Temporal trends and weekend–weekday differences for benzene and 1,3-butadiene in Houston, Texas

Abstract

Temporal trends and weekend–weekday differences in benzene and 1,3-butadiene concentrations were investigated at a number of sites in the Houston, Texas area using monitoring data collected by the Texas Commission on Environmental Quality (TCEQ). The dataset included two networks, the Community Air Toxics Monitoring Network (CATMN), which has 24-h measurements every six days, and the Photochemical Assessment Monitoring System (PAMS), which includes hourly measurements. The period for data analysis was 1997–2004. The CATMN network had the larger number of stations. Over this time period, the network average benzene concentration at the CATMN stations was 0.93 ppb with individual stations ranging within a factor of about two of this network average. The network average 1,3-butadiene concentration was 0.57 ppb with most individual stations ranging within a factor of about six of this network average. These ranges suggest more spatial variability for 1,3-butadiene concentrations compared to benzene concentrations. Variations in meteorology can influence benzene and 1,3-butadiene concentrations making it difficult to examine the true concentration trends. Therefore, a generalized additive model (GAM) was used in a time-series analysis to adjust the benzene and 1,3-butadiene concentrations for variations caused by daily differences in meteorology. The analysis revealed very little difference in the weekend–weekday concentrations, except for a block of time during the morning rush hour where weekday concentrations were slightly higher. The GAM analysis showed that there was a significant decrease in the benzene and 1,3-butadiene concentrations throughout the trend period of 1997–2004. The average annual percent decrease across the sites was 1.7% for benzene for a meteorologically adjusted model (MA model) and 3.2% for a model without meteorological adjustment (raw trend or RT model). For 1,3-butadiene, there was a 3.7% annual decrease for MA model and 5.1% annual decrease for the RT model. For the five stations with data for each of the 8-year period, the mean overall benzene reduction over the 8-year trend period was 20% for the MA models and 29% for the RT models. For 1,3-butadiene, the reduction was 30% for the MA models and 35% for the RT models.