Temporal, spatial and meteorological variations in hourly PM2.5 concentration extremes in New York City

Variations in the extreme percentiles of empirical hourly PM2.5 concentration distributions from a unique high-density network of 20 stations within New York City are statistically analyzed. Significant diurnal, seasonal and day-of-week variations are noted, with the highest concentrations typically found between 7:00 and 9:00 a.m., during summer, and on weekdays. The lowest concentrations are generally found during early morning hours (4:00-6:00 a.m.), in winter and on weekends. The amplitudes of these seasonal and diurnal cycles vary with percentile, with less pronounced cycles for the lowest and in some cases highest percentiles. The diurnal and day-of-week patterns suggest that although anthropogenic factors may be primarily responsible for the observed diurnal cycle, meteorological conditions also have some influence. There is little spatial variation in concentration across the city. Highly significant between-station correlations are obtained for all seasons. However, lower correlation is found in winter. Meteorologically, the highest PM2.5 concentrations occur with moderate southwesterly winds and high temperatures and humidity during summer. These conditions are related to the westward expansion of the Bermuda high-pressure system. Calm winds are conducive to the highest winter particulate concentrations. Relatively strong northerly winds are typically associated with the lowest PM2.5 concentrations. It appears that regional-scale processes dominate day-to-day changes in particulate concentrations across the city. (C) 2004 Elsevier Ltd. All rights reserved.