I have a time series which is the number of weekly flu cases from 2010 to early 2018 in one county. I want to remove seasonality from my data so I can have a clearer data to infer the relationship with environmental factor PM2.5. I know if the amplitude over time is constant I can use additive model to extract seasonality and if it changes (decrease or increase over time) I use multiplicative model.
Can you please help me what time of decomposition I can use to remove the seasonality from the data?
Also if I use additive model, when I subtract the seasonal component from the flu cases, I get a lot of negative values which do not make sense in terms of count. Is there any thing I can do about it?