral interactions can also convert binding sites on bio-char surface, which can also influence herbicide sorption capacity of bio-char amended soil, both positive and negative impacts reported previously.On the other hand, bio-char amendment can reduce the effectiveness of pesticides in soil and has been shown to reduce the bio-availability of herbicides to weeds in soils.

This could require increased inputs of herbicides and increased costs of agricultural management. However, the increased adsorption capacity, if managed correctly, could possibly provide a mechanism that would permit a slow release source of herbicide from bio-char and thus lengthen the period of effectiveness of the herbicide application. Based on both lab and field scale experiments, the transport of herbicides in soil depends not only on soil properties but also climatic conditions, especially hydrological processes, such as rainfall events and soil moisture condition. These two factors can also impact the long term effects of bio-char soil amendment and interact in the ageing of bio-char. Sorption capacity of aged bio-char has been observed in some cases to decrease with time and, in other cases, remain similar to the behavior of freshly added bio-char.Based on the results above, herbicide application rates may need to be adjusted depending on how a particular bio-char ages and particular environmental conditions; this topic deserves more research. The deliberate setting of fires as a tool for agricultural management has a long history that remains ubiquitous around the world today . In modern agriculture, the principal benefit from these fires takes the form of avoided labor costs otherwise required to clear brush, remove crop residues, and manage invasive plant species . At the same time, these fires generate considerable smoke comprised of a number of pollutants that are known to be harmful to human health . Yet, the direct study of the causal relationship between agricultural fires on human health has been greatly hampered by concerns of endogeneity and the competing benefits and costs from local fires. One notable exception is the recent study by Rangel and Vogl , which examines the impacts of sugarcane harvest fires in Brazil on infant health by exploiting wind direction for empirical identification. Given the emergent literature showing that pollution can also harm a range of other human capital outcomes , the goal of this paper is to examine the impacts of agricultural fires on one important component of human capital – cognitive performance.

Our analysis of impacts on young and healthy adults in a high-stakes environment, generalizes and extends evidence from a recent working paper that examines the impact of fires on survey-based measures of cognitive decline amongst the elderly in China . More specifically, we exploit high-resolution satellite data on agricultural fires in the granary regions of China and a unique geocoded dataset on test performance on the Chinese National College Entrance Examination to investigate the impacts of fires on cognitive performance. This setting is attractive for a number of reasons. First, the majority of agricultural fires take place in the developing world where environmental controls are less stringent and the returns to human capital are generally substantial. China, in particular, is the largest grain producer in the world, with approximately one-third of all grain cropland managed through burning practices. 1 Second, the NCEE is one of the most important institutions in China. It is taken by all seniors in high school and the exam score is almost the sole determinant of admission to institutions of higher learning in China. As such, the NCEE serves as a critical channel for social mobility with important implications for earnings over the life cycle . Test takers face high-powered incentives to do as well as possible on the test and thus any impact from agricultural fires is likely to represent an impact on cognitive performance rather than effort. Finally,square pots several features of the NCEE make it particularly well suited to causal inference. The exam date is fixed, and thus self-selection on test dates are impossible. Fortuitously for our research design, the exam takes place during the height of the agricultural burning season. Moreover, students must take the exam in the county of their household registration , rendering self-selection on exam locations virtually impossible. Our NCEE data includes test scores for the universe of students who were admitted into colleges and universities between 2005–2011 from the granary regions which form the basis of our study. Despite the many virtues of our empirical setting, identifying the causal effect of agricultural fires on cognitive performance is challenging for reasons alluded to earlier. Agricultural fires are designed to reduce labor demands and improve farm profitability, both of which could also impact test performance. For example, if some agricultural labor is typically supplied by students, agricultural fires could improve test performance by providing them with more time to prepare for their exams. To address concerns of this type, we follow the approach recently pioneered by Rangel and Vogl , and leverage exogenous variation in local wind direction during the exam period. Specifically, we compare the effect of upwind and downwind fires on students’ test scores, and interpret that difference as the causal effect of pollution exposure on students’ cognitive performance net of economic impacts. The implicit assumption under this approach is that, ceteris parabus, students upwind and downwind of the fire are differentially exposed to its pollution but share equally in its economic influences. Our results suggest that a one-standard-deviation increase in the difference between upwind and downwind fires during the NCEE decreases the total exam score by 1.42 percent of a standard deviation , and further decreases the probability of getting into first-tier universities by 0.51 percent of a standard deviation. These impacts are entirely contemporaneous. Fires one to four weeks before the exam have no impact on performance. Reassuringly, neither do fires one to four weeks after the exam. The results are robust to alternative approaches for assigning pollution to test takers as well as a number of other specification checks. While a lack of pollution data from our study period does not allow us to utilize fires as an instrumental variable, data from a more recent period suggests that, consistent with evidence from Israel these cognitive impairments are likely the result of exposure to fine and coarse particulate matter. Together, these results suggest that agricultural fires impose non-trivial external costs on the citizens living near them. They also contribute to ongoing debates about the appropriate role of standardized testing in determining access to higher education and employment opportunities .

While our analysis is based on NCEE test performance, the impacts are likely much broader, touching all aspects of life that rely on sharp thinking and careful calculations. Indeed, the impacts in lower-stakes environs may well be larger as the incentives to succumb to the fatigue and lack of focus that also typically accompanies exposure to pollution are greater, and thus more likely to exacerbate any impacts on cognitive decision making. Given the importance of human capital for economic growth , these impacts should play an important role in the calculus of developing country policy makers when designing rules to manage the use of agricultural fires. The rest of the paper is organized as follows. In Section 2, we provide more background on the institutional setting. In Section 3 we describe each of the elements in our merged dataset. Section 4 describes our empirical strategy followed by our results in Section 5. Section 6 offers some concluding remarks. The practice of burning crop residues after an agricultural harvest in order to cheaply prepare the land for the next planting is commonplace across the developing world . While such burning can greatly reduce labor costs to farmers and potentially help with pest management, it also generates considerable particulate matter pollution . Particulate matter consists of airborne solid and liquid particles that can remain suspended in the air for extended periods of time and travel lengthy distances.These risks arise primarily from changes in pulmonary and cardiovascular functioning , which may, in turn, impair cognitive performance due to increased fatigue and decreased focus. Particles at the finer end of the spectrum are particularly important in our empirical setting since they are small enough to be absorbed into the bloodstream and can even become embedded deep within the brain stem . This can lead to inflammation of the central nervous system, cortical stress, and cerebrovascular damage . As such, greater exposure to fine particles is associated with lower intelligence and diminished performance over a range of cognitive domains . Consistent with this epidemiological evidence, a recent study of Israeli teenagers found that students perform worse on high-stakes exams on days with higher PM levels . As the name suggests, the NCEE is a national exam used to determine admission into higher education institutions at the undergraduate level in China. It is held annually on June 7th and 8th, and is generally taken by students in their last year of high school. In contrast to college testing in the U.S., it is almost the sole determinant for higher education admission in China.