Do Atmospheric Non-Methane Hydrocarbons Concentrations Show Long-term Trends? Results from a 15-Year Auto-GC Time Series.

Speciated C₂-C₁₁ non-methane hydrocarbons (NMHC) have been measured online on an hourly basis at Lake Jackson, Texas, close to the Gulf of Mexico. Altogether 48 NMHCs along with NO, NO₂, NO_x, O₃ have been collected continuously from January 2004 - December 2018 under the auspices of the Texas Commission on Environmental Quality.

Based on HYSPLIT backtrajectories data was screened for background conditions representing marine wind sectors. The data set represents a combination of marine air masses mixed with local biogenic emissions. The data analysis addresses photochemical processing of air masses as reflected in the relationship of ln(n-butane/ethane) vs. ln(propane/ethane) and ln(i-butane/ethane) vs. ln(n-butane/ethane). In addition, key NMHC relationships for radical chemistry, e.g. i-butane vs n-butane for OH and Cl chemistry and i-pentane vs. n-pentane for NO₃ chemistry, are discussed.

Seasonal analysis revealed a clear trend with maximum NMHC mixing ratios in winter time and lowest mixing ratios in summer reflecting the impact of photochemical processes in summer. Propene equivalents were highest during summertime, with significant contributions from alkenes, including isoprene. The relation of propane/ethane vs ethane indicates seasonal variation with lowest values (i.e. most aged air masses) in winter.

An analysis of the atmospheric oxidation capacity using daily summer-time data (the time of maximum OH impact) and an analysis of the temporal trend of potential NMHC emission changes using daily night-time winter data (the time of minimum OH impact) will be presented.