Long term (2005–2016) study of formaldehyde (HCHO) columns from satellite data in two regions in the south of Mexico. Evidence of the impact of agricultural activity

Abstract

In order to determine the driver factors and the principal months of enhancement of HCHO over the Oaxaca and the Chiapas regions, important agricultural areas located in the south of Mexico, HCHO columns from the Ozone Monitoring Instrument (OMI) were monthly averaged (HCHOOMI) and used in conjunction with monthly averaged isoprene emissions (isoprene), monthly NOx emissions from biomass burning (NOx_bb), and monthly averaged NOx emissions from soils with their different sources (biomes: NOx_bio, fertilization and manure: NOx_fer, deposition of N: NOx_dep, pulses: NOx_pul, and total: NOx_tot) from January 2005 to December 2016. Based on scatterplots grouped by month due to the seasonality of the variables, Spearman rank correlations, and multiple linear regression models, we determined that isoprene was an important driver of HCHOOMI in the Oaxaca region, contributing both to its seasonality and peak presence during April and May; whereas NOx_bb was a relevant source in the enhancement of HCHOOMI during April and May, months in which the fire season occurs in the south of Mexico. In the Chiapas region, we determined that both isoprene and NOx_fer were important drivers of the seasonality of HCHOOMI and its peak months during April and May. NOx_bb was predominantly involved in the HCHOOMI enhancement during April and May. The overall multiple linear regression model for each study region was significant (F-statistic = 21.35, p<0.0001 for the Oaxaca region; F-statistic = 101.59, p<0.0001 for the Chiapas region) and explained 72% of the variation of HCHOOMI in the Oaxaca region, 85% in the Chiapas region. Based on the multiple linear regression model for each study region, monthly averaged modeled HCHO columns (HCHOMOD) were obtained. With the estimated regression coefficients, the contribution (in %) of the relevant emission fluxes for each region were calculated in order to know which source was the predominant one in the exacerbation of HCHOMOD. During the peak months of HCHOMOD (April and May) in the Oaxaca region, isoprene predominates over NOx_bb. In the Chiapas region both NOx_bb and NOx_fer caused the exacerbation of HCHOMOD during April and May, whereas NOx_fer was the major variable that caused the increase of HCHOMOD during June, July, and August, especially from 2014 to 2016. Overall, in this region the emissions of NOx were more important in the increase of HCHOMOD than isoprene. NOx_bb and NOx_fer were enhanced principally due to the agricultural activity taking place in the Oaxaca and Chiapas regions. Thus, this work demonstrates the impact of agricultural activity on HCHO columns observed by the OMI instrument. In addition, this work shows that HCHO columns can be modeled using HCHO columns obtained from satellite data and emission inventories. The analysis performed here can be applied to other similar regions not only in Mexico but elsewhere around the world.