A high-resolution modeling study of a heat wave-driven ozone exceedance event in New York City and surrounding regions

Abstract

Heat-wave is one of key meteorological conditions causing the National Ambient Air Quality Standard (NAAQS) exceedance event in urban environment. In this study, the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem) was used to investigate an unusual heat-wave driven ozone (O3) exceedance event occurring on May 17–19, 2017 in New York City (NYC) and surrounding areas. The WRF/Chem simulations were conducted over the three nested domains with the finest grid spacing of 1.3 km. The simulations were evaluated with various observational data including in situ measurements of air quality and meteorological variables as well as the Lidar-measured vertical profiles. Overall, the WRF/Chem successfully captured the diurnal variations of air quality (e.g., O3 and nitrogen oxides (NOx)) and meteorological fields during the event. However, the O3 was under-predicted during the daytime peak hours but over-predicted during the nighttime. The under-predictions of O3 were highly associated with over-estimation of the planetary-boundary-layer (PBL) heights and uncertainties of emissions (e.g., NOx, and volatile organic compounds, VOCs), whereas the over-predictions were likely attributed to underestimation of the PBL heights and strong vertical mixing. Several findings are obtained from this study. First, the PBL growth plays a critical role in the development of O3 episode. The maximum PBLH and the largest PBL growth rates on a high O3 day are higher than those on a low O3 day. A strong low-level jet (LLJ) in the lower atmospheric boundary layer with increasing boundary-layer height was observed during the event. Second, the isoprene emissions calculated by the biogenic emission model are under-estimated significantly in the WRF/Chem model, which leads to the underprediction of daily O3 peak values. Third, the process analysis indicates that the local chemical productions are the dominant contributor with the contributions of 63–68% during the ozone-exceedance event.