Appendix E

Connecticut Department of Energy and Environmental Protection’s Review 

of the Seasonality of High PM2.5 Days.
E ‐ 1

Connecticut Department of Energy and Environmental Protection’s 

Review of the Seasonality of High PM2.5 Days 

This appendix explains the data sources, methodology and conclusions relating to an analysis of the 

seasonality of peak PM2.5 concentrations. The purpose of this analysis was to determine if the highest 

monitored PM2.5 concentrations in Connecticut’s portion of the NY-NJ-CT PM2.5 nonattainment area are 

limited to one season or can occur over two or more seasons during the year. If high PM2.5 levels only 

occur in a single season, then inventory development in support of PM2.5 redesignation efforts should be 

focused on that season. If high PM2.5 levels occur in multiple seasons, then an annual inventory can be 

used to support redesignation. 

Methodology: 

Connecticut reviewed fully QA’d and certified PM2.5 data from the 2004-2010 period, for the seven 

southwestern Connecticut PM2.5 sites (Bridgeport, Danbury, Norwalk, New Haven Criscuolo Park, New 

Haven State St, Westport and Waterbury). For each year, daily PM2.5 concentrations were ranked for each 

site from highest to lowest concentration, with the appropriate season identified for each day (i.e., Winter: 

Dec-Jan; Spring: Mar-May; Summer: Jun-Aug; Fall: Sep-Nov). Seasonal frequency during the 2004-

2010 period was then determined for two groupings of data: 1) The highest 2% of daily values each year 

(since the daily PM2.5 NAAQS is based on the 98th %-ile value) and 2) The highest 10 days each year (to 

provide a more robust comparison). The highest 10 day analysis was repeated for the most three recent 

years of data (2008-2010) to determine if recent large-scale regional reductions in sulfur dioxide 

emissions have influenced the seasonal distribution of high PM2.5 levels (sulfate is typically the largest 

contributor to high summer PM2.5 values, with less contribution during the winter1

). 

Results and Conclusions: 

Table 1 shows the seasonal distribution for the highest 2% of daily values2

 at each monitor for the period 

from 2004 through 2010. Overall, 49% of the highest PM2.5 days occurred during the summer, with 

winter exhibiting a secondary peak at 35%, while 12% of the highest PM2.5 days occurred during the fall 

season. There was minor variability between sites, with Danbury experiencing more high PM2.5 days 

during the winter (43% of the total number of high PM2.5 days) than the summer (38%) and Norwalk’s 

spring value (10%) at least double the value for the other six sites. 

                                                           

1 Bell, Michele et al. (2007) Spatial and Temporal Variation in PM2.5 Chemical Composition in the United States for

Health Effects Studies. Environmental Health Perspectives. v15(7) p 989‐995. 2 For sites with once‐in‐three‐day sampling, 2% generally represents the 3 highest daily values each year.  For sites

with every‐day sampling, 2% generally represents the 8 highest daily values each year.   
E ‐ 2

Table 1. Seasonal Distribution of the Highest 2% of Annual Daily PM2.5 Concentrations (2004-2010) 

Season Waterbury 

New Haven 

State St 

New Haven 

Criscuolo Westport Norwalk Danbury Bridgeport 

Overall

Average 

Summer 48% 48% 46% 57% 48% 38% 62% 49% 

Fall 14% 14% 11% 5% 14% 14% 10% 12% 

Winter 38% 38% 39% 33% 29% 43% 24% 35% 

Spring 0% 0% 4% 5% 10% 5% 5% 4% 

Table 2 summarizes the seasonal distribution based on the annual highest 10 daily values at each site for 

the full 7 year period (2004-2010). Although the same general bimodal distribution (i.e., summer/winter 

peaks) is observed as was found for the top 2% of PM2.5 levels, the results based on the highest 10 days 

each year reflect a greater occurrence of spring days than in Table 1 (12% versus 4%). 

When the highest 10 day approach is narrowed to the most recent three-year period (2008-2010), as 

shown in Table 3, the overall bimodal results shift such that winter becomes the predominant season 

(45% of the total number of high PM2.5 days), with summer exhibiting the secondary peak (36%). In the 

Northeast, the sulfate contribution to PM2.5 levels is typically greater in the summer than in the winter 

season due to temperature and chemistry factors. Thus, a possible reason for the recently observed 

seasonal shift could be more stringent regional sulfur emission limits imposed by the CAIR program on 

power plants. 

Table 2. Seasonal Distribution of Annual Top 10 Daily PM2.5 Concentrations (2004-2010) 

Season Waterbury 

New Haven 

State St 

New Haven

Criscuolo Westport Norwalk Danbury Bridgeport 

Overall

Average 

Summer 43% 46% 49% 43% 46% 43% 49% 45% 

Fall 17% 14% 10% 10% 13% 13% 13% 13% 

Winter 30% 29% 36% 36% 30% 31% 20% 30% 

Spring 10% 11% 6% 11% 11% 13% 19% 12% 

Table 3. Seasonal Distribution of Annual Top 10 Daily PM2.5 Concentrations (2008-2010) 

Season Waterbury 

New Haven 

State St 

New Haven 

Criscuolo Westport Norwalk Danbury Bridgeport 

Overall

Average 

Summer 37% 37% 23% 27% 43% 37% 50% 36% 

Fall 10% 10% 10% 10% 10% 7% 7% 9% 

Winter 43% 40% 60% 57% 37% 43% 33% 45% 

Spring 10% 13% 7% 7% 10% 13% 10% 10% 

Based on this analysis, high PM2.5 levels occur throughout the year in Connecticut portion of the NY-NJCT area, with the highest frequency of events in the summer and winter seasons. Since high PM2.5 levels 

occur in multiple seasons, annual inventories should be used to support the PM2.5 redesignation effort.