**1. Introduction**

Agricultural burning is the controlled incineration of biomass before and after harvesting. It is a common practice worldwide to harvest and to control and eliminate fungi and pests, reduce the erosion and maintain the soil quality for future crops at the lowest cost [1]. Despite the technological progress, currently, ~60% of the harvesting processes worldwide take place manually, which leads to the biomass burning over large areas of cultivated land (>1 ha per burning event) [2]. This practice has been widely studied for the case of wildfires, which has several implications on climate, atmospheric composition and air quality [3]. Presently, around 8600 Tg/year of biomass are burned globally, from which ~2000 Tg/year are related to agricultural crops [4]. Table 1 lists the main crops of interest for the environmental authorities. They correspond to those crops carried out extensively where open burning is a common practice. Now, the agricultural burning is mainly associated with industrial sugarcane crops, in countries such as Brazil, Colombia, Guatemala, India, Mexico and Costa Rica. During the harvesting period, biomass burning produces fine and ultrafine particles (particles with aerodynamic diameter *d* < 30 μm, and *d* < 100 nm, respectively) [2,5]. It also contributes to the emissions of carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), hydrocarbons (HC), nitrogen oxides (NOx), and other toxic compounds, such as Polycyclic aromatic hydrocarbons (PAHS) and Volatile organic compounds (VOCs), which are ozone precursors [6]. Agricultural burning could lead to short-term

(~1-day) episodes of air pollution, due to the capability of the emitted pollutants of being transported over a large spatial scale [7] and to their contribution to the formation of secondary pollutants such as ozone [8,9]. As a result, agricultural burning can cause adverse health e ffects on the people living nearby the burning areas [10,11].


**Table 1.** Emission factors and loading factors for di fferent crops [12].

\* PM, particulate matter with aerodynamic diameter d < 30 μm; PM2.5, particulate matter with aerodynamic diameter d < 2.5 μm.

Prado et al. [13] reported that, during the harvesting period in Mendonça, Brazil, the concentration of particulate matter registered in the atmosphere of urban areas, near to sugarcane fields, was almost 2.5-times higher than the World Health Organization air quality recommendation for short-term human exposure (24 h) [14]. Wagner et al. [15] measured ambient particle concentrations and particle type downwind, upwind and at several distances from agricultural burns in Imperial Valley, California. They reported significantly high PM10 and PM2.5 concentrations at locations less than 3.2 km from the nearest burning. Mugica et al. [16] estimated sugarcane-burning emissions in Mexican municipalities, and reported exceedances on the PM2.5 Mexican emission standards by at least 5.4 times, with an average of 86 ± 22 μg m<sup>−</sup>3. Their measurements were used to adjust the parameters of their Gaussian dispersion model, with which they studied 25 additional burning episodes. They observed concentrations up to 1000 μg/m<sup>3</sup> in urban areas when the wind blew towards those urban areas during the burning episodes.

Biomass burning is poorly regulated worldwide. Environmental authorities require the identification of the agricultural burning influence area as a legal requirement to start any regulatory actions to control this activity [17]. The influence area is defined as the largest area where the concentration of any pollutant exceeds the local National Atmospheric Air Quality Standards (NAAQS). The extent of the influence area depends on multiple factors including the size and geometry of the burning area, the local meteorological conditions and the pollutant considered. The influence area can be determined experimentally or by using any well-accepted dispersion model.

The experimental determination of the agricultural burning influence area possesses technical challenges due to the need for a large number of simultaneous measurements required for each variable affecting the dispersion phenomenon. Carney et al. [18] proposed a methodology to estimate the influence area as a cone whose orientation is aligned to the predominant wind direction. Aiming to advance this work, Hiscox et al. [17] measured the size and dispersion of smoke plumes during four sugarcane burning events during pre- and post-harvesting periods in Louisiana, USA, using a scanning, elastic-backscatter LiDAR (Laser Imaging Detection and Ranging). Their results show that particle concentration exceeded the NAAQS at distances of up to 300 m from the source, and that the vertical extension of the plume was about 2 km. They also found that wind speed and atmospheric stability conditions could make the plumes to travel distances greater than 45 km. Based on these studies, the USEPA developed guidelines that limit the meteorological conditions under which land cultivators of this region can burn [18]. The conclusions obtained with these experimental works are valid for the characteristics of the particular region, type of crops and meteorological conditions under which researchers conducted their experiments. The lack of generality of these conclusions limits the possibility of using them for an eventual policy to control agricultural burning in other regions.

Alternatively, air dispersion models can be used for estimating the size of the influence area under varying scenarios of meteorological conditions, crops types and area sizes. We propose the use of AERMOD for this purpose. It is a steady-state Gaussian dispersion model developed by the American Meteorological Society and the USEPA (The United States Environmental Protection Agency) for regulatory purposes. Gaussian dispersion models assume that pollutant concentration, downwind the source, follows a normal distribution in the horizontal and vertical direction. The main challenges of using this model for the study of the environmental impact of agricultural burning are:


The use of AERMOD for the determination of the agricultural burning influence areas has not been extensively employed. In this work, we systematically used AERMOD to study the dispersion of the pollutants emitted from short-term agricultural burning events, under varying conditions of emission rates, meteorological conditions, sizes and geometries of the burning areas. Then, based on the obtained results of pollutant concentration downwind: (i) we identified the size of the generated influence area; and (ii) we proposed alternatives of public policy to control this activity.
